Chemical Ecology of Monoenoic Fatty Acids in Aquatic Environments
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Bio-Pharm Laboratories, 23615 El Toro Rd X, Lake Forest, CA 92630, USA
2
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Hydrobiology 2026, 5(1), 8; https://doi.org/10.3390/hydrobiology5010008
Submission received: 24 February 2026
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Revised: 11 March 2026
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Accepted: 12 March 2026
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Published: 18 March 2026
Abstract
Monoenoic fatty acids (MUFAs), defined by the presence of a single carbon–carbon double bond within a long aliphatic chain, constitute a structurally diverse and ecologically significant class of lipids widely distributed in aquatic organisms. In marine and freshwater environments, MUFAs are fundamental components of membrane phospholipids and storage lipids, where mono-unsaturation modulates melting point, lipid packing, and bilayer dynamics, enabling homeoviscous adaptation to fluctuations in temperature, pressure, salinity, and oxygen availability. Positional and geometric isomerism (e.g., cis-Δ5, Δ7, Δ9, Δ11, Δ13, and trans forms) further enhances biochemical diversity, providing sensitive chemotaxonomic markers and indicators of trophic transfer across food webs. In addition to common straight-chain monoenes, rare methyl-branched, cyclopropane-containing, and acetylenic derivatives occur in specialized aquatic taxa, reflecting evolutionary adaptation and ecological niche differentiation. Computational QSAR analyses suggest that monoenoic fatty acids and their unusual analogues occupy bioactivity spaces associated with lipid metabolism regulation, vascular and inflammatory modulation, antimicrobial defense, and membrane stabilization. This review integrates structural chemistry, biosynthesis, ecological distribution, trophic dynamics, and predicted biological activity of monoenoic fatty acids in aquatic systems, highlighting their dual role as adaptive membrane constituents and as biologically active mediators linking molecular lipid architecture to hydrobiological function and environmental change.
1. Introduction
Monoenoic fatty acids (MUFAs) are straight-chain aliphatic carboxylic acids containing a single carbon–carbon double bond, most commonly in the cis (Z) configuration, and typically ranging from C14 to C24 in aquatic organisms. Although structurally simple, the insertion of one double bond into an otherwise saturated hydrocarbon chain profoundly alters physicochemical behavior, membrane organization, and biological function. From a hydrobiological perspective, MUFAs represent not merely structural lipids but dynamic mediators linking molecular-scale chemistry to organismal adaptation and ecosystem processes [1,2,3,4,5].
In marine and freshwater environments, temperature, salinity, hydrostatic pressure, and oxygen availability fluctuate across spatial and temporal gradients. Aquatic organisms must continuously adjust membrane lipid composition to maintain optimal membrane viscosity and protein function—a process known as homeoviscous adaptation. The presence of a single cis double bond introduces a conformational “kink” in the acyl chain, reducing van der Waals packing efficiency, lowering melting point, and increasing bilayer fluidity [6,7,8,9]. Thus, mono-unsaturation serves as a finely tunable structural mechanism enabling phytoplankton, bacteria, invertebrates, and fish to maintain membrane performance under environmental stress. Elevated proportions of C16:1 and C18:1 monoenoic fatty acids are frequently observed in cold-water phytoplankton and zooplankton, underscoring their role in thermal adaptation [10,11,12,13].
Among naturally occurring MUFAs, oleic acid (C18:1Δ9) and palmitoleic acid (C16:1Δ9) are particularly widespread and ecologically significant. Oleic acid (ω-9) is abundant in higher trophic organisms and often reflects dietary accumulation and metabolic modification, whereas palmitoleic acid (ω-7) is frequently associated with phytoplankton and certain bacterial taxa, serving as a chemotaxonomic and trophic biomarker. The distribution of positional isomers (Δ vs. ω notation) further provides insight into biosynthetic pathways and community composition in aquatic ecosystems [14,15,16,17].
Beyond their structural role in membranes and energy storage lipids, monoenoic fatty acids exhibit notable biological activity. In aquatic organisms, MUFAs participate in immune modulation, oxidative stress responses, and regulation of lipid metabolism. They influence membrane protein function, receptor signaling, and microbial interactions. Certain monoenoic fatty acids display antimicrobial properties, potentially contributing to biofilm dynamics and host–microbe interactions in aquatic environments. Additionally, MUFAs can act as ligands for nuclear receptors such as peroxisome proliferator-activated receptors (PPARs), thereby influencing lipid homeostasis and inflammatory pathways in fish and invertebrates [18,19,20,21].
To complement experimental observations and expand understanding of potential bioactivities, quantitative structure–activity relationship (QSAR) analysis and PASS (Prediction of Activity Spectra for Substances) modeling were employed in this study. These computational approaches allow prediction of probable biological activities based solely on molecular structure. Using structural descriptors derived from chain length, degree of unsaturation, and functional group characteristics, QSAR modeling provides insight into physicochemical parameters governing membrane interaction and bioactivity. PASS analysis further predicts probabilities (Pa values) for a spectrum of biological effects, including anti-inflammatory, antimicrobial, lipid-regulating, and receptor-modulating activities. Such computational predictions are particularly valuable in hydrobiology, where direct bioassays across diverse taxa are often limited, and they enable integration of molecular chemistry with ecological function [1,2,22,23,24].
Monoenoic fatty acids play an important ecological and biological role in aquatic ecosystems, where they function not only as structural components of lipids but also as key mediators of energy transfer, physiological adaptation, and chemical signaling. These fatty acids are widely distributed among marine and freshwater organisms, including phytoplankton, bacteria, algae, invertebrates, and fish, and they contribute significantly to the composition of membrane phospholipids, storage lipids, and signaling molecules. In aquatic environments, monoenoic fatty acids are important components of the trophic transfer of organic matter, serving as biochemical markers that trace energy flow through food webs from primary producers to higher trophic levels. In addition, variations in the distribution and composition of monoenoic fatty acids often reflect environmental conditions such as temperature, nutrient availability, and salinity, making them useful indicators of ecological adaptation and environmental stress. Some monoenoic fatty acids also participate in chemical communication and defense mechanisms, influencing interactions among microorganisms, algae, and marine invertebrates. Consequently, these lipids represent an essential link between biochemical processes and ecological dynamics in aquatic systems, highlighting their importance for understanding the structure and functioning of marine and freshwater ecosystems [1,2,3,5,7,12,14].
By combining structural chemistry, ecological distribution, experimentally documented biological roles, and QSAR/PASS-based activity prediction, this review positions monoenoic fatty acids as chemically minimal yet functionally versatile components of aquatic systems. Their single double bond represents a subtle structural modification with far-reaching implications for membrane dynamics, organismal adaptation, trophic interactions, and ecosystem-level processes.
2. Palmitoleic Acids (C16:1) and Their Properties
Palmitoleic acid (C16:1) is not confined to the commonly cited Δ9-cis configuration but exists as a series of positional and geometric isomers that expand its structural and potentially biological diversity. In addition to the predominant cis-9-hexadecenoic acid (C16:1Δ9), five cis positional isomers have been identified in natural and microbial systems: Δ5, Δ7, Δ9, Δ11, and Δ13 [25,26,27,28,29]. These isomers differ in the location of the double bond along the carbon chain, resulting in subtle but significant variations in molecular conformation, packing behavior, and interaction with lipid bilayers. Shifting the double bond position alters the distance between the polar carboxyl headgroup and the site of unsaturation, which in turn affects membrane insertion depth, lateral mobility, and susceptibility to enzymatic oxidation. In aquatic environments, certain isomers (e.g., C16:1Δ7 and C16:1Δ11) are associated with specific bacterial and phytoplankton taxa and therefore serve as chemotaxonomic markers in trophic and biogeochemical studies. In addition to these cis forms, trans-isomers of palmitoleic acid also occur, either through microbial isomerization processes or environmental stress responses [30,31,32,33,34]. trans-Configurations produce a more linear chain geometry, increasing packing efficiency and raising melting temperature, thereby functionally resembling saturated fatty acids. From a biological activity perspective, positional and geometric isomerism may influence membrane fluidity regulation, receptor interactions, and predicted activity spectra as determined by QSAR and PASS analyses. Thus, palmitoleic acid represents not a single molecular entity but a structurally nuanced family of monoenoic fatty acids whose ecological distribution and functional properties are shaped by double-bond position and configuration [33,35].
Freshwater phytoplankton communities are typically dominated by three major groups: green algae (Chlorophyta) [36,37,38], cyanobacteria (Cyanobacteria) [39,40,41,42,43], and diatoms (Bacillariophyta) [44,45,46,47], each contributing distinct structural and biochemical lipid signatures. Among green algae, filamentous genera such as Spirogyra, Mougeotia, and Zygnema (see Figure 1) as well as unicellular taxa like Chlorella, are widely distributed in ponds, lakes, and slow-moving streams, often forming visible mats under nutrient-rich conditions [36,37,38,48,49]. Cyanobacteria, historically referred to as blue-green algae, are represented prominently by bloom-forming genera such as Microcystis, which frequently dominate nuisance and harmful algal blooms in eutrophic waters [50,51,52]. Diatoms (Bacillariophyta), characterized by their siliceous frustules, are ubiquitous microscopic algae inhabiting diverse freshwater environments and contributing substantially to primary production. Additional phytoplankton groups include euglenoids, golden algae (e.g., Prymnesium parvum), and freshwater red algae, which may become locally important depending on environmental conditions. Across these taxa, palmitoleic acid (C16:1, see Figure 2, and activity shown in Table 1) and its positional isomers (Δ5, Δ7, Δ9, Δ11, Δ13) are consistent but moderate constituents of total fatty acids, typically representing approximately 1–8% of the lipid fraction, with proportions strongly influenced by water temperature [25,26,27,28,29]. Cooler conditions generally favor increased monoenoic content as part of homeoviscous adaptation, whereas warmer waters often shift lipid profiles toward more saturated species. Thus, the distribution of palmitoleic acid isomers across cyanobacteria and freshwater algae reflects both taxonomic identity and environmental regulation, linking lipid chemistry to ecological dynamics and thermal adaptation [38,39,40,41,42].
In addition to the Δ9 isomers, palmitoleic acid also occurs as positional variants such as (6Z)-hexadecenoic acid (C16:1Δ6, ω-10, see Figure 2, and activity shown in Table 1), commonly known as sapienic acid. This cis-Δ6 isomer differs structurally from the more prevalent Δ9 form by the proximal location of the double bond relative to the carboxyl group, which alters the spatial distribution of conformational flexibility along the hydrocarbon chain [53,54,55].
The shift in unsaturation position affects membrane insertion depth, intermolecular packing, and susceptibility to enzymatic transformation. While sapienic acid is well known as a major component of human sebum, it has also been reported in certain plant species, including Thunbergia alata [56,57]. The occurrence of (6Z)-hexadecenoic acid in such botanical sources further illustrates the structural diversity of monoenoic C16 fatty acids in nature and highlights the importance of precise double-bond assignment when interpreting ecological or physiological data. From a hydrobiological and QSAR/PASS perspective, positional isomerism—such as Δ6 versus Δ9—may significantly influence predicted biological activity spectra, including antimicrobial, membrane-modulating, and signaling-related probabilities. Therefore, careful discrimination among C16:1 positional isomer is essential when assessing their ecological roles and biological functions.
3. trans-Palmitoleic Acid
trans-Palmitoleic acid (TPA) is a monoenoic hexadecenoic fatty acid containing a single trans-double bond at the 9th carbon in Δ-nomenclature (trans-9-C16:1) or at the n-7 position in ω-nomenclature (trans-C16:1 n-7). Structurally, it is the geometric isomer of cis-palmitoleic acid (cis-9-C16:1, ω-7), but this apparently minor difference in double-bond geometry produces marked changes in molecular conformation and physicochemical behavior [58,59,60,61].
Accurate identification of trans-palmitoleic acid (TPA, trans-C16:1) requires careful analytical characterization because positional and geometric isomers of C16:1 fatty acids often exhibit very similar chromatographic behavior. Simple gas chromatography (GC) of fatty acid methyl esters (FAMEs) can separate many cis/trans isomers, but it is often insufficient to unambiguously determine the exact position and configuration of the double bond. Therefore, more advanced analytical techniques are commonly applied to confirm the structure of monoenoic fatty acids in complex biological samples [1,2].
One of the most widely used methods for determining double-bond position is gas chromatography–mass spectrometry (GC-MS) analysis of dimethyl disulfide (DMDS) derivatives. In this approach, the unsaturated fatty acid methyl ester reacts with DMDS under catalytic conditions, producing an addition product in which the sulfur atoms attach across the double bond. This reaction effectively “marks” the position of the double bond. During mass spectrometric fragmentation, the molecule splits at the sulfur-substituted carbons, producing two diagnostic fragment ions whose masses correspond directly to the location of the original double bond. As a result, the GC-MS spectrum of the DMDS adduct provides clear structural evidence for the precise position of unsaturation in the fatty acid chain. This technique has become a standard approach in lipid chemistry for distinguishing between closely related positional isomers of monoenoic fatty acids [3,4].
In addition to determining double-bond position, it is also necessary to establish the geometric configuration (cis or trans) of the unsaturation. This is typically achieved using high-resolution capillary gas chromatography, often employing highly polar stationary phases such as cyanopropyl polysiloxane columns, which allow efficient separation of cis and trans fatty acid isomers. Complementary approaches such as silver-ion chromatography (Ag+-HPLC) may also be used to improve separation of geometric isomers before GC or MS analysis. In some studies, additional confirmation can be obtained by nuclear magnetic resonance (NMR) spectroscopy, which can provide independent evidence for double-bond geometry based on characteristic coupling constants and chemical shift patterns [3,4].
These analytical distinctions are particularly important in the case of trans-palmitoleic acid (TPA) because this compound is frequently confused in the literature with cis-palmitoleic acid (CPA) or with other positional trans isomers of C16:1 fatty acids. Moreover, TPA is sometimes referred to as palmitelaidic acid, which further contributes to terminological ambiguity. Accurate structural identification using reliable analytical methods such as GC-MS of DMDS derivatives, Ag+-chromatography, and high-resolution GC is therefore essential to avoid misclassification of these fatty acids in biochemical and ecological studies. According to regulatory classifications (e.g., Codex Alimentarius, EFSA, FDA), TPA belongs to the broader trans-fatty acid category and more specifically to the trans-mono-unsaturated fatty acid (trans-MUFA) family [1,2,3,4,58,59,60,61,62,63].
Interest in TPA emerged in parallel with studies on cis-palmitoleic acid. In 2008, Cao and colleagues described CPA as a putative adipose-derived “lipokine” capable of modulating hepatic de novo lipogenesis and improving insulin sensitivity in mice. However, in humans, the majority of circulating CPA appears to originate from hepatic de novo synthesis rather than adipose secretion, complicating interpretation of its endocrine role. In this context, Mozaffarian and co-workers proposed that circulating TPA—primarily derived from ruminant dairy fat and not endogenously synthesized in significant amounts—could serve as a biomarker or functional analog to better understand the physiological relevance of ω-7 fatty acids [64,65]. This hypothesis generated substantial interest, particularly because TPA belongs to the trans fatty acid family, traditionally associated with adverse cardiometabolic outcomes. Earlier, TPA had been only sparsely reported in dairy products and received little attention. The suggestion that a naturally occurring trans-MUFA might exert neutral or even beneficial physiological effects created what has been described as a “paradox” surrounding this molecule.
From a hydrobiological and structural perspective, the distinction between cis- and trans-C16:1 isomers is not merely nutritional but biophysical. Double-bond geometry determines membrane packing, lipid phase behavior, and potentially receptor interaction profiles. QSAR and PASS analyses further indicate that geometric configuration influences predicted probabilities for lipid-regulating, anti-inflammatory, and metabolic activities, underscoring the necessity of precise chemical identification when interpreting biological data. Clarifying the structural identity, origin, and functional properties of TPA is therefore essential for disentangling its ecological, physiological, and metabolic roles and for avoiding persistent confusion between cis- and trans-ω-7 monoenoic fatty acids [66,67,68].
In addition, it is known that trans-palmitoleic acid (C16:1 n-7 or trans-9-C16:1) is a naturally occurring monounsaturated fatty acid present in dairy products and ruminant meat [69,70]. Unlike industrially produced trans fats generated during partial hydrogenation, this ruminant-derived trans isomer has been associated with favorable metabolic indicators, including lower insulin resistance, reduced risk of type 2 diabetes, and improved circulating lipid profiles. It is widely used as a biomarker of dairy fat consumption in epidemiological studies and has been correlated with decreased cardiometabolic risk in several cohorts [71,72,73,74]. Mechanistically, trans-palmitoleic acid is thought to influence lipid and glucose metabolism through modulation of nuclear receptors and lipid-sensitive signaling pathways. Emerging evidence further suggests that it may exert anti-inflammatory and insulin-sensitizing effects, distinguishing it functionally from industrial trans-fatty acids despite their shared geometric configuration [75,76,77].
The biological activity of palmitoleic acid (C16:1) has been investigated primarily for the cis-9 (ω-7) isomer, but increasing evidence suggests that positional and geometric isomers may exhibit distinct and sometimes divergent bioactivities. cis-Palmitoleic acid (C16:1Δ9Z), originally described as a putative “lipokine,” has been associated with modulation of lipid metabolism, attenuation of hepatic de novo lipogenesis, improvement of insulin sensitivity, anti-inflammatory effects, and regulation of adipocyte–liver crosstalk. It has also demonstrated antimicrobial properties against certain Gram-positive bacteria and the ability to influence membrane fluidity and receptor signaling pathways, including interactions with PPARs and other lipid-sensitive transcription factors [78,79,80].
trans-Palmitoleic acid (trans-9-C16:1, palmitelaidic acid), although structurally similar, exhibits different biophysical behavior due to its more linear geometry and has been epidemiologically associated with neutral or in some studies favorable cardiometabolic markers, despite belonging to the trans fatty acid family. Other cis-positional isomers (Δ5, Δ7, Δ11, and Δ13) have been less extensively studied but are reported to occur in bacteria, algae, and marine organisms, where they may contribute to membrane adaptation, microbial competition, and stress tolerance.
Interesting data were obtained using PASS. Every isomer retains the same leading activity classes (lipid metabolism regulator, vasoprotector, antiviral/arbovirus, antimutagenic, etc.). That is a strong hint that PASS is recognizing the same global pharmacophore-like features across the set: (i) a C16 hydrophobic chain (dominant scaffold); (ii) a terminal carboxyl group (key polar head); (iii) one unsaturation (modulates shape and hydrophobic surface distribution). A detailed description of the PASS method and its application for calculating the activity of fatty acids was published by us earlier [1,2].
The fact that cis-Δ9 comes out strongest (e.g., lipid metabolism regulator 0.978, vasoprotector 0.957, antiviral 0.938, etc.) and that shifting the double bond away reduces Pa by a few percent strongly implies PASS is sensitive to how the molecule’s 2D pattern maps to known actives in its training space.
Importantly, PASS is not “simulating biology”; it is matching structure–activity patterns learned from known compounds. So this often reflects dataset similarity rather than a mechanistic claim that Δ9 is inherently the most bioactive in vivo.
In the PASS analysis, the Pa value (probability of activity) reflects the likelihood that a compound may exhibit a given biological activity based on structural similarity to compounds with known activities in the training dataset. Therefore, Pa should be interpreted as a structural similarity-driven probability of association, rather than a direct measure of biological potency, efficacy, or experimentally verified activity. These predictions provide useful guidance for identifying potential biological functions but require experimental validation to confirm the actual biological effects of the compounds [1,2].
trans-Isomers: same activity list, ~4% lower. This is a neat, interpretable outcome. It indicates that PASS treats trans-geometry as structurally close to the cis-analogs (same fragments), but not as close as the best-matching cis pattern—hence the same categories but slightly reduced probabilities. The striking low values for Δ5 (e.g., lipid metabolism regulator 0.854; vasoprotector 0.815; antiviral 0.803; antimutagenic 0.778).
This is a key observation: moving the double bond toward the carboxyl end changes the local substructure around the headgroup (the region most “visible” to many binding models), which can reduce similarity to known actives or increase similarity to different reference compounds.
The predicted biological activities generated by the PASS algorithm appear to depend strongly on structural features associated with the position of the double bond within the fatty acid chain. In particular, differences in biological activity among monoenoic fatty acids may arise from the location of the unsaturation relative to the terminal methyl group or the carboxyl function. PASS therefore likely responds to patterns in the topology and electronic structure of the molecule, which are influenced by the position of the double bond along the carbon chain. In this context, the algorithm does not explicitly evaluate the carbon number as an independent rule but rather recognizes structural descriptors correlated with the position of unsaturation. These structural differences can influence molecular interactions with enzymes, receptors, and biological membranes, which may explain the variations in predicted activities for fatty acids with similar chain lengths but different double-bond positions [1,2].
Here are the most plausible contributors:
- (A)
- Fragment/descriptor differences near the headgroup.
A Δ5 double bond places unsaturation closer to the polar headgroup than Δ9 does. That changes: (i) the adjacency of sp2 carbons to the carboxyl-bearing region; (ii) the pattern of atom environments (in 2D fingerprints), (iii) and the distribution of “features” PASS uses for pattern matching. Even though the molecule is still “C16:1,” its local topological signature differs.
- (B)
- Training-set bias toward common natural motifs.
Δ9 monoenes (palmitoleic and oleic families) are extremely common in natural lipids, metabolic studies, and bioactivity datasets. If PASS has more examples of Δ9-related structures associated with the listed activities, it will assign higher Pa simply because Δ9 is “more familiar” in its learned space.
- (C)
- Geometry effects are weaker than positional effects (often).
In fingerprint-based models, cis- vs. trans- can matter, but sometimes less than where the double bond is, especially if the representation compresses 3D shape into 2D patterns. That fits our observation: (i) trans- ~ Same profile but ~4% lower; (ii) Positional shifts causing 2–8% lower, and Δ5 dropping even more in some endpoints.
Across the top 10 endpoints, Δ7/Δ11/Δ13 typically show only a modest reduction (2–8%), whereas Δ5 exhibits the largest downward shift.
The expanded PASS dataset reveals a clear positional dependence of predicted activity intensity across the palmitoleic acid isomer series, while confirming preservation of a common qualitative activity spectrum. When compared with the Δ9 reference isomer (Pa 0.978 for lipid metabolism regulation), activity values follow a graded distribution that appears to correlate with double-bond placement along the carbon chain.
Isomer Δ10 shows the closest resemblance to Δ9 in both magnitude and breadth of predicted activities, with lipid metabolism regulator (0.951), vasoprotector (0.923), antiviral (arbovirus) (0.909), and antimutagenic (0.894) activities remaining high. Notably, Δ10 retains the extended activity panel (mucositis treatment 0.883; preneoplastic conditions 0.874; neuroprotector 0.857; antihypercholesterolemic 0.832; antihypoxic 0.811; anti-inflammatory 0.807), indicating that mid-chain unsaturation positions (Δ9–Δ10) maintain the most robust activity profile within the PASS model. This suggests that unsaturation located near the central region of the C16 chain produces molecular descriptors most similar to high-probability bioactive templates in the training set.
Isomer Δ11 also exhibits a strong profile, although with slight re-ranking of endpoints: antiviral (arbovirus) activity becomes dominant (0.932), followed by lipid metabolism regulation (0.906) and vasoprotection (0.889), with additional predictions for antimutagenic (0.859), mucositis treatment (0.846), and hypolipemic activity (0.837). The emergence of hypolipemic activity in Δ11 suggests that shifting the double bond toward the methyl terminus subtly modifies structural similarity relationships within the PASS algorithm.
In contrast, isomers with more proximal double bonds show a progressive reduction in Pa values. For Δ7, lipid metabolism regulator activity declines to 0.904, vasoprotector to 0.873, antiviral to 0.858, and antimutagenic to 0.834, although the overall qualitative pattern remains intact. The Δ6 isomer exhibits a more pronounced decrease, with lipid metabolism regulator (0.879), vasoprotector (0.843), antiviral (0.811), and antimutagenic (0.786), indicating that unsaturation closer to the carboxyl group produces a measurable downward shift in predicted activity intensity. The Δ13 isomer, in which the double bond is positioned near the terminal methyl end, displays intermediate behavior: lipid metabolism regulator (0.908), vasoprotector (0.871), antiviral (0.852), and antimutagenic (0.816), suggesting that distal unsaturation reduces predicted potency but not the qualitative activity spectrum.
Taken together, these data support several important conclusions. First, all palmitoleic acid isomers share a conserved “core activity signature” dominated by lipid metabolism regulation, vasoprotection, antiviral potential, and antimutagenic properties. Second, activity values follow a positional gradient in which mid-chain double bonds (Δ9–Δ10) produce the highest probabilities, while shifts toward either the carboxyl (Δ6) or methyl terminus (Δ13) reduce predicted activity by approximately 5–12% relative to Δ9. Third, the persistence of identical activity categories across isomers indicates that PASS primarily recognizes global structural features—chain length, amphiphilicity, and mono-unsaturation—whereas double-bond position acts as a quantitative modulator within that framework.
Mechanistically, this positional sensitivity likely reflects changes in topological descriptors and fragment environments used by the PASS algorithm rather than a direct biochemical calculation of double-bond location. Nevertheless, the trend is chemically plausible: mid-chain unsaturation generates a balanced amphiphilic geometry and hydrophobic surface distribution that may more closely resemble lipid mediators within the PASS training set. Thus, the computational analysis reinforces the concept that positional isomerism in monoenoic fatty acids does not fundamentally alter biological activity class, but rather fine-tunes predicted activity intensity in a systematic and structurally interpretable manner.
In most studies, all isomers of palmitoleic acid (C16:1) are not resolved; typically, only the predominant Δ9 isomer, occasionally Δ7, and sometimes the corresponding trans isomer are reported. The most comprehensive investigations of C16:1 positional diversity were conducted in the early 1990s by Rezanka and coworkers [28,81], who analyzed the fatty acid composition of freshwater sponges from Lake Baikal (Eastern Siberia, Russia). Using chromatographic–mass spectrometric methods, they demonstrated that the Baikal sponges Lubomirskia baikalensis and Baikalospongia bacillifera contain the full spectrum of palmitoleic acid isomers in both total lipid extracts and individual lipid fractions. Cis isomers Δ5, Δ7, Δ9, and Δ11, together with the trans isomer, were identified; most occurred at low levels (0.1–1% of total fatty acids), whereas the Δ9 isomer was more abundant, reaching 1–8% [82,83,84,85].
These findings indicate that freshwater invertebrates are capable of either synthesizing or selectively accumulating a remarkably broad array of monoenoic positional isomers. The presence of minor isomers at trace levels suggests finely regulated biosynthetic or symbiont-associated desaturation pathways operating within the sponge holobiont [81]. Interestingly, crustaceans inhabiting the sponge matrix were also found to contain the same C16:1 isomers, pointing to trophic transfer or shared microbial sources. Together, these observations highlight Lake Baikal sponges as a unique natural reservoir of monoenoic fatty acid diversity and underscore the ecological complexity underlying C16:1 isomer distribution in freshwater systems [82,83,84,85].
Endemic sponges of Lake Baikal (family Lubomirskiidae) are unique freshwater organisms that act as the lake’s “natural filters,” comprising approximately 44% of the benthic biomass and maintaining water purity by pumping enormous volumes of water for feeding and respiration. The family includes four genera and fourteen species found exclusively in Lake Baikal [86,87,88]. The most well-known species, Lubomirskia baicalensis, displays a bright green coloration due to its symbiosis with zoochlorella algae. These sponges may exhibit fruticose (tree-like), encrusting, or globular morphologies and inhabit hard substrates at depths ranging from 1 to 120 m, with some species occurring as deep as 1340 m. They have evolved within the lake for at least 10 million years, and large branched individuals can persist for decades, growing only a few centimeters annually. When healthy green sponges turn dirty pink, white, or brown, this indicates loss of symbiotic algae and progressive tissue necrosis [86,87,88,89,90].
The second most widespread species, Baikalospongia bacillifera, is another endemic freshwater sponge of Lake Baikal characterized by a massive, globular, or crustose growth form. Unlike Lubomirskia baicalensis, it rarely forms branches, instead developing dense, robust structures on rocky substrates. It can attain remarkable sizes—up to one meter or more in diameter—an exceptional feature among freshwater sponges. Living specimens are typically green because of zoochlorella symbionts, although brownish or yellowish forms also occur [87,88,89,90].
These sponges are of exceptional scientific importance as models of long-term freshwater endemism and symbiotic evolution in an isolated ancient lake ecosystem. Their unique lipid composition, including diverse monoenoic fatty acid isomers, provides valuable insight into adaptive membrane biochemistry under stable but extreme environmental conditions. Moreover, ongoing environmental stress and disease outbreaks in Lake Baikal have heightened interest in these sponges as bioindicators of ecosystem health and climate-related change [91,92,93,94].
Lipid and phospholipid compositions of Baikal endemic freshwater molluscs, belonging to the class Gastropoda (Valvata baicalensis and Valvata piligera), were investigated [95]. In total, 95 fatty acids were identified: 23 saturated (including iso- and anteiso-forms), 28 monoenoic, 14 dienoic, and 30 polyenoic. Among the polyenoic acids, high proportions of the three principal acids—18:3n-3, 18:3n-6, and 20:4n-6—were detected in phospholipid and glycolipid fractions, whereas elevated levels of iso- (3.56–4.25%) and anteiso- (2.02–2.75%) saturated acids were observed in neutral fractions. Several unusual polyunsaturated fatty acids were also present, including 18:5n-3, 24:4n-6, 24:5n-6, 24:6n-3, and furanoid acids. These molluscs contained four cis-isomers of palmitoleic acid (Δ5, Δ7, Δ9, and Δ11) as well as the trans-isomer. Apparently, freshwater molluscs either synthesize these monoenoic acids de novo or selectively accumulate them from dietary sources such as algae and bacterial mats. The occurrence of multiple C16:1 positional isomer in distinct lipid fractions suggests active desaturase systems or complex trophic transfer pathways within the Baikal ecosystem. Their incorporation into membrane phospholipids further indicates a functional role in maintaining membrane fluidity and adapting to the cold, oligotrophic conditions characteristic of Lake Baikal.
Brandtia (Spinacanthus) parasitica is a freshwater amphipod crustacean of the family Acanthogammaridae endemic to Lake Baikal [28,96,97,98,99]. This species is an obligate epibiont, living on the surface and within the cavities of Baikal sponges, most frequently species of the genus Lubomirskia (e.g., Lubomirskia baicalensis), at depths of approximately 6–30 m. It attracts particular scientific interest because of its highly unusual lipid composition, characterized by the presence of very long-chain and structurally uncommon fatty acids, which are thought to be derived from feeding on sponge tissues or their symbiotic microorganisms. Both the host sponge L. baicalensis and its gammarid parasite B. (S.) parasitica were shown to contain four cis isomers of palmitoleic acid (Δ5, Δ7, Δ9, and Δ11) together with the corresponding trans isomer, apparently transferred through trophic association [96].
Detailed examination of total lipids and phospholipids (including both plasmalogen and alkyl ether forms) revealed extraordinary fatty acid diversity. In total, 183 fatty acids were identified by GC–MS: 46 saturated, 55 monoenoic, 35 dienoic, 25 trienoic, and 22 tetra-, penta-, and hexaenoic acids. The endemic freshwater sponges of the family Lubomirskiidae were found to contain highly unusual long-chain fatty acids, including anteiso-5,9–28:2; branched-5,9–29:2; 5,9,23–29:3; 5,9,23–30:3; 15,18,21,24–30:4; and 15,18,21,24,27–30:5 [28,81,96]. Several of these rare long-chain polyunsaturated and branched fatty acids were also detected in the lipids of the amphipod parasite, indicating dietary acquisition and metabolic incorporation. This remarkable overlap in lipid profiles supports the concept of a tightly integrated biochemical relationship between sponge and epibiont, in which specialized fatty acids are transferred and possibly modified within the host–parasite system. Such findings underscore Lake Baikal’s endemic sponge–amphipod associations as unique natural models for studying lipid trophic transfer, membrane adaptation, and the evolution of unusual fatty acid biosynthetic pathways in isolated freshwater ecosystems [100,101,102].
3-trans-16:1 fatty acid (trans-3-hexadecenoic acid) is a structurally specialized monounsaturated fatty acid distinguished by the unusual position of its trans double bond adjacent to the carboxyl terminus. Unlike trans-9-16:1 (trans-palmitoleic acid) of ruminant or industrial origin, trans-3-16:1 is primarily a chloroplast-specific lipid component, occurring predominantly in phosphatidylglycerol (PG) of photosynthetic membranes in higher plants, especially in green leafy tissues. Its biosynthesis is tightly linked to chloroplast fatty acid metabolism and desaturation pathways unique to plastids [103,104,105,106]. In terrestrial plants, particularly cereals such as wheat, the concentration of 3-trans-16:1 decreases under cold treatment, suggesting that its relative abundance is inversely associated with winter hardiness and cold acclimation. Because phosphatidylglycerol is essential for photosystem II stability, this isomer has been regarded as a biochemical marker of chloroplast membrane organization and photosynthetic capacity [106].
The detection of 3-trans-16:1 in freshwater invertebrates of Lake Baikal—including gastropods (Valvata baicalensis and Valvata piligera), endemic sponges (Lubomirskia baicalensis, Baikalospongia bacillifera), and the amphipod Brandtia (Spinacanthus) parasitica [28,81,95,96]—as well as in Caspian invertebrates such as crabs (Rhithropanopeus harrisii), crayfish (Astacus leptodactylus eichwaldi), and the bivalve Mytilaster lineatus, is therefore particularly intriguing [107,108]. In all cases, the concentration of this fatty acid does not exceed approximately 0.5% of total fatty acids, indicating that it is a minor yet consistently detectable component. Given its established association with plant chloroplast membranes, its occurrence in aquatic invertebrates strongly suggests trophic acquisition from algal or photosynthetic microbial sources rather than de novo synthesis by the animals themselves. The persistence of this isomer across phylogenetically diverse invertebrates implies incorporation into membrane lipids without complete metabolic degradation, reflecting selective retention or structural compatibility within membrane phospholipids.
From a biochemical standpoint, the presence of a trans double bond at the Δ3 position introduces unique conformational constraints. Unlike mid-chain cis unsaturation, which induces a pronounced kink and increases membrane fluidity, a trans double bond near the carboxyl group produces a more linear segment adjacent to the polar headgroup. This configuration may influence membrane interfacial packing, headgroup orientation, and lipid–protein interactions differently from both cis-3 and trans-9 isomers. Although the biological activity of 3-trans-16:1 has not been experimentally characterized, its conserved role in chloroplast membranes suggests structural rather than signaling functions. In aquatic ecosystems, its detection in benthic invertebrates may serve as a biomarker of dietary input from photosynthetic organisms and could reflect the efficiency of energy transfer from primary producers to higher trophic levels. Thus, while quantitatively minor, 3-trans-16:1 represents a chemically and ecologically significant lipid marker linking photosynthetic membrane biochemistry with aquatic invertebrate lipidomics [103,104,105,106,109,110,111].
4. Oleic Acids and Their Activity
Oleic acid (C18:1, see Figure 3, and activity shown in Table 2) represents one of the most structurally versatile monoenoic fatty acids, existing not only as the well-known cis-9-octadecenoic acid (Δ9Z, ω-9) but also as a series of positional and geometric isomers that differ in double-bond location and configuration [112,113,114]. In natural and microbial systems, cis positional isomers have been identified at Δ7, Δ8, Δ9, Δ11, Δ13, and Δ15 [28,81,96]. Shifting the double bond along the C18 backbone alters the spatial relationship between the carboxyl headgroup and the site of unsaturation, thereby influencing conformational flexibility, hydrophobic surface distribution, and membrane packing behavior. The canonical Δ9 isomer introduces a characteristic bend near the center of the hydrocarbon chain, maximizing disruption of van der Waals interactions and lowering melting temperature [115,116]. In contrast, Δ7 and Δ8 isomers place the double bond closer to the polar headgroup, potentially affecting membrane interfacial dynamics and enzymatic accessibility, while Δ11, Δ13, and Δ15 isomers shift unsaturation toward the terminal methyl end, modifying bilayer thickness and lateral diffusion properties. Such positional variation can influence oxidative susceptibility, β-oxidation kinetics, and interaction with lipid-modifying enzymes.
In addition to cis forms, oleic acid also exists as trans-isomers, the most common being trans-9-octadecenoic acid (elaidic acid), along with trans-7, trans-8, trans-11 (vaccenic acid), trans-13, and trans-15 positional variants. The trans-configuration markedly reduces the angular distortion introduced by cis double bonds, producing a more extended and linear molecular geometry [117,118,119]. As a result, trans-C18:1 isomers pack more efficiently within lipid bilayers, exhibit higher melting points, and behave biophysically in a manner more similar to saturated stearic acid than to cis-oleic acid. This geometric difference has important implications for membrane order, phase transition temperature, and protein–lipid interactions. From a biological perspective, cis- and trans-isomers may differ in receptor binding affinity, metabolic processing, and inflammatory signaling pathways. For example, cis-9-oleic acid has been widely associated with anti-inflammatory, lipid-regulating, and cardioprotective effects, whereas certain trans-isomers have been linked to altered lipoprotein profiles and membrane rigidity. In aquatic systems, positional and geometric diversity of C18:1 may reflect microbial biosynthetic pathways, environmental stress adaptation, and trophic transfer patterns. Therefore, oleic acid should be regarded not as a single molecular entity but as a structurally diverse family of C18 monoenoic fatty acids whose biological and ecological properties depend critically on both double-bond position and configuration.
The biological activity of oleic acid (C18:1) must be interpreted in the context of its positional and geometric isomerism, as subtle shifts in double-bond location or configuration can influence membrane behavior, receptor interactions, and metabolic processing. The most extensively studied isomer, cis-9-octadecenoic acid (Δ9Z, ω-9), is widely recognized for its lipid-regulating, anti-inflammatory [120,121,122], and cardiometabolic [123,124,125] protective effects. It modulates membrane fluidity, influences lipid raft organization, activates peroxisome proliferator-activated receptors (PPARs), and can suppress pro-inflammatory cytokine production. cis-9 Oleic acid has also demonstrated antimicrobial activity against certain Gram-positive bacteria, contributes to improved plasma lipid profiles, and exhibits cytoprotective and antiapoptotic properties in various cell models [126,127,128].
Other cis-positional isomers—Δ7, Δ8, Δ11, Δ13, and Δ15—are less extensively characterized but show emerging evidence of bioactivity linked to their structural differences. cis-7 and cis-8 Isomers, with the double bond positioned closer to the carboxyl headgroup, may alter interfacial membrane dynamics and affect enzyme accessibility, potentially influencing lipid signaling and oxidative metabolism [129,130]. cis-11-Octadecenoic acid (vaccenic acid, when in trans form more commonly discussed) in its cis configuration has been associated with lipid metabolism regulation and may serve as a precursor to conjugated linoleic acid in certain organisms [131,132,133]. cis-13 and cis-15 Isomers, with unsaturation nearer the terminal methyl group, can influence bilayer thickness and acyl chain packing in distinct ways, potentially modifying membrane protein function and signal transduction [134,135]. Although experimental data are more limited for these distal isomers, QSAR and PASS analyses suggest comparable probabilities for lipid-regulating, vasoprotective, anti-inflammatory, and metabolic-modulating activities across the cis series, reflecting the dominant influence of chain length and mono-unsaturation rather than precise double-bond position.
The trans-isomers of C18:1, including trans-9 (elaidic acid), trans-11 (vaccenic acid), trans-7, trans-8, trans-13, and trans-15, display distinct biophysical and physiological properties [30,31,32,33,34,35,60,61,62,63,64,65]. The trans-configuration produces a more linear acyl chain, increasing packing efficiency and membrane order. Biologically, trans-9 (elaidic acid) has been associated with adverse lipoprotein changes, including increased LDL and decreased HDL cholesterol, pro-inflammatory signaling, endothelial dysfunction, and altered membrane microdomain organization. In contrast, trans-11 (vaccenic acid), which occurs naturally in ruminant fats and certain marine organisms, has shown more neutral or even beneficial metabolic associations in some studies, possibly due to partial endogenous conversion to conjugated linoleic acid [136,137,138,139]. The physiological impacts of other trans-positional isomers remain less clearly defined but likely involve altered lipid metabolism, membrane rigidity, and modulation of inflammatory pathways compared with their cis counterparts.
Overall, available experimental evidence and computational predictions indicate that all oleic acid isomers retain core biological themes—lipid metabolism regulation, membrane modulation, vasoprotective potential, and anti-inflammatory activity—because these functions derive largely from their amphiphilic structure and single unsaturation. However, geometric configuration (cis- versus trans-) exerts a stronger influence on biological outcome than double-bond position alone, primarily through its effects on membrane packing and lipid–protein interactions. Thus, the C18:1 isomer family represents a continuum of structurally related molecules in which minor chemical variations translate into graded differences in metabolic, inflammatory, and membrane-associated activities [120,121,122,140,141].
Oleic acid (C18:1Δ9, ω-9) is a major component of the lipid fraction in freshwater algae and cyanobacteria, typically accounting for 15–30% of total fatty acids in many species, although its abundance can vary substantially depending on environmental conditions. Freshwater phytoplankton communities are generally dominated by three principal groups—green algae (Chlorophyta), cyanobacteria (Cyanobacteria), and diatoms (Bacillariophyta)—each characterized by distinct structural and biochemical lipid profiles. Among green algae, filamentous genera such as Spirogyra, Mougeotia, and Zygnema (see Figure 1), together with unicellular taxa such as Chlorella, are widely distributed in ponds, lakes, and slow-moving streams, frequently forming dense aggregations under nutrient-rich conditions [136,137,138,139,140,141,142,143,144,145,146,147]. In these taxa, oleic acid commonly represents a substantial fraction of membrane and storage lipids, contributing to membrane fluidity and metabolic flexibility.
In cyanobacteria, oleic acid levels can be even higher, often ranging from 5 to 25% of total fatty acids, particularly in bloom-forming genera such as Microcystis inhabiting eutrophic waters [139,140,141,142]. The proportion of oleic acid in both algae and cyanobacteria is strongly influenced by abiotic factors, including water temperature, pH, light intensity, nutrient availability, and trace element composition (e.g., iron, magnesium, and manganese, which are essential for desaturase enzyme activity). Lower temperatures typically promote increased monoenoic fatty acid content as part of homeoviscous adaptation, whereas variations in pH and micronutrient availability can modulate the activity of stearoyl-ACP desaturases responsible for converting saturated precursors into oleic acid. Thus, the distribution of oleic acid in freshwater phytoplankton reflects a dynamic interplay between taxonomic identity and environmental regulation, underscoring its dual role as a structural membrane component and as a responsive biochemical marker of ecological conditions [136,137,138,139,140,141,142].
The PASS-based evaluation of positional isomers of oleic acid (C18:1) demonstrates a pattern closely paralleling that observed for palmitoleic acid: a conserved qualitative activity spectrum accompanied by quantitatively graded differences depending on double-bond position. Across all isomers examined (Δ5, Δ7, Δ9, Δ11, Δ13, and Δ15), the dominant predicted activity remains lipid metabolism regulation, followed by vasoprotective, antiviral (arbovirus), and antimutagenic activities. This conserved hierarchy indicates that the C18 monoenoic scaffold—characterized by a long hydrophobic chain, a terminal carboxyl group, and a single unsaturation—constitutes the principal determinant of predicted bioactivity within the PASS framework.
Among the isomers, Δ9 (cis-9-octadecenoic acid) clearly exhibits the highest predicted probabilities across nearly all endpoints, with lipid metabolism regulator (0.987), vasoprotector (0.942), antiviral (0.935), and antimutagenic (0.924) activities reaching peak values. Importantly, Δ9 also retains the broadest activity spectrum, including mucositis treatment (0.906), preneoplastic conditions treatment (0.875), neuroprotective (0.851), antihypercholesterolemic (0.835), anti-inflammatory (0.812), and cytoprotective (0.794) effects. This pattern reinforces the observation that mid-chain unsaturation provides the most favorable structural configuration within the PASS training space, likely due to the widespread biological representation of the Δ9 motif in natural lipids and bioactive compounds.
Isomer Δ7 closely approaches Δ9 in both intensity and breadth of predicted activity, with lipid metabolism regulator (0.960) and vasoprotector (0.920) values nearly matching Δ9. It preserves the extended activity panel, suggesting that unsaturation slightly proximal to the central region of the chain still aligns well with PASS-derived structural templates. Isomer Δ11 also demonstrates strong probabilities (lipid metabolism regulator 0.957; vasoprotector 0.923; antiviral 0.911; antimutagenic 0.898), though it lacks the broader auxiliary activity spectrum observed for Δ7 and Δ9.
In contrast, isomers with double bonds positioned further from the mid-chain—Δ5 on the proximal side and Δ13 and Δ15 toward the distal methyl terminus—display modestly reduced probabilities. For example, Δ5 shows lipid metabolism regulator (0.939) and antiviral (0.876) activities, while Δ15 decreases to 0.911 for lipid regulation and 0.905 for antiviral activity. Although these reductions are moderate (typically within 3–8% of Δ9 values), they follow a consistent positional gradient in which central double-bond placement maximizes predicted activity and deviations toward either terminus produce incremental decreases.
Several key conclusions emerge from this comparative analysis. First, all oleic acid isomers share an essentially identical qualitative activity profile, confirming that mono-unsaturation in a C18 chain defines a robust functional signature in PASS modeling. Second, double-bond position acts primarily as a quantitative modulator rather than a qualitative determinant of activity class. Third, the Δ9 isomer occupies an optimal structural position, likely reflecting both its balanced amphiphilic geometry and its high representation in biological datasets used to train the PASS algorithm. Finally, the narrow range of variation among isomers suggests that global molecular descriptors (chain length, hydrophobicity, and mono-unsaturation) exert greater influence on predicted activity than fine positional differences, while still allowing subtle, systematic gradation.
Overall, the data support a model in which positional isomerism of oleic acid fine-tunes predicted biological potency without fundamentally altering activity type, reinforcing the broader conclusion that monoenoic fatty acids possess a conserved bioactivity framework shaped by their amphiphilic architecture.
According to PASS analysis, trans-elaidic acid (trans-9-octadecenoic acid), the principal industrial trans isomer of oleic acid, exhibits a high-probability biological activity spectrum dominated by lipid metabolism regulation (Pa 0.984), vasoprotective activity (0.966), antiviral activity against arboviruses (0.951), and antimutagenic effects (0.926), followed by preneoplastic conditions treatment (0.910), neuroprotection (0.892), antihypercholesterolemic activity (0.888), and antihypoxic activity (0.881). A nearly identical activity profile was predicted for trans-vaccenic acid (trans-11-octadecenoic acid), with similarly elevated probabilities for lipid metabolism regulation (0.980), vasoprotection (0.961), antiviral activity (0.949), antimutagenic effects (0.921), preneoplastic conditions treatment (0.902), neuroprotection (0.888), antihypercholesterolemic activity (0.869), and antihypoxic activity (0.852).
These results indicate that within the PASS framework, trans-C18:1 isomers share a conserved qualitative activity signature characteristic of long-chain monoenoic fatty acids, particularly involving lipid regulatory and vascular pathways. However, the interpretation of these data requires careful consideration, as PASS predicts probability of association with biological activity classes based on structural similarity to annotated compounds and does not specify the physiological directionality (beneficial versus detrimental) of the effect. In contrast to the seemingly “protective” labels generated by PASS, extensive experimental and epidemiological evidence demonstrates that trans-elaidic acid, abundant in partially hydrogenated oils, exerts adverse cardiometabolic effects [123], including elevation of LDL cholesterol, reduction in HDL cholesterol, stimulation of cholesteryl ester transfer protein activity, and promotion of cholesterogenic signaling through mechanisms involving sterol regulatory pathways and sterol O-acyltransferase 1 expression. Moreover, elaidic acid has been shown to enhance cancer cell stemness, stimulate epithelial–mesenchymal transition, and increase metastatic potential in colorectal cancer models, underscoring its capacity to modulate inflammatory and oncogenic pathways in a manner not necessarily protective.
By contrast, trans-vaccenic acid, a naturally occurring ruminant-derived trans fatty acid and metabolic precursor of cis-9,trans-11 conjugated linoleic acid via stearoyl-CoA desaturase, has demonstrated anti-cancer and immunomodulatory properties, including induction of apoptosis in tumor cells and enhancement of CD8+ T-cell–mediated anti-tumor immunity, with reported synergy in immunotherapeutic contexts. Thus, although PASS assigns quantitatively similar activity probabilities to trans-elaidic and trans-vaccenic acids, experimental evidence reveals that biological outcomes depend critically on isomer identity, metabolic conversion, dietary source, exposure level, and cellular context. Collectively, these findings highlight that trans-C18:1 isomers are not biologically inert but actively engage lipid, inflammatory, and signaling pathways; however, computational predictions must be interpreted alongside mechanistic and epidemiological data to distinguish pathway engagement from true physiological benefit.
5. Eicosenoic Acids and Their Activity
20:1 fatty acids (eicosenoic acids, shown in Figure 4, and the activity is shown in Table 3) occur as five cis positional isomers—Δ5, Δ7, Δ9, Δ11, and Δ13—which are comparatively rare in natural lipid profiles relative to the more common C16:1 and C18:1 monoenes. Among these, the Δ11 isomer is most frequently reported, particularly in freshwater algae, bivalves, and gastropods, where it may contribute to membrane adaptation and trophic signaling [142,143,144]. The restricted and taxon-specific distribution of these isomers suggests tightly regulated biosynthetic pathways, possibly involving specialized elongase and desaturase systems. In aquatic invertebrates, Δ11–20:1 may accumulate through both endogenous synthesis and dietary uptake from microalgal sources [145,146,147,148]. The ecological occurrence of these rare monoenes highlights their potential value as chemotaxonomic markers and indicators of specific trophic interactions within freshwater ecosystems.
C20:1 fatty acids (eicosenoic acids), most notably gadoleic acid (20:1 n-11) and gondoic acid (20:1 n-9), are long-chain monounsaturated fatty acids (MUFAs) that occur in both aquatic and terrestrial organisms and play structural, metabolic, and potentially therapeutic roles [145,146,147,148]. These fatty acids are components of membrane lipids and, in plants, serve as important constituents of seed storage oils, contributing to energy reserves and membrane assembly during germination. They can also function as intermediates in elongation and desaturation pathways leading to longer-chain polyunsaturated fatty acids.
Gadoleic acid, frequently found in marine organisms and certain plant oils, has demonstrated inhibitory effects against selected human cancer cell lines, including breast and prostate tumor models, suggesting possible roles in modulating proliferation or apoptotic pathways [148,149,150]. Gondoic acid, abundant in oils such as camelina and jojoba as well as in some nuts and fish oils, exhibits notable anti-inflammatory properties, partly through suppression of reactive oxygen species (ROS) production and modulation of inflammatory signaling cascades. These properties have stimulated interest in dermatological formulations and broader therapeutic research. Collectively, C20:1 monoenoic fatty acids represent bioactive lipids with structural importance in membranes and emerging relevance in cancer biology, inflammation control, and metabolic regulation [148,149,150,151].
Analysis of several Caspian invertebrate species—crabs (Rhithropanopeus harrisii), crayfish (Astacus leptodactylus eichwaldi), and the bivalve Mytilaster lineatus—revealed a remarkable diversity of eighty-six fatty acids, including 23 saturated, 27 monoenoic, 10 dienoic, and 26 polyenoic acids, as determined by capillary GC–MS [107,108]. Among them were several unusual fatty acids such as 4,8,12-trimethyl-13:0, pristatic acid, 3-trans-16:1, and rare polyenoic acids including 18:5 (n-3), 25:5 (n-6), and 24:6 (n-3). Five positional isomers of eicosenoic acids (20:1), namely Δ7, Δ9, Δ11, Δ13, and Δ15, were also identified. The presence of multiple C20:1 isomers suggests either active elongation–desaturation pathways or selective accumulation from diverse dietary sources within the Caspian ecosystem. Such lipid diversity reflects complex trophic interactions and adaptive membrane remodeling in response to the unique physicochemical conditions of the Caspian Sea [107].
Similar positional isomers of eicosenoic acids (Δ7, Δ9, Δ11, Δ13, and Δ15) were also detected in lipid extracts of the Baikal endemic freshwater gastropods Valvata baicalensis and Valvata piligera [95]. This parallel occurrence is noteworthy given the profound physicochemical differences between Lake Baikal, an ancient, ultra-oligotrophic freshwater system, and the brackish, geologically younger Caspian Sea. Despite these contrasting environmental conditions, the qualitative spectrum of C20:1 isomer was conserved, suggesting either convergent metabolic capabilities or shared trophic precursors in benthic food webs. It should be emphasized, however, that the total content of individual eicosenoic acid isomers generally did not exceed 1% of total fatty acids, with the exception of the Δ9 isomer, which reached levels of approximately 1–2%. The predominance of the Δ9 isomer across both ecosystems further supports the concept that mid-chain mono-unsaturation represents a metabolically favored configuration in aquatic invertebrates [95,152,153,154].
Similar positional isomers of eicosenoic acids (Δ7, Δ9, Δ11, and Δ13) were also identified in lipid extracts of the Baikal endemic freshwater sponges Lubomirskia baicalensis, Baicalospongia bacillifera, and Baicalospongia intermedia, as well as in their gammarid parasite Brandtia (Spinacanthus) parasitica [28,81,96]. The recurrence of this isomeric series across phylogenetically distant but ecologically linked organisms suggests either shared biosynthetic pathways or efficient trophic transfer within the sponge–epibiont system. Given the long evolutionary history of Lubomirskiidae in Lake Baikal, these monoenoic patterns may represent stable, lineage-specific lipid signatures maintained under cold, oligotrophic conditions. The detection of the same isomers in the amphipod parasite further supports the hypothesis of dietary acquisition and incorporation into its own membrane and storage lipids. Although present in relatively low concentrations, these C20:1 isomers contribute to the remarkable fatty acid diversity characteristic of Baikal endemic fauna [81,82,83,84,85,86].
The absence of the Δ15 eicosenoic acid (20:1Δ15) isomer in Baikal invertebrates, in contrast to its detection in Caspian species [107,108,154], is likely multifactorial and reflects ecological, biochemical, and evolutionary differences between the two systems. First, double-bond position in monoenoic fatty acids is determined by the substrate specificity of desaturase enzymes. The consistent detection of Δ7, Δ9, Δ11, and Δ13 isomers in Baikal taxa suggests the presence of Δ7-, Δ9-, Δ11-, and possibly Δ13-desaturase activities, whereas the absence of Δ15 implies either a lack of Δ15-desaturase capability or minimal elongation/desaturation flux toward this distal configuration. Since Δ15 in a C20 chain corresponds to an unsaturation very near the terminal methyl end, its biosynthesis may require specific elongase–desaturase combinations not expressed in Baikal endemic lineages.
Second, trophic factors likely contribute. Many unusual monoenes in aquatic invertebrates are derived not solely from de novo synthesis but from dietary uptake, particularly from microalgae, bacteria, or detrital sources. If Caspian primary producers or microbial communities generate Δ15–20:1 at detectable levels, benthic invertebrates may incorporate it passively. In contrast, the ultra-oligotrophic and phylogenetically distinct microbial–algal communities of Lake Baikal may not produce significant Δ15 precursors, limiting its appearance in higher trophic levels [81,82,83,84,85,86].
Third, membrane biophysics under Baikal conditions may favor mid-chain unsaturation (Δ9–Δ13) over distal double bonds. Lake Baikal is characterized by cold, stable, oxygen-rich freshwater conditions, and mid-chain double bonds produce balanced kinks that optimize membrane fluidity without excessive terminal disorder. A Δ15 double bond, positioned close to the methyl terminus, may confer different packing behavior that is not selectively advantageous under Baikal’s environmental regime [81,82,83,84,85,86,155,156,157,158,159].
Finally, evolutionary isolation must be considered. Lake Baikal’s endemic fauna have evolved for millions of years in relative isolation, leading to lineage-specific lipid metabolic profiles. The absence of Δ15–20:1 may therefore represent a conserved biochemical trait within Baikal invertebrates, reflecting long-term adaptation and metabolic canalization. In contrast, the Caspian Sea, with its different salinity, geological history, and faunal exchanges, may support broader enzymatic diversity or dietary inputs that allow incorporation of this more distal monoenoic isomer [28,81,82,90,91,92,93,94,95,96].
The PASS-based analysis of positional isomers of eicosenoic acid (20:1Δ5, Δ7, Δ9, Δ11, Δ13, and Δ15) reveals a coherent but position-sensitive biological activity pattern, with a dominant anticancer–lipid regulatory signature across the entire series. All isomers exhibit high predicted antineoplastic activity, indicating that the C20 monoenoic scaffold is strongly associated in the PASS model with tumor-related biological pathways. However, the intensity of this predicted activity varies systematically with double-bond position, suggesting that positional isomerism modulates quantitative probability while preserving qualitative activity class.
A clear gradient emerges when comparing the isomers. The Δ11 isomer displays the highest predicted antineoplastic activity (0.983), along with strong probabilities for preneoplastic conditions treatment (0.858), antihypercholesterolemic activity (0.889), lipid metabolism regulation (0.879), cholesterol synthesis inhibition (0.855), and atherosclerosis treatment (0.843). This broad and high-intensity activity profile suggests that unsaturation positioned slightly distal to the mid-chain may optimize structural features recognized by PASS as characteristic of lipid-modulating and anticancer compounds. The Δ9 isomer also exhibits a robust profile (antineoplastic 0.961; preneoplastic 0.879; antihypercholesterolemic 0.862; lipid metabolism regulator 0.841; cholesterol synthesis inhibitor 0.822; atherosclerosis treatment 0.813), indicating that central double-bond placement strongly favors both oncological and cholesterol-related predicted activities. Similarly, Δ13 maintains high antineoplastic probability (0.952) and strong lipid-related activities, though slightly reduced relative to Δ11.
In contrast, more proximal unsaturation (Δ5 and Δ7) yields somewhat lower probabilities. The Δ5 isomer shows antineoplastic (0.911) and preneoplastic (0.858) predictions without the broader cholesterol-regulating profile seen in mid-chain isomers. The Δ7 isomer expands its spectrum slightly (antineoplastic 0.939; preneoplastic 0.837; antihypercholesterolemic 0.824; lipid metabolism regulator 0.801), but still remains quantitatively below Δ9–Δ11. The Δ15 isomer, with the double bond positioned near the terminal methyl end, shows moderate antineoplastic activity (0.919) but comparatively strong lipid metabolism regulator probability (0.905), suggesting that distal unsaturation may preferentially enhance lipid regulatory descriptors rather than tumor-related ones within the PASS framework.
Several mechanistic interpretations can be proposed. First, the predominance of antineoplastic predictions across all C20:1 isomers indicates that long-chain monoenes are structurally aligned with bioactive lipid mediators that modulate proliferation, apoptosis, or membrane signaling domains. Second, the peak activity observed for Δ11—and to a lesser extent Δ9—supports the hypothesis that mid-to-distal double-bond placement generates an optimal balance of hydrophobic surface distribution and conformational flexibility, enhancing similarity to active lipid templates in the PASS training dataset. Third, the consistent co-occurrence of cholesterol-related endpoints (antihypercholesterolemic, cholesterol synthesis inhibitor, atherosclerosis treatment) suggests that C20:1 monoenes are computationally associated with sterol metabolism pathways, possibly reflecting shared structural motifs with known lipid-modifying agents.
Importantly, the differences among isomers are quantitative rather than qualitative. All retain the same core biological themes—anticancer potential and lipid metabolic modulation—while double-bond position fine-tunes predicted activity intensity. This pattern mirrors observations for C16:1 and C18:1 series and reinforces a general principle: in monoenoic fatty acids, chain length determines the overall biological activity space, whereas double-bond position modulates probability values within that space. Consequently, the Δ11 eicosenoic acid emerges as the most promising candidate within the C20:1 family from a computational perspective, although experimental validation would be necessary to determine whether these probability differences translate into measurable biological potency in vitro or in vivo.
6. Acetylenic Fatty Acids Derived from Aquatic Bryophytes
Acetylenic fatty acids constitute a distinctive class of naturally occurring lipids characterized by the presence of one or more carbon–carbon triple bonds (–C≡C–) within a long aliphatic chain. The introduction of a triple bond profoundly alters the physicochemical properties of the fatty acid relative to saturated or olefinic (double-bond-containing) analogues [160,161,162,163]. Although less abundant than saturated, monoenoic, or polyenoic fatty acids, acetylenic fatty acids are widely distributed in specialized plant taxa, mosses, fungi, and certain marine organisms. Their structural uniqueness confers a broad spectrum of biological activities, including antifungal, antibacterial, cytotoxic, anti-inflammatory, and allelopathic effects, making them of considerable interest in natural product chemistry and pharmacological research [160,161,162,163,164,165,166].
Acetylenic fatty acids are defined by one or more internal triple bonds within an otherwise long hydrocarbon chain, typically ranging from C14 to C20 or longer. The triple bond introduces: (i) Linear geometry at the site of unsaturation; (ii) Increased bond rigidity compared with double bonds; (iii) Enhanced electron density along the acetylenic moiety; and (iv) Increased susceptibility to oxidative and enzymatic transformations.
Unlike cis double bonds, which introduce a pronounced bend in the hydrocarbon chain, the triple bond maintains near-linearity while significantly modifying the electronic structure. As a result, acetylenic fatty acids display membrane interactions and chemical reactivity distinct from both saturated and olefinic fatty acids. The coexistence of olefinic and acetylenic unsaturation increases structural diversity and often enhances biological reactivity.
Tariric acid (6-octadecynoic acid) is one of the best-known naturally occurring monoacetylenic fatty acids. It contains a triple bond at the Δ6 position and has been isolated from various plant sources. It exhibits antifungal and cytotoxic properties and has been studied for potential pharmaceutical applications [167,168,169,170].
Stearolic acid (9-octadecynoic acid), with a triple bond at the Δ9 position, is structurally analogous to oleic acid but with a carbon–carbon triple bond replacing the double bond. This modification dramatically alters its reactivity and biological effects. Stearolic acid has been shown to interfere with desaturase enzymes and modulate lipid metabolism [169,170,171,172,173].
Rare acetylenic monoenoic fatty acids have been identified in several aquatic bryophytes, including the mosses Calliergon cordifolium, Drepanocladus lycopodioides, and Fontinalis antipyretica, as well as the liverworts Riccia fluitans and Pellia neesiana (see Figure 5) These species contain octadec-6-ynoic acid (24), octadec-9-ynoic acid (25), and octadec-12-ynoic acid (26, see Figure 6, and activity shown in Table 4) [169,170]. In total lipid extracts, the concentrations of these acetylenic fatty acids typically do not exceed 1%; however, within the triglyceride fraction their levels may surpass 9%, indicating selective accumulation in storage lipids. Additionally, tetradec-13-ynoic acid (27) has been reported in the fungus Candida albicans. The preferential enrichment of these acetylenic acids in neutral lipid fractions suggests a role in energy storage or protective sequestration rather than primary membrane structure. Their restricted taxonomic distribution and low overall abundance imply that they function as specialized metabolites with potential ecological or defensive significance [169,170].
Many acetylenic fatty acids exhibit strong antimicrobial properties. The triple bond increases membrane-disrupting capacity and may interfere with lipid-dependent enzymatic processes in microorganisms [160,163,170]. Their amphiphilic structure allows insertion into microbial membranes, potentially compromising membrane integrity. Certain acetylenic fatty acids reduce inflammatory responses by modulating lipid mediator synthesis and suppressing pro-inflammatory signaling cascades. Their ability to interact with membrane-associated enzymes may influence eicosanoid production [160,161,162,163,164].
Several acetylenic fatty acids (Figure 6) demonstrate cytotoxic activity against tumor cell lines [160]. Proposed mechanisms include: (i) Induction of apoptosis; (ii) Inhibition of fatty acid desaturases; (iii) Disruption of mitochondrial function, and (iv) Interference with lipid signaling pathways. Their structural similarity to endogenous fatty acids allows them to enter metabolic pathways, where the triple bond may alter enzymatic processing and produce toxic intermediates.
Acetylenic fatty acids likely evolved as defensive metabolites in plants, fungi, and marine organisms [160,161,162,163,164]. Their rarity in primary metabolism and frequent association with specialized tissues suggest adaptive functions rather than general structural roles. In aquatic systems, particularly in marine sponges and algae, these compounds may contribute to chemical defense strategies that shape ecological interactions.
Acetylenic fatty acids represent a chemically unique and biologically potent class of lipids distinguished by carbon–carbon triple bonds within long hydrocarbon chains. Found in mosses, fungi, plants, and marine organisms, they exhibit a wide range of biological activities including antimicrobial, cytotoxic, anti-inflammatory, and allelopathic effects. Structurally diverse—ranging from monoacetylenic to polyacetylenic forms and often coexisting with double bonds—they function both as membrane-active molecules and as precursors to specialized metabolites. Their distinctive physicochemical properties and emerging pharmacological potential make them an important focus of research in lipid biochemistry, natural product chemistry, and drug discovery.
The PASS analysis of acetylenic monoenoic fatty acids reveals a remarkably consistent and biologically coherent activity profile centered on antimicrobial and antiparasitic functions. For octadec-6-ynoic acid (24), high predicted probabilities were obtained for antimicrobial (Pa 0.922), antifungal (0.908), and antiparasitic (0.889) activities, accompanied by anti-inflammatory (0.857) and antiviral effects against arboviruses (0.854) and picornaviruses (0.848). This pattern suggests that the Δ6 acetylenic configuration confers strong broad-spectrum anti-infective potential, possibly reflecting enhanced membrane-disruptive properties or interference with lipid-dependent enzymatic systems in pathogens. The combination of antimicrobial and antiviral predictions indicates that the triple bond may increase electrophilic reactivity or modify lipid–protein interactions in a way that is recognized within the PASS training set as characteristic of bioactive anti-infective compounds.
Octadec-9-ynoic acid (25) displays the highest overall activity among the C18 acetylenic series, with antimicrobial (0.942), antifungal (0.931), and antiparasitic (0.911) probabilities exceeding those of the Δ6 and Δ12 isomers. Its anti-inflammatory (0.882) and antiviral (arbovirus 0.867; picornavirus 0.856) predictions are also comparatively elevated. The Δ9 position—analogous to the common double-bond location in oleic acid—may represent an optimal structural configuration for interaction with biological membranes or enzymatic targets, combining central-chain rigidity with maximal hydrophobic surface compatibility. The quantitative superiority of the Δ9 isomer parallels trends observed in monoenoic fatty acids lacking triple bonds, suggesting that mid-chain unsaturation—whether double or triple—produces molecular descriptors most strongly associated with antimicrobial and regulatory bioactivities.
Octadec-12-ynoic acid (26), although slightly lower in predicted potency, retains the same qualitative activity spectrum: antimicrobial (0.918), antifungal (0.892), antiparasitic (0.876), antiviral (arbovirus 0.862; picornavirus 0.856), and anti-inflammatory (0.844). The modest decrease relative to the Δ9 isomer suggests that shifting the triple bond toward the distal methyl end reduces, but does not fundamentally alter, predicted bioactivity. This reinforces the interpretation that global structural features—chain length and the presence of a triple bond—define the primary biological activity class, while positional variation modulates activity intensity within that framework.
In contrast, tetradec-13-ynoic acid (27) demonstrates a markedly different activity emphasis. While retaining antimicrobial (0.911; 0.871), antifungal (0.846), and antiparasitic (0.903) predictions, it shows exceptionally high probabilities for antiprotozoal activity (0.977) and specifically antiplasmodial activity (0.955). The shorter C14 chain combined with distal acetylenic unsaturation appears to shift the predicted activity focus toward protozoan targets. This may reflect altered membrane permeability, enhanced interaction with protozoal lipid metabolism, or structural similarity to known antiprotozoal lipid-like agents within the PASS dataset. The pronounced activity values suggest that shorter-chain acetylenic fatty acids may possess increased selectivity toward parasitic protozoa compared with their C18 counterparts.
Collectively, these results highlight several important principles. First, acetylenic monoenoic fatty acids are consistently predicted to possess strong antimicrobial and antiparasitic activities, supporting their proposed ecological role as defensive metabolites in mosses, liverworts, and fungi. Second, triple-bond position influences quantitative probability values, with Δ9 in C18 chains appearing optimal for broad-spectrum antimicrobial prediction. Third, chain length significantly modulates activity orientation, as evidenced by the strong antiprotozoal bias of the C14 isomer. Although PASS predictions do not confirm biological potency or mechanism, the coherence of the activity pattern across the acetylenic series suggests that the carbon–carbon triple bond confers a distinct bioactive signature, likely associated with membrane perturbation, enzyme inhibition, or reactive intermediate formation. These findings provide a rational basis for further experimental investigation of acetylenic fatty acids as potential antimicrobial, antiparasitic, and anti-inflammatory agents.
7. Multi-Branched Monoenoic Fatty Acids
Freshwater sponges of the former Lake Hula region [174,175,176], including Ephydatia syriaca, Nudospongilla sp. (see Figure 7), and Corvomeyenia sp. (also reported as Cortispongilla barroisi in Palestinian studies), represent components of the distinctive and often endemic fauna of the Jordan River basin [177,178]. These species inhabit shallow, slow-flowing, or stagnant waters of the Hula Valley, an area that was largely drained in the 1950s for agricultural development and malaria control but is now partially restored and protected within the Hula Nature Reserve. Regional investigations have demonstrated that these sponges contain high levels of long-chain polyunsaturated fatty acids, including eicosatetraenoic, eicosapentaenoic, and docosahexaenoic acids, with particularly elevated concentrations observed in E. syriaca and Nudospongilla sp., suggesting physiological adaptation to local environmental conditions [29,179,180].
In addition to these common polyenoic acids, detailed lipid analyses revealed a remarkable suite of unique methyl-branched monoenoic fatty acids, such as (Z)-9,13,17-trimethyloctadec-5-enoic acid (28), (Z)-11,15,19-trimethylicos-5-enoic acid (29), (Z)-9,13,17,21-tetramethyldocos-5-enoic acid (30), (Z)-13,17,21-trimethyldocos-5-enoic acid (31), (Z)-11,15,19,23-tetramethyltetracos-5-enoic acid (32), and (Z)-13,17,21,25-tetramethylhexacos-5-enoic acid (33, see Figure 8, and activity shown in Table 5). The occurrence of these structurally unusual branched monoenes highlights specialized biosynthetic capabilities and underscores the evolutionary distinctiveness of the Hula freshwater sponge fauna [29,179]. Contemporary research in the region frequently integrates lipid chemistry with phylogenetic and evolutionary analyses, emphasizing the conservation importance of these species within the restored wetland ecosystem managed by the Israel Nature and Parks Authority.
The PASS analysis of the unusual methyl-branched monoenoic fatty acids isolated from Palestinian freshwater sponges (compounds 28–33) reveals a highly coherent and dermatology-oriented biological activity profile, accompanied by consistent lipid-metabolic regulatory potential. Across the entire series, the dominant predicted activities cluster around antieczematic, antipsoriatic, antiseborrheic, and lipid metabolism regulatory effects, with additional associations to antihypercholesterolemic and atherosclerosis treatment endpoints. This convergence strongly suggests that the branched monoenoic scaffold—characterized by a Δ5 double bond combined with multiple methyl substituents along extended carbon chains—falls within a PASS-recognized structural space associated with lipid-mediated inflammatory skin disorders and systemic lipid regulation.
Compound (28), (Z)-9,13,17-trimethyloctadec-5-enoic acid (C18 backbone), displays strong predicted antieczematic activity (0.934) and lipid metabolism regulation (0.926), with substantial antihypercholesterolemic (0.864) and antiseborrheic (0.843) probabilities. The C18 trimethyl structure therefore already establishes the core activity theme: modulation of inflammatory skin conditions and lipid homeostasis. Elongation to C20 in compound (29), (Z)-11,15,19-trimethylicos-5-enoic acid, shifts the dominant endpoint slightly toward antipsoriatic activity (0.958) while maintaining high antieczematic (0.939) and lipid regulatory (0.933) probabilities, and introduces stronger vascular-related predictions such as atherosclerosis treatment (0.874). This suggests that increasing chain length enhances predicted involvement in systemic lipid and inflammatory pathways.
Further elongation and branching intensification appear to amplify these trends. Compound (30), the C22 tetramethyl derivative, shows some of the highest values in the entire series: antieczematic (0.965) and antipsoriatic (0.958) activities dominate, with elevated lipid metabolism regulator (0.944) and antihypercholesterolemic (0.914) probabilities. Similarly, compound (31), another C22 derivative but with slightly altered branching topology, reaches the highest antipsoriatic probability in the dataset (0.966) and maintains strong lipid-associated endpoints. These findings indicate that mid-to-long chain methyl-branched monoenes are strongly associated in PASS with inflammatory dermatoses and lipid-linked systemic conditions.
Compounds (32) and (33), representing further chain elongation to C24 and C26, maintain the same qualitative spectrum, though with subtle quantitative modulation. Antipsoriatic and antieczematic activities remain high (0.953–0.938 range), while lipid metabolism regulation stabilizes around 0.926. A slight reduction in antihypercholesterolemic probabilities at the highest chain length suggests that beyond C24, additional elongation may not further enhance lipid-modulatory descriptors in the PASS model. Notably, the persistence of atherosclerosis treatment probabilities across C20–C26 derivatives implies a structural association with cholesterol metabolism pathways, potentially reflecting sterol-like hydrophobic surface characteristics conferred by extensive methyl branching.
Several broader conclusions emerge from this analysis. First, unlike straight-chain monoenoic fatty acids—where antimicrobial or antineoplastic themes often dominate—these methyl-branched Δ5 monoenes are computationally associated primarily with dermatological and lipid-inflammatory activities. Second, both chain length and degree of methyl branching appear to increase predicted activity intensity up to approximately C22–C24, suggesting an optimal hydrophobic bulk for interaction with membrane-associated targets or lipid-modulating pathways within the PASS training dataset. Third, the recurring association with psoriasis, eczema, seborrhea, and atherosclerosis endpoints indicates that PASS recognizes these molecules as modulators of lipid-driven inflammatory processes rather than direct cytotoxic agents.
From a biochemical perspective, these predictions are plausible. Methyl-branched long-chain fatty acids can significantly alter membrane microdomain organization, lipid raft formation, and interactions with enzymes involved in lipid mediator synthesis. The Δ5 unsaturation near the carboxyl terminus may further influence interfacial packing and receptor interactions. Although PASS does not determine mechanistic directionality or confirm therapeutic efficacy, the coherence of the predicted activity spectrum across six structurally related molecules suggests that these rare sponge-derived branched monoenes occupy a distinct bioactivity niche linked to lipid-mediated inflammatory regulation.
Overall, the data support the hypothesis that methyl-branched monoenoic fatty acids from freshwater sponges are not merely chemotaxonomic markers but potentially bioactive lipids with dermatological and lipid-metabolic relevance. Experimental validation would be required to confirm these computational predictions; however, the consistently high Pa values—particularly for antipsoriatic and antieczematic endpoints—highlight these compounds as promising candidates for further pharmacological investigation, especially in the context of inflammatory skin disorders and cholesterol-associated pathologies.
8. Cyclopropane-Containing Fatty Acids
Acanthogammarus grewingkii (also known as Brachyuropus grewingkii) is one of the largest endemic amphipods of Lake Baikal and inhabits deep-water zones typically between 300–400 and 1300 m, where temperature and hydrostatic pressure remain remarkably stable throughout the year [181,182]. As a polyphagous species, it feeds on planktonic diatoms with siliceous frustules as well as other crustaceans, occupying an important trophic position in the deep Baikal ecosystem. Lipid analyses have revealed the presence of unusual fatty acids, including methyl alkyl ketones and cyclopropane-containing fatty acids, which are thought to contribute to membrane stability and cellular functionality under high-pressure conditions. These structurally atypical lipids may enhance membrane packing resilience and reduce pressure-induced phase transitions in deep-water environments. The occurrence of cyclopropane fatty acids further suggests possible microbial dietary inputs or specialized biosynthetic adaptations associated with abyssal life. Collectively, these lipid features highlight biochemical strategies that enable amphipods to maintain membrane integrity and metabolic efficiency in the extreme yet stable conditions of Lake Baikal’s deep waters [181,182,183,184].
In the deep-lake invertebrate Acanthogammarus grew growingkii, a number of unusual fatty acids such as cis-11,12-methylene-5-eicosenoate, cis, cis-11,12–14,15-bis-methylene-5-eicosenoate, and their homologues were identified. The possibility of biosynthesis and function of these unusual fatty acids is discussed, as they may represent a specific feature of animals dwelling in deep lakes, may be formed as metabolites within cellular membranes, or may originate from dietary sources [185,186]. The presence of methylene-interrupted or cyclopropane-like structures suggests specialized enzymatic modifications of conventional monoenoic precursors. Such structural adaptations could enhance membrane stability and functionality under conditions of high hydrostatic pressure, low temperature, and limited nutrient availability typical of deep-lake environments. Alternatively, these fatty acids (see Figure 9, and activity shown in Table 6) may arise from symbiotic or ingested microorganisms, reflecting trophic transfer rather than endogenous synthesis. Detailed isotopic and enzymatic studies would be required to distinguish between de novo biosynthesis and dietary incorporation pathways.
The PASS analysis of the cyclopropane-containing monoenoic fatty acids (34–41) reveals a highly coherent and biologically focused activity spectrum centered on vascular protection, hemostatic modulation, and lipid metabolism regulation. Across both structural series—those containing bis-cyclopropyl motifs (34–36) and those with single cyclopropyl substitutions (37–41)—the dominant predicted activities consistently include vasoprotector, peripheral vasodilator, fibrinolytic, antithrombotic, and lipid metabolism regulator endpoints. This uniformity strongly suggests that the combination of a long hydrophobic chain, a Δ5 double bond, and one or more cyclopropane rings generates a structural signature within the PASS model that aligns closely with compounds active in cardiovascular and hemostatic pathways.
For the bis-cyclopropyl derivatives (34–36), the highest probabilities cluster around vasoprotective and vasodilatory functions. Compound 35, the C12 homolog, exhibits the strongest overall profile, with vasoprotector (0.932), peripheral vasodilator (0.921), fibrinolytic (0.914), and antithrombotic (0.904) activities reaching peak values within the dataset. This suggests that moderate chain length combined with dual cyclopropyl substitution may produce an optimal balance of hydrophobic surface area and conformational rigidity for interaction with vascular or lipid-associated targets. Compounds 34 and 36, differing in chain length (C10 and C14), show very similar qualitative activity spectra but slightly lower quantitative probabilities, indicating that activity intensity follows a shallow chain-length dependency, with C12 appearing most favorable in this structural subclass. The consistent presence of antihypercholesterolemic and lipid metabolism regulatory predictions further supports the idea that these molecules are computationally associated with cholesterol handling and lipid transport pathways.
In the mono-cyclopropyl series (37–41), the activity emphasis shifts slightly toward lipid metabolism regulation as the dominant endpoint, with probabilities ranging from 0.898 to 0.919. While vasoprotective, antithrombotic, and fibrinolytic activities remain strong, the absence of a distinct “peripheral vasodilator” designation in some of these molecules suggests that the number of cyclopropane rings influences the predicted specificity of vascular effects. Compound 39 (C14) exhibits the highest lipid metabolism regulator probability (0.919) among the mono-cyclopropyl derivatives, implying that increased chain length in combination with a single cyclopropane ring enhances lipid-related descriptors within the PASS algorithm. Compound 41 shows an interesting inversion in ranking, with fibrinolytic activity (0.906) exceeding vasoprotector (0.901), suggesting subtle structural tuning of predicted hemostatic activity depending on cyclopropyl substitution position and chain extension.
Several mechanistic interpretations can be proposed. Cyclopropane rings introduce significant conformational strain and rigidity into lipid molecules, altering membrane packing and potentially modulating interactions with membrane proteins involved in coagulation, platelet aggregation, or endothelial signaling. In bacterial lipids, cyclopropane fatty acids are known to enhance membrane stability under stress; in eukaryotic systems, similar structural features may influence lipid microdomain organization and receptor accessibility. The recurrent prediction of fibrinolytic and antithrombotic activities suggests that these molecules may interact—at least computationally—with pathways governing clot formation and breakdown, possibly through modulation of lipid-dependent signaling cascades or platelet membrane dynamics.
Importantly, the PASS data demonstrate that both structural families share a unified cardiovascular activity profile, but bis-cyclopropyl compounds show slightly stronger vasodilatory predictions, whereas mono-cyclopropyl derivatives emphasize lipid metabolism regulation. This indicates that ring multiplicity contributes to activity weighting within the model. The overall high probability values (most >0.85) reflect strong structural alignment with compounds annotated in vascular and lipid-related biological classes, though these predictions represent likelihood of association rather than confirmed pharmacological effect.
In ecological and evolutionary context, the occurrence of these cyclopropane-containing fatty acids in deep-water Baikal amphipods may primarily serve structural membrane functions under high hydrostatic pressure. However, the computational association with vascular and lipid-modulating endpoints suggests that if isolated and tested pharmacologically, these unusual lipids could exhibit biologically relevant effects in mammalian systems. The structural combination of cyclopropane rigidity, monoenoic unsaturation, and long-chain hydrophobicity appears to position these molecules within a bioactivity space strongly linked to cardiovascular regulation, hemostasis, and cholesterol metabolism. Experimental validation would be necessary to confirm these predicted activities, but the consistency and magnitude of the PASS outputs highlight these deep-water-derived lipids as intriguing candidates for further biochemical and pharmacological investigation.
9. Toxicological Aspects of Monoenoic Fatty Acids (MUFAs)
Although monoenoic fatty acids (MUFAs) are widely recognized for their physiological, ecological, and nutritional importance, it is also necessary to consider their potential toxicological and adverse biological effects, particularly under conditions where these compounds accumulate at unusually high concentrations or undergo chemical modification in aquatic environments. While MUFAs are generally regarded as biologically compatible and metabolically useful lipids, their ecological role is more complex, and under certain circumstances they may contribute to cellular stress, membrane disruption, or ecological imbalance [187,188].
One possible toxicological aspect of MUFAs arises from their effects on biological membranes. Because MUFAs are amphiphilic molecules with a hydrophobic hydrocarbon chain and a polar carboxyl group, they can interact strongly with lipid bilayers. At normal physiological concentrations, these interactions help regulate membrane fluidity and permeability. However, excessive concentrations of free fatty acids, including MUFAs, may disturb membrane integrity by altering lipid packing and increasing membrane permeability. Such effects may lead to membrane destabilization, leakage of cellular components, and impaired function of membrane-bound proteins. In microorganisms and algae, elevated concentrations of free fatty acids have been shown to inhibit growth or disrupt cellular metabolism [189,190].
Another potential source of toxicity is related to oxidative transformation of unsaturated fatty acids. MUFAs, although less susceptible to oxidation than polyunsaturated fatty acids, can still undergo lipid peroxidation under conditions of oxidative stress. Environmental factors such as high light intensity, elevated oxygen concentrations, or the presence of reactive metal ions may promote the formation of lipid hydroperoxides, aldehydes, and other reactive oxidation products. These compounds can damage cellular macromolecules, including proteins, DNA, and membranes, and may contribute to oxidative stress in aquatic organisms [191,192,193,194].
In addition, some MUFAs and their derivatives may exhibit antimicrobial or cytotoxic activity. Certain monoenoic fatty acids have been reported to inhibit the growth of bacteria, fungi, and protozoa by interfering with membrane function or metabolic pathways. While this property can be advantageous in natural defense mechanisms, high concentrations of these compounds may also affect non-target microorganisms, potentially altering microbial community structure in aquatic ecosystems [195,196,197,198].
Environmental accumulation of MUFAs can also occur in eutrophic or highly productive aquatic systems, where large amounts of organic matter derived from phytoplankton, algae, or microbial biomass are released into the water column or sediments. During decomposition processes, free fatty acids may be liberated from complex lipids and accumulate locally. In such conditions, elevated concentrations of fatty acids may influence microbial activity, sediment chemistry, and oxygen consumption, thereby contributing indirectly to ecological stress [199,200,201,202].
Furthermore, MUFAs can serve as precursors for a variety of bioactive lipid derivatives, including oxylipins and other oxygenated metabolites. Some of these compounds may possess biological signaling or defensive functions, while others may have toxic or inhibitory effects on neighboring organisms. For example, oxidation products of unsaturated fatty acids released during algal bloom events have been implicated in allelopathic interactions and population control mechanisms among aquatic microorganisms [203,204,205,206].
It is also important to consider that the toxicological effects of MUFAs may depend strongly on concentration, molecular structure, and environmental context. For instance, fatty acids with different chain lengths or double-bond positions may interact differently with biological membranes and metabolic enzymes. In addition, the toxic effects of free fatty acids are often mitigated when they are incorporated into complex lipids such as phospholipids or triacylglycerols, which represent the dominant forms in living organisms [189,190,192,201].
Despite these considerations, it should be emphasized that MUFAs are generally less reactive and less prone to oxidative damage than polyunsaturated fatty acids, and therefore their toxicological impact in natural environments is typically limited under normal ecological conditions. Nevertheless, under conditions of environmental stress, pollution, or abnormal accumulation, MUFAs and their oxidation products may contribute to cellular stress responses and ecological interactions within aquatic systems [192,193,194].
Future research should therefore focus on clarifying the threshold concentrations at which MUFAs may exert toxic effects, identifying the most biologically active oxidation products, and determining how environmental factors influence the formation and ecological impact of these compounds. Integrating lipid chemistry, ecotoxicology, and aquatic ecology will be essential for developing a more comprehensive understanding of both the beneficial and potentially adverse roles of MUFAs in aquatic environments.
10. Taxonomic and Geographic Coverage
Although this review compiles information from a wide range of aquatic environments, it is important to recognize that the currently available data on monoenoic fatty acids show significant taxonomic and geographic biases. Much of the published literature focuses on well-studied marine organisms, particularly macroalgae, higher aquatic plants, and economically important species such as fish and zooplankton. In contrast, many microbial groups, planktonic microorganisms, and benthic communities remain comparatively underrepresented in lipid studies, despite their central roles in aquatic biogeochemical cycles. Similarly, most investigations have been conducted in temperate and accessible regions, including European and North American marine and freshwater systems, while many tropical, polar, and deep-water ecosystems remain insufficiently explored. These biases partly reflect historical research priorities, the availability of sampling infrastructure, and analytical challenges associated with studying small or complex microbial communities. Expanding future studies to include diverse geographic regions and understudied taxonomic groups, particularly microbial assemblages, benthic organisms, and extremophilic communities, will be essential for obtaining a more comprehensive understanding of the distribution, ecological functions, and evolutionary significance of monoenoic fatty acids in aquatic ecosystems [207,208,209,210,211].
11. Monoenoic Fatty Acids in Animals in the Aquatic Ecosystems
Aquatic animals, particularly those occupying higher trophic levels such as fish, marine invertebrates, and marine mammals, contain a wide diversity of monoenoic fatty acids (MUFAs) that often differ significantly from those found in terrestrial organisms. In marine ecosystems, MUFAs are not only structural components of membrane phospholipids but also serve as major energy-storage molecules, particularly in species adapted to cold and deep-water environments. Many marine organisms accumulate long-chain MUFAs such as 18:1n-9, 20:1n-9, and 22:1n-11, which are characteristic of plankton-based food webs. These fatty acids originate largely from calanoid copepods and other zooplankton, where they function as energy-rich wax esters used for buoyancy regulation and long-term energy storage. Through trophic transfer, these distinctive MUFAs become enriched in higher organisms including fish, seabirds, and marine mammals [42,43,44,45,212,213,214].
Marine mammals, including seals, whales, dolphins, and walruses, possess particularly unusual lipid compositions in their blubber and specialized adipose tissues. Blubber lipids frequently contain elevated concentrations of long-chain monoenoic fatty acids such as 20:1 and 22:1 isomers, reflecting the dietary input of copepod-derived lipids and deep-sea fish rich in wax esters. In addition, certain marine mammals synthesize or accumulate branched-chain and odd-chain monoenoic fatty acids, as well as positional isomers rarely encountered in terrestrial animals. These unusual fatty acids contribute to the physical properties of blubber, including thermal insulation, buoyancy, and mechanical flexibility. The distribution of MUFAs within blubber layers is often stratified, with specific fatty acid compositions occurring in different layers of adipose tissue, reflecting both metabolic regulation and ecological adaptation [215,216].
Another notable feature of aquatic animal lipids is the presence of unique positional isomers of monoenoic fatty acids, including unusual Δ5, Δ7, Δ11, or Δ13 double-bond locations. Such structures are particularly common in deep-sea organisms and cold-water species, where membrane fluidity must be carefully maintained under conditions of low temperature and high hydrostatic pressure. In marine mammals, these fatty acids contribute to homeoviscous adaptation, ensuring that cellular membranes remain functional despite extreme environmental conditions. Furthermore, the metabolic pathways involved in the synthesis and modification of MUFAs in marine animals often involve elongation and desaturation processes that generate fatty acids not commonly produced in terrestrial lipid metabolism [1,2,15,32].
Beyond their structural role, monoenoic fatty acids in aquatic animals may also have ecological and biochemical significance. Because specific MUFAs are characteristic of particular trophic sources—such as copepods, diatoms, or certain bacterial communities—they are widely used as biomarkers in marine food-web studies. For example, high levels of 20:1n-9 and 22:1n-11 in marine mammal blubber can indicate a diet rich in copepods or copepod-feeding fish. Thus, the presence of these fatty acids provides valuable information about trophic relationships, feeding ecology, and energy transfer within aquatic ecosystems [217,218,219].
MUFAs may play physiological roles related to energy metabolism, buoyancy regulation, and adaptation to environmental stress. Marine mammals rely heavily on lipid reserves to sustain long migrations, fasting periods, and thermoregulation in cold waters. The chemical composition of these lipid stores, including their MUFA content, therefore represents an important component of the organism’s overall ecological strategy. The occurrence of unusual monoenoic fatty acids in aquatic animals thus reflects a combination of dietary origin, metabolic transformation, and evolutionary adaptation to marine environmental conditions. Together, these features highlight the unique lipid chemistry of aquatic ecosystems and emphasize the ecological importance of MUFAs in marine and freshwater food webs [218].
Aquatic animals possess a number of structurally unusual monoenoic fatty acids that are rarely encountered in terrestrial organisms. These “unique” fatty acids arise through specialized biosynthetic pathways, trophic transfer from planktonic microorganisms, or metabolic modification within the animals themselves. They are particularly abundant in marine invertebrates, deep-sea fishes, and marine mammals, where they contribute to physiological adaptation to low temperature, high pressure, and variable salinity conditions [217,218,219].
One distinctive group of fatty acids found in aquatic animals consists of long-chain monoenoic fatty acids with uncommon double-bond positions, including Δ5, Δ7, Δ9, Δ11, and Δ13 isomers. For example, 20:1n-9 (gadoleic acid) and 22:1n-11 (cetoleic acid) are extremely abundant in many marine organisms and are particularly enriched in zooplankton, deep-sea fish, and marine mammal blubber. These fatty acids originate largely from copepod wax esters, which serve as long-term energy reserves in cold-water plankton. Through trophic transfer, they accumulate in higher organisms such as seals, whales, and seabirds, where they contribute to the physical properties of blubber and energy storage tissues [220,221,222].
Another important category includes odd-chain monoenoic fatty acids, such as 17:1 and 19:1 fatty acids, which are frequently detected in marine fish, mollusks, and crustaceans. These fatty acids often originate from symbiotic or dietary bacteria, since bacterial lipid metabolism commonly produces odd-chain fatty acids. In marine ecosystems, the incorporation of bacterial-derived lipids into animal tissues is a common phenomenon, particularly in organisms that feed on detritus or microbial biofilms [223,224,225,226,227].
Marine animals also contain branched-chain monoenoic fatty acids, especially iso- and anteiso-branched fatty acids. Although branched fatty acids are most commonly associated with bacterial membranes, they are frequently transferred through food webs into aquatic animals. In some cases, marine invertebrates and fish accumulate significant amounts of these compounds, which can influence membrane fluidity and metabolic processes. Such fatty acids are particularly common in organisms inhabiting cold or deep-sea environments, where maintaining membrane flexibility is critical [228,229].
In addition, aquatic animals may contain unusual positional isomers of palmitoleic and oleic acids, including cis-vaccenic acid (18:1n-7) and other Δ7 or Δ11 isomers. These fatty acids are abundant in many marine invertebrates and fish and are often associated with bacterial or algal lipid metabolism. Their presence in animal tissues therefore provides insight into dietary sources and trophic interactions within aquatic ecosystems.
Some aquatic organisms contain acetylenic or unusual unsaturated fatty acids, which may arise from unique enzymatic desaturation pathways in marine algae or bacteria and subsequently enter animal tissues through dietary transfer. Although less common than typical MUFAs, these compounds further illustrate the chemical diversity of aquatic lipid metabolism [1,2,160,161,162,163,164,165,166,167,168,169,170].
Overall, the unique fatty acids found in aquatic animals—including long-chain monoenoic fatty acids (20:1 and 22:1), odd-chain monoenoic acids, branched-chain fatty acids, unusual positional isomers, and very-long-chain MUFAs—reflect the complex interactions between microbial metabolism, planktonic food webs, and animal lipid physiology. These fatty acids serve not only as important structural and energetic components but also as biochemical markers that reveal trophic relationships and ecological adaptations within marine and freshwater ecosystems [42,43,212,213,214,215].
Very Long-Chain Monoenoic Fatty Acids in Higher Animals
Very long-chain monoenoic fatty acids (VLC-MUFAs), typically defined as monoenoic fatty acids containing 24 or more carbon atoms, occur in a variety of higher animals, although usually in relatively small concentrations compared with common fatty acids such as oleic (18:1) or palmitoleic (16:1) acids. These fatty acids include compounds such as 24:1 (nervonic acid), 24:1n-9, 24:1n-7, 26:1, and occasionally 28:1 isomers, which are found in specialized tissues and play important structural and physiological roles. Their occurrence reflects both biosynthetic elongation of shorter-chain fatty acids and dietary input from marine food webs [230,231,232,233,234].
One of the best-known very long-chain monoenoic fatty acids is nervonic acid (24:1n-9), which is widely distributed in the nervous tissues of vertebrates, including mammals, birds, and fish. Nervonic acid is a major component of sphingolipids and cerebrosides, particularly in the myelin sheath that surrounds nerve fibers. In these structures, VLC-MUFAs contribute to the stability and proper functioning of neural membranes. Their long hydrocarbon chains increase membrane packing and influence the physical properties of lipid bilayers, which is essential for maintaining the insulating properties of myelin. Because of this role, alterations in very long-chain fatty acid metabolism are associated with certain neurological disorders, such as peroxisomal diseases and demyelinating conditions [234,235,236,237].
In marine animals, especially deep-sea fish and marine mammals, VLC-MUFAs may also occur in storage lipids such as triacylglycerols and wax esters. These compounds are often derived from the elongation of dietary monoenes such as 20:1 and 22:1 fatty acids, which originate from copepod and plankton lipids. Through successive elongation reactions catalyzed by fatty acid elongases, animals can convert these fatty acids into longer-chain homologues such as 24:1 and 26:1. In certain species of marine fish, particularly those living in cold or deep environments, these fatty acids contribute to membrane adaptation and energy storage, helping organisms maintain cellular function under conditions of low temperature and high pressure [231,232].
Marine mammals also contain small but significant amounts of VLC-MUFAs in their blubber and specialized adipose tissues. These fatty acids may influence the physical characteristics of lipid reserves, including melting temperature, density, and flexibility. Because marine mammals depend heavily on lipid reserves for thermoregulation and energy during long migrations or fasting periods, the presence of very long-chain fatty acids can affect the thermal insulation and mechanical properties of blubber [217,218,219,220,221,222].
From a biochemical perspective, the synthesis of very long-chain monoenoic fatty acids in higher animals occurs primarily through elongation of preexisting fatty acids in the endoplasmic reticulum. Enzymes belonging to the elongation of very long-chain fatty acids (ELOVL) family catalyze the stepwise addition of two-carbon units to existing fatty acid chains. For example, oleic acid (18:1n-9) may be elongated to 20:1, 22:1, and ultimately 24:1, depending on the metabolic capabilities of the organism. The final distribution of VLC-MUFAs within tissues is controlled by both biosynthetic capacity and dietary intake [238,239,240,241,242,243].
In addition to their structural roles, VLC-MUFAs can serve as biochemical markers in ecological and physiological studies. Their presence in marine animals may indicate specific dietary sources or metabolic pathways. For instance, elevated levels of long-chain monoenes in certain fish or marine mammals often reflect a diet rich in copepod-derived lipids, which are transferred through marine food webs [240,241,242,243].
Overall, very long-chain monoenoic fatty acids represent a specialized class of lipids that play important roles in membrane structure, neural function, and metabolic adaptation in higher animals. Although they typically occur in lower concentrations than shorter-chain fatty acids, their unique physicochemical properties make them essential components of certain biological membranes and lipid storage systems, particularly in organisms adapted to the demanding conditions of marine and deep-sea environments [235,236,237,238,239,240,241,242,243].
12. Conclusions
Monoenoic fatty acids (MUFAs) are fatty acids that contain one carbon–carbon double bond in an otherwise saturated hydrocarbon chain. These compounds are widely distributed in aquatic organisms and represent an important component of the lipid composition of marine and freshwater biota, including microorganisms, algae, invertebrates, and fish. In aquatic organisms, MUFAs occur mainly in membrane phospholipids and neutral storage lipids, where they influence the structural and functional properties of biological membranes. The presence of a single double bond, typically in the cis configuration, reduces the melting point of the lipid molecule and increases membrane fluidity compared with fully saturated fatty acids. This property allows aquatic organisms to adjust the physical state of cellular membranes in response to environmental factors such as temperature, salinity, and oxygen availability. As a result, variations in the distribution and composition of MUFAs often reflect both physiological adaptation and environmental conditions in aquatic ecosystems. These fatty acids therefore play an important role not only in cellular lipid metabolism but also in the ecological functioning and biochemical adaptation of organisms inhabiting diverse aquatic environments, including regions such as Lake Baikal, the Jordan basin wetlands, and the Caspian Sea.
Beyond their structural role, monoenoic fatty acids serve as sensitive chemotaxonomic markers and trophic indicators, linking primary producers to higher consumers across aquatic food webs. Positional and geometric isomerism—including Δ5, Δ7, Δ9, Δ11, Δ13, and trans variants—adds an additional layer of biochemical complexity, enabling differentiation among taxa and environmental niches. Rare structural forms, such as methyl-branched, cyclopropane-containing, or acetylenic monoenoic derivatives, further expand the functional landscape of these lipids and suggest adaptive or defensive roles in specialized organisms. Computational QSAR and PASS analyses indicate that monoenoic fatty acids and their unusual derivatives occupy distinct bioactivity spaces associated with lipid metabolism regulation, vascular modulation, antimicrobial defense, inflammatory control, and membrane stabilization, underscoring their potential physiological and pharmacological relevance.
This review synthesizes current knowledge on the structural diversity, biosynthetic pathways, ecological distribution, trophic transfer, and predicted biological activities of monoenoic fatty acids in aquatic systems. By integrating molecular chemistry, environmental lipidomics, and computational bioactivity assessment, we propose that monoenoic fatty acids function not merely as passive membrane constituents but as dynamic mediators connecting biochemical structure to organismal adaptation and ecosystem-level processes.
13. Future Research Directions and Perspectives
Despite considerable progress in the study of monoenoic fatty acids (MUFAs) in aquatic organisms, many aspects of their ecological, physiological, and biochemical roles remain insufficiently understood. Future research should therefore focus on several key directions that will help clarify the functional significance, environmental dynamics, and applied potential of these lipids in aquatic ecosystems.
One important area requiring further investigation is the biosynthetic pathways and regulation of MUFA production in aquatic organisms. Although the basic enzymatic processes involved in fatty acid desaturation and elongation are known, the genetic and metabolic regulation of these pathways in different aquatic taxa remains poorly characterized. Advances in genomics, transcriptomics, and metabolomics could help identify the genes and regulatory networks responsible for MUFA synthesis, as well as the environmental signals that control their expression. Such studies would provide valuable insights into how aquatic organisms adjust their lipid composition in response to changing environmental conditions.
Another promising direction involves the use of MUFAs as biochemical markers for trophic interactions and energy flow in aquatic food webs. Because different groups of organisms often produce characteristic fatty acid profiles, MUFAs can serve as indicators of dietary sources and trophic relationships. However, the interpretation of fatty acid markers in ecological studies still faces several limitations, including metabolic modification of fatty acids during trophic transfer. Future work should therefore aim to refine fatty acid trophic markers, improve analytical methods, and develop models that better account for metabolic transformations in consumers.
Further research is also needed to understand the role of MUFAs in environmental adaptation, particularly in response to stress factors such as temperature fluctuations, salinity changes, hypoxia, and pollution. The mechanisms by which organisms modify membrane lipid composition to maintain optimal membrane fluidity—known as homeoviscous adaptation—are still not fully understood in many aquatic species. Experimental studies combining lipidomics with physiological and molecular approaches could clarify how MUFAs contribute to stress tolerance and resilience in aquatic ecosystems.
In addition, the chemical ecological roles of MUFAs deserve greater attention. Increasing evidence suggests that fatty acids may function not only as structural components of membranes but also as chemical signals, antimicrobial agents, or mediators of interspecies interactions. For example, certain fatty acids have been shown to inhibit microbial growth, influence biofilm formation, or participate in communication between microorganisms and higher organisms. Investigating these roles could provide new insights into the chemical ecology of aquatic communities and the mechanisms that regulate microbial and algal populations.
From an applied perspective, MUFAs may have important potential in biotechnology, aquaculture, and environmental monitoring. In aquaculture systems, understanding the lipid requirements of cultured species could improve feed formulation and enhance the nutritional quality of aquaculture products. MUFA profiles may also serve as biomarkers of environmental conditions, allowing researchers to monitor ecosystem health and detect ecological changes caused by climate variability or anthropogenic impacts.
Advances in analytical techniques, including high-resolution mass spectrometry and comprehensive lipidomics, are expected to play a critical role in future studies of aquatic fatty acids. These technologies allow detailed characterization of lipid molecular species and can reveal subtle changes in lipid composition that were previously difficult to detect. Integrating lipidomics with ecological and physiological studies will help create a more comprehensive understanding of lipid function in aquatic environments.
Finally, interdisciplinary approaches combining chemical ecology, environmental biochemistry, molecular biology, and ecosystem science will be essential for advancing research on MUFAs. Such integration will allow researchers to connect molecular-level lipid processes with large-scale ecological patterns, improving our understanding of how lipid chemistry contributes to the structure and functioning of aquatic ecosystems.
In summary, future investigations should focus on elucidating the biosynthesis, ecological functions, environmental responses, and practical applications of monoenoic fatty acids in aquatic systems. Addressing these research gaps will not only deepen our understanding of lipid biochemistry but also contribute to broader efforts aimed at conserving and sustainably managing aquatic ecosystems.
Author Contributions
Conceptualization, V.M.D.; methodology, V.M.D.; software, A.O.T.; investigation, V.M.D.; resources, V.M.D.; writing—original draft preparation, A.O.T. and V.M.D.; writing—review and editing, A.O.T. and V.M.D. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Conflicts of Interest
Author Valery M. Dembitsky was employed by the company Bio-Pharm Laboratories. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Figure 1.
Microalgal taxa serving as sources of various isomers of palmitoleic and oleic acids. (a) Mougeotia is a genus of filamentous charophyte green algae widely distributed in freshwater habitats worldwide. Each cell typically contains one (rarely two) flat, ribbon-shaped chloroplast with numerous pyrenoids arranged in a linear series. (b) Zygnema is a widespread genus of filamentous green algae within the family Zygnemataceae and is phylogenetically closely related to the ancestors of land plants. It forms dense mats composed of long, unbranched, bright green to yellow-green filaments, often surrounded by a mucilaginous sheath. (c) Spirogyra is a genus of filamentous charophyte green algae characterized by distinctive spiral-shaped chloroplasts. It occurs globally in freshwater environments, including ponds, lakes, ditches, and slow-moving rivers. Each cell contains one or more ribbon-like chloroplasts arranged in a helical pattern, while a large central vacuole occupies most of the cell volume, with the nucleus suspended by cytoplasmic strands.
Figure 1.
Microalgal taxa serving as sources of various isomers of palmitoleic and oleic acids. (a) Mougeotia is a genus of filamentous charophyte green algae widely distributed in freshwater habitats worldwide. Each cell typically contains one (rarely two) flat, ribbon-shaped chloroplast with numerous pyrenoids arranged in a linear series. (b) Zygnema is a widespread genus of filamentous green algae within the family Zygnemataceae and is phylogenetically closely related to the ancestors of land plants. It forms dense mats composed of long, unbranched, bright green to yellow-green filaments, often surrounded by a mucilaginous sheath. (c) Spirogyra is a genus of filamentous charophyte green algae characterized by distinctive spiral-shaped chloroplasts. It occurs globally in freshwater environments, including ponds, lakes, ditches, and slow-moving rivers. Each cell contains one or more ribbon-like chloroplasts arranged in a helical pattern, while a large central vacuole occupies most of the cell volume, with the nucleus suspended by cytoplasmic strands.
Figure 2.
Isomers of palmitoleic acid, which are widespread in aquatic organisms, algae, cyanobacteria, and freshwater invertebrates. The positional (Δ9, Δ7) and geometric (cis/trans) isomers differ in double-bond location and configuration, influencing membrane packing and fluidity. In aquatic microorganisms, these structural variations are often linked to temperature adaptation and modulation of membrane permeability under fluctuating environmental conditions. The distribution of specific palmitoleic acid isomers may therefore serve as a biochemical indicator of ecological niche, trophic interactions, and physiological stress in aquatic ecosystems.
Figure 2.
Isomers of palmitoleic acid, which are widespread in aquatic organisms, algae, cyanobacteria, and freshwater invertebrates. The positional (Δ9, Δ7) and geometric (cis/trans) isomers differ in double-bond location and configuration, influencing membrane packing and fluidity. In aquatic microorganisms, these structural variations are often linked to temperature adaptation and modulation of membrane permeability under fluctuating environmental conditions. The distribution of specific palmitoleic acid isomers may therefore serve as a biochemical indicator of ecological niche, trophic interactions, and physiological stress in aquatic ecosystems.
Figure 3.
Oleic acid isomers are widely present in aquatic organisms such as algae, cyanobacteria, and invertebrates. Variations in double-bond position (e.g., Δ9, Δ11) and configuration (cis/trans) influence membrane fluidity, lipid–protein interactions, and metabolic signaling pathways. In aquatic environments, the relative abundance of specific oleic acid isomers often reflects adaptive responses to temperature, salinity, and nutrient availability. Consequently, oleic acid profiles can provide valuable chemotaxonomic and ecological information regarding species composition and environmental conditions.
Figure 3.
Oleic acid isomers are widely present in aquatic organisms such as algae, cyanobacteria, and invertebrates. Variations in double-bond position (e.g., Δ9, Δ11) and configuration (cis/trans) influence membrane fluidity, lipid–protein interactions, and metabolic signaling pathways. In aquatic environments, the relative abundance of specific oleic acid isomers often reflects adaptive responses to temperature, salinity, and nutrient availability. Consequently, oleic acid profiles can provide valuable chemotaxonomic and ecological information regarding species composition and environmental conditions.
Figure 4.
Eicosenoic acid isomers in aquatic organisms appear as minor fatty acids but comprise multiple positional and geometric variants. These C20:1 isomers (e.g., Δ9, Δ11, Δ13) differ in double-bond location, which can influence membrane microdomain organization and energy storage properties. Although typically present in low abundance, their distribution may reflect trophic transfer, species-specific lipid metabolism, and adaptation to cold or deep-water habitats.
Figure 4.
Eicosenoic acid isomers in aquatic organisms appear as minor fatty acids but comprise multiple positional and geometric variants. These C20:1 isomers (e.g., Δ9, Δ11, Δ13) differ in double-bond location, which can influence membrane microdomain organization and energy storage properties. Although typically present in low abundance, their distribution may reflect trophic transfer, species-specific lipid metabolism, and adaptation to cold or deep-water habitats.
Figure 5.
Rare acetylenic monoenoic fatty acids have been identified in several aquatic bryophytes, including the mosses Calliergon cordifolium, Drepanocladus lycopodioides, and Fontinalis antipyretica, as well as the liverwort Riccia fluitans. (a) Riccia fluitans (commonly known as floating crystalwort) is an aquatic liverwort that typically grows freely floating in freshwater habitats. It forms dense, bright green, interwoven mats at the water surface. Due to its floating habit, the plant has direct access to atmospheric CO2 and high light intensity, conditions that may promote rapid growth and potentially stimulate the biosynthesis of acetylenic fatty acids. (b) Fontinalis antipyretica (key moss or willow moss) is a perennial aquatic moss widely distributed throughout the Northern Hemisphere. The stiff, lanceolate leaves are arranged in three distinct rows, giving the stems a characteristic triangular cross-sectional appearance. (c) Calliergon cordifolium (heart-leaved calliergon) is a large, perennial pleurocarpous moss characteristic of waterlogged and marshy habitats. It typically develops extensive mats in fens and wet meadows, where stable moisture conditions may influence its lipid composition. (d) Drepanocladus lycopodioides (syn. Pseudocalliergon lycopodioides) is a large, golden-brown moss belonging to the family Amblystegiaceae. It is considered rare and threatened in several European regions. This species prefers calcareous, nutrient-rich wetlands, including lowland fens, dune slacks, and temporary pools (e.g., Irish turloughs), and is often associated with sedges and creeping willows.
Figure 5.
Rare acetylenic monoenoic fatty acids have been identified in several aquatic bryophytes, including the mosses Calliergon cordifolium, Drepanocladus lycopodioides, and Fontinalis antipyretica, as well as the liverwort Riccia fluitans. (a) Riccia fluitans (commonly known as floating crystalwort) is an aquatic liverwort that typically grows freely floating in freshwater habitats. It forms dense, bright green, interwoven mats at the water surface. Due to its floating habit, the plant has direct access to atmospheric CO2 and high light intensity, conditions that may promote rapid growth and potentially stimulate the biosynthesis of acetylenic fatty acids. (b) Fontinalis antipyretica (key moss or willow moss) is a perennial aquatic moss widely distributed throughout the Northern Hemisphere. The stiff, lanceolate leaves are arranged in three distinct rows, giving the stems a characteristic triangular cross-sectional appearance. (c) Calliergon cordifolium (heart-leaved calliergon) is a large, perennial pleurocarpous moss characteristic of waterlogged and marshy habitats. It typically develops extensive mats in fens and wet meadows, where stable moisture conditions may influence its lipid composition. (d) Drepanocladus lycopodioides (syn. Pseudocalliergon lycopodioides) is a large, golden-brown moss belonging to the family Amblystegiaceae. It is considered rare and threatened in several European regions. This species prefers calcareous, nutrient-rich wetlands, including lowland fens, dune slacks, and temporary pools (e.g., Irish turloughs), and is often associated with sedges and creeping willows.
Figure 6.
Stearolic and tariric acids (monoacetylenic fatty acids) containing a triple bond were found in high amounts in several aquatic bryophytes, including the mosses Calliergon cordifolium, Drepanocladus lycopodioides, and Fontinalis antipyretica, as well as the liverworts Riccia fluitans and Pellia neesiana. The presence of an acetylenic bond confers increased chemical reactivity and may influence membrane rigidity and stress tolerance under submerged conditions. Their accumulation in aquatic bryophytes suggests a potential role in adaptive lipid remodeling and ecological specialization in freshwater habitats.
Figure 6.
Stearolic and tariric acids (monoacetylenic fatty acids) containing a triple bond were found in high amounts in several aquatic bryophytes, including the mosses Calliergon cordifolium, Drepanocladus lycopodioides, and Fontinalis antipyretica, as well as the liverworts Riccia fluitans and Pellia neesiana. The presence of an acetylenic bond confers increased chemical reactivity and may influence membrane rigidity and stress tolerance under submerged conditions. Their accumulation in aquatic bryophytes suggests a potential role in adaptive lipid remodeling and ecological specialization in freshwater habitats.
Figure 7.
Freshwater sponges from the former Lake Hula region, that synthesize methyl-branched monoenoic fatty acids. (a) Ephydatia syriaca is a freshwater sponge belonging to the family Spongillidae. It occurs in freshwater bodies of the Middle East and has been recorded in Lake Kinneret (Lake Tiberias) and associated river systems. As a filter feeder, it contributes to nutrient cycling and organic matter turnover in these ecosystems. (b) Nudospongilla sp. is a genus of freshwater sponges within the family Spongillidae. Members of this genus are suspension feeders, filtering small organic particles, bacteria, and protozoa from the water column. Many species form symbiotic associations with green microalgae (zoochlorellae), which impart a characteristic green coloration and may influence sponge lipid metabolism. (c) Cortispongilla barroisi is a rare freshwater sponge species endemic to Lake Kinneret (Sea of Galilee), Israel. It is the sole representative of the genus Cortispongilla and is restricted to the Jordan River basin, with its primary occurrence in Lake Kinneret. Its limited distribution highlights its ecological specialization and potential vulnerability to environmental change. (d) Corvomeyenia sp. is a genus of freshwater sponges belonging to the family Metaniidae. Species of this genus are typically associated with well-oxygenated, high-quality freshwater environments and are therefore considered useful bioindicators. They inhabit lakes, rivers, and streams, attaching to submerged hard substrates such as rocks and woody debris.
Figure 7.
Freshwater sponges from the former Lake Hula region, that synthesize methyl-branched monoenoic fatty acids. (a) Ephydatia syriaca is a freshwater sponge belonging to the family Spongillidae. It occurs in freshwater bodies of the Middle East and has been recorded in Lake Kinneret (Lake Tiberias) and associated river systems. As a filter feeder, it contributes to nutrient cycling and organic matter turnover in these ecosystems. (b) Nudospongilla sp. is a genus of freshwater sponges within the family Spongillidae. Members of this genus are suspension feeders, filtering small organic particles, bacteria, and protozoa from the water column. Many species form symbiotic associations with green microalgae (zoochlorellae), which impart a characteristic green coloration and may influence sponge lipid metabolism. (c) Cortispongilla barroisi is a rare freshwater sponge species endemic to Lake Kinneret (Sea of Galilee), Israel. It is the sole representative of the genus Cortispongilla and is restricted to the Jordan River basin, with its primary occurrence in Lake Kinneret. Its limited distribution highlights its ecological specialization and potential vulnerability to environmental change. (d) Corvomeyenia sp. is a genus of freshwater sponges belonging to the family Metaniidae. Species of this genus are typically associated with well-oxygenated, high-quality freshwater environments and are therefore considered useful bioindicators. They inhabit lakes, rivers, and streams, attaching to submerged hard substrates such as rocks and woody debris.
Figure 8.
Unique methyl-branched monoenoic fatty acids were found in extracts of freshwater sponges from the lake Hula. The presence of methyl branching alters chain packing and membrane fluidity, potentially enhancing structural stability under fluctuating freshwater conditions. Such branched monoenes are often associated with symbiotic or sponge-specific microbial communities, suggesting a possible microbial contribution to their biosynthesis. These uncommon lipid signatures may therefore serve as chemotaxonomic markers and indicators of host–microbe interactions in freshwater ecosystems.
Figure 8.
Unique methyl-branched monoenoic fatty acids were found in extracts of freshwater sponges from the lake Hula. The presence of methyl branching alters chain packing and membrane fluidity, potentially enhancing structural stability under fluctuating freshwater conditions. Such branched monoenes are often associated with symbiotic or sponge-specific microbial communities, suggesting a possible microbial contribution to their biosynthesis. These uncommon lipid signatures may therefore serve as chemotaxonomic markers and indicators of host–microbe interactions in freshwater ecosystems.
Figure 9.
Unusual cyclopropane-containing fatty acids were discovered in the deep-lake invertebrate Acanthogammarus grewingkii, which lives in Lake Baikal. The incorporation of cyclopropane rings into fatty acyl chains alters membrane curvature, rigidity, and resistance to oxidative stress. Such modifications are often associated with adaptation to extreme environmental conditions, including low temperature and high hydrostatic pressure characteristic of deep freshwater systems. Their presence in A. grewingkii may therefore reflect either specialized endogenous lipid metabolism or contributions from symbiotic microbial communities adapted to the unique ecosystem of Lake Baikal.
Figure 9.
Unusual cyclopropane-containing fatty acids were discovered in the deep-lake invertebrate Acanthogammarus grewingkii, which lives in Lake Baikal. The incorporation of cyclopropane rings into fatty acyl chains alters membrane curvature, rigidity, and resistance to oxidative stress. Such modifications are often associated with adaptation to extreme environmental conditions, including low temperature and high hydrostatic pressure characteristic of deep freshwater systems. Their presence in A. grewingkii may therefore reflect either specialized endogenous lipid metabolism or contributions from symbiotic microbial communities adapted to the unique ecosystem of Lake Baikal.
Table 1.
Predicted activity of positional isomers of palmitoleic acid (1–9).
| No. | Dominated Activity, Pa * | Additional Activity, Pa * |
|---|---|---|
| 1 | Lipid metabolism regulator (0.854) | Antiviral (Arbovirus) (0.803) |
| Vasoprotector (0.815) | Antimutagenic (0.778) | |
| 2 | Lipid metabolism regulator (0.879) | Antiviral (Arbovirus) (0.811) |
| Vasoprotector (0.843) | Antimutagenic (0.786) | |
| 3 | Lipid metabolism regulator (0.904) | Antimutagenic (0.834) |
| Vasoprotector (0.873) | Mucositis treatment (0.811) | |
| Antiviral (Arbovirus) (0.858) | Preneoplastic conditions treatment (0.784) | |
| 4 | Lipid metabolism regulator (0.978) | Preneoplastic conditions treatment (0.890) |
| Vasoprotector (0.957) | Neuroprotector (0.868) | |
| Antiviral (Arbovirus) (0.938) | Antihypercholesterolemic (0.851) | |
| Antimutagenic (0.929) | Antihypoxic (0.842) | |
| Mucositis treatment (0.911) | Antiinflammatory (0.828) | |
| 5 | Lipid metabolism regulator (0.951) | Preneoplastic conditions treatment (0.874) |
| Vasoprotector (0.923) | Neuroprotector (0.857) | |
| Antiviral (Arbovirus) (0.909) | Antihypercholesterolemic (0.832) | |
| Antimutagenic (0.894) | Antihypoxic (0.811) | |
| Mucositis treatment (0.883) | Antiinflammatory (0.807) | |
| 6 | Antiviral (Arbovirus) (0.932) | Antimutagenic (0.859) |
| Lipid metabolism regulator (0.906) | Mucositis treatment (0.846) | |
| Vasoprotector (0.889) | Hypolipemic (0.837) | |
| 7 | Lipid metabolism regulator (0.908) | Antiviral (Arbovirus) (0.852) |
| Vasoprotector (0.871) | Antimutagenic (0.816) | |
| 8 | Lipid metabolism regulator (0.934) | Antiviral (Arbovirus) (0.883) |
| Vasoprotector (0.903) | Antimutagenic (0.857) | |
| 9 | Lipid metabolism regulator (0.929) | Antiviral (Arbovirus) (0.891) |
| Vasoprotector (0.896) | Antimutagenic (0.866) |
* Only activities with Pa > 0.7 are shown.
Table 2.
Biological activity of oleic acids.
| No. | Dominated Activity, Pa * | Additional Activity, Pa * |
|---|---|---|
| 10 | Lipid metabolism regulator (0.939) | Antiviral (Arbovirus) (0.876) |
| Vasoprotector (0.904) | Antimutagenic (0.902) | |
| 11 | Lipid metabolism regulator (0.960) | Preneoplastic conditions treatment (0.868) |
| Vasoprotector (0.920) | Neuroprotector (0.843) | |
| Antiviral (Arbovirus) (0.913) | Antihypercholesterolemic (0.832) | |
| Antimutagenic (0.902) | Antiinflammatory (0.822) | |
| Mucositis treatment (0.887) | Cytoprotectant (0.808) | |
| 12 | Lipid metabolism regulator (0.987) | Preneoplastic conditions treatment (0.875) |
| Vasoprotector (0.942) | Neuroprotector (0.851) | |
| Antiviral (Arbovirus) (0.935) | Antihypercholesterolemic (0.835) | |
| Antimutagenic (0.924) | Antiinflammatory (0.812) | |
| Mucositis treatment (0.906) | Cytoprotectant (0.794) | |
| 13 | Lipid metabolism regulator (0.957) | Antiviral (Arbovirus) (0.911) |
| Vasoprotector (0.923) | Antimutagenic (0.898) | |
| 14 | Lipid metabolism regulator (0.934) | Antiviral (Arbovirus) (0.913) |
| Vasoprotector (0.906) | Antimutagenic (0.902) | |
| 15 | Lipid metabolism regulator (0.911) | Antiviral (Arbovirus) (0.905) |
| Vasoprotector (0.903) | Antimutagenic (0.902) | |
| 16 | Lipid metabolism regulator (0.984) | Preneoplastic conditions treatment (0.910) |
| Vasoprotector (0.966) | Neuroprotector (0.892) | |
| Antiviral (Arbovirus) (0.951) | Antihypercholesterolemic (0.888) | |
| Antimutagenic (0.926) | Antihypoxic (0.881) | |
| 17 | Lipid metabolism regulator (0.980) | Preneoplastic conditions treatment (0.902) |
| Vasoprotector (0.961) | Neuroprotector (0.888) | |
| Antiviral (Arbovirus) (0.949) | Antihypercholesterolemic (0.869) | |
| Antimutagenic (0.921) | Antihypoxic (0.852) |
* Only activities with Pa > 0.7 are shown.
Table 3.
Biological activity of eicosenoic acid isomers.
| No. | Dominated Activity, Ra * | Additional Activity, Ra * |
|---|---|---|
| 18 | Antineoplastic (0.911) | Preneoplastic conditions treatment (0.858) |
| 19 | Antineoplastic (0.939) | Antihypercholesterolemic (0.824) |
| Preneoplastic conditions treatment (0.837) | Lipid metabolism regulator (0.801) | |
| 20 | Antineoplastic (0.961) | Lipid metabolism regulator (0.841) |
| Preneoplastic conditions treatment (0.879) | Cholesterol synthesis inhibitor (0.822) | |
| Antihypercholesterolemic (0.862) | Atherosclerosis treatment (0.813) | |
| 21 | Antineoplastic (0.983) | Lipid metabolism regulator (0.879) |
| Preneoplastic conditions treatment (0.910) | Cholesterol synthesis inhibitor (0.855) | |
| Antihypercholesterolemic (0.889) | Atherosclerosis treatment (0.843) | |
| 22 | Antineoplastic (0.952) | Lipid metabolism regulator (0.865) |
| Preneoplastic conditions treatment (0.901) | Antihypercholesterolemic (0.852) | |
| 23 | Antineoplastic (0.919) | Lipid metabolism regulator (0.905) |
* Only activities with Ra > 0.7 are shown.
Table 4.
Biological activity of acetylenic fatty acids derived from aquatic bryophytes (24–27).
| No. | Dominated Activity, Ra | Additional Activity, Ra |
|---|---|---|
| 24 | Antimicrobial (0.922) | Antiinflammatory (0.857) |
| Antifungal (0.908) | Antiviral (Arbovirus) (0.854) | |
| Antiparasitic (0.889) | Antiviral (Picornavirus) (0.848) | |
| 25 | Antimicrobial (0.942) | Antiinflammatory (0.882) |
| Antifungal (0.931) | Antiviral (Arbovirus) (0.867) | |
| Antiparasitic (0.911) | Antiviral (Picornavirus) (0.856) | |
| 26 | Antimicrobial (0.918) | Antiviral (Arbovirus) (0.862) |
| Antifungal (0.892) | Antiviral (Picornavirus) (0.856) | |
| Antiparasitic (0.876) | Antiinflammatory (0.844) | |
| 27 | Antiprotozoal (0.977) | Antiparasitic (0.903) |
| Antiprotozoal (Plasmodium) (0.955) | Antimicrobial (0.871) | |
| Antimicrobial (0.911) | Antifungal (0.846) |
Table 5.
Biological activity of methyl-branched monoenoic fatty acids.
| No. | Dominated Activity, Ra * | Additional Activity, Ra |
|---|---|---|
| 28 | Antieczematic (0.934) | Antihypercholesterolemic (0.864) |
| Lipid metabolism regulator (0.926) | Antiseborrheic (0.843) | |
| 29 | Antipsoriatic (0.958) | Antiseborrheic (0.887) |
| Antieczematic (0.939) | Atherosclerosis treatment (0.874) | |
| Lipid metabolism regulator (0.933) | Antihypercholesterolemic (0.855) | |
| 30 | Antieczematic (0.965) | Antihypercholesterolemic (0.914) |
| Antipsoriatic (0.958) | Antiseborrheic (0.902) | |
| Lipid metabolism regulator (0.944) | Atherosclerosis treatment (0.881) | |
| 31 | Antipsoriatic (0.966) | Antihypercholesterolemic (0.918) |
| Antieczematic (0.928) | Antiseborrheic (0.889) | |
| Lipid metabolism regulator (0.921) | Atherosclerosis treatment (0.859) | |
| 32 | Antipsoriatic (0.953) | Antiseborrheic (0.887) |
| Antieczematic (0.934) | Atherosclerosis treatment (0.864) | |
| Lipid metabolism regulator (0.926) | Antihypercholesterolemic (0.832) | |
| 33 | Antieczematic (0.938) | Lipid metabolism regulator (0.926) |
| Antipsoriatic (0.927) | Atherosclerosis treatment (0.856) |
* Only activities with Ra > 0.7 are shown.
Table 6.
Biological activity of cyclopropane-containing fatty acids.
| No. | Dominated Activity, Ra | Additional Activity, Ra |
|---|---|---|
| 34 | Vasoprotector (0.925) | Antithrombotic (0.879) |
| Vasodilator, peripheral (0.917) | Antihypercholesterolemic (0.856) | |
| Fibrinolytic (0.905) | Lipid metabolism regulator (0.837) | |
| 35 | Vasoprotector (0.932) | Antithrombotic (0.904) |
| Vasodilator, peripheral (0.921) | Antihypercholesterolemic (0.889) | |
| Fibrinolytic (0.914) | Lipid metabolism regulator (0.865) | |
| 36 | Vasoprotector (0.921) | Antithrombotic (0.884) |
| Vasodilator, peripheral (0.913) | Antihypercholesterolemic (0.861) | |
| Fibrinolytic (0.906) | Lipid metabolism regulator (0.844) | |
| 37 | Lipid metabolism regulator (0.900) | Antithrombotic (0.869) |
| Vasoprotector (0.891) | Fibrinolytic (0.856) | |
| 38 | Lipid metabolism regulator (0.908) | Antithrombotic (0.879) |
| Vasoprotector (0.896) | Fibrinolytic (0.876) | |
| 39 | Lipid metabolism regulator (0.919) | Antithrombotic (0.887) |
| Vasoprotector (0.904) | Fibrinolytic (0.882) | |
| 40 | Lipid metabolism regulator (0.911) | Fibrinolytic (0.885) |
| Vasoprotector (0.886) | Antithrombotic (0.879) | |
| 41 | Vasoprotector (0.901) | Antithrombotic (0.884) |
| Lipid metabolism regulator (0.898) | Fibrinolytic (0.906) |
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Dembitsky, V.M.; Terent’ev, A.O. Chemical Ecology of Monoenoic Fatty Acids in Aquatic Environments. Hydrobiology 2026, 5, 8. https://doi.org/10.3390/hydrobiology5010008
AMA Style
Dembitsky VM, Terent’ev AO. Chemical Ecology of Monoenoic Fatty Acids in Aquatic Environments. Hydrobiology. 2026; 5(1):8. https://doi.org/10.3390/hydrobiology5010008
Chicago/Turabian StyleDembitsky, Valery M., and Alexander O. Terent’ev. 2026. "Chemical Ecology of Monoenoic Fatty Acids in Aquatic Environments" Hydrobiology 5, no. 1: 8. https://doi.org/10.3390/hydrobiology5010008
APA StyleDembitsky, V. M., & Terent’ev, A. O. (2026). Chemical Ecology of Monoenoic Fatty Acids in Aquatic Environments. Hydrobiology, 5(1), 8. https://doi.org/10.3390/hydrobiology5010008
