Bioactivities of Lipid Extracts and Complex Lipids from Seaweeds: Current Knowledge and Future Prospects

While complex lipids of seaweeds are known to display important phytochemical properties, their full potential is yet to be explored. This review summarizes the findings of a systematic survey of scientific publications spanning over the years 2000 to January 2021 retrieved from Web of Science (WoS) and Scopus databases to map the state of the art and identify knowledge gaps on the relationship between the complex lipids of seaweeds and their reported bioactivities. Eligible publications (270 in total) were classified in five categories according to the type of studies using seaweeds as raw biomass (category 1); studies using organic extracts (category 2); studies using organic extracts with identified complex lipids (category 3); studies of extracts enriched in isolated groups or classes of complex lipids (category 4); and studies of isolated complex lipids molecular species (category 5), organized by seaweed phyla and reported bioactivities. Studies that identified the molecular composition of these bioactive compounds in detail (29 in total) were selected and described according to their bioactivities (antitumor, anti-inflammatory, antimicrobial, and others). Overall, to date, the value for seaweeds in terms of health and wellness effects were found to be mostly based on empirical knowledge. Although lipids from seaweeds are little explored, the published work showed the potential of lipid extracts, fractions, and complex lipids from seaweeds as functional ingredients for the food and feed, cosmeceutical, and pharmaceutical industries. This knowledge will boost the use of the chemical diversity of seaweeds for innovative value-added products and new biotechnological applications.


Introduction
Marine macroalgae, popularly known as seaweeds, have emerged as one of the contributors to achieve United Nations sustainable development goals (SDG) [1]. Indeed, algae can be used in healthy and sustainable diets, thereby meeting the farm to fork strategy, which is the core of the European Green Deal [2,3]. Moreover, they are a rich source of nutrients and valuable bioactive phytochemicals that act as preventive agents against non-communicable diseases [4] and that can contribute to overcome multiple societal challenges, such as the ongoing fight on obesity [5] and on the issues caused by antimicrobial resistance in microorganisms [6,7]. Additionally, their chemical diversity can also be paramount to fight infectious viral diseases and allow a higher efficiency when tackling future pandemic situations [7,8]. The exploitation of seaweeds as marine resources for new high value-added products thus contributes to increase their economic relevance on multiple niche markets [9]. The authors have performed a systematic review of scientific literature to establish the state of the art of our knowledge on naturally occurring bioactive complex lipids from seaweeds. The information here assembled provides new insights on how studies are being performed and allows the identification of gaps in knowledge that still need attention in upcoming years.

Methods
This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA-P) guidelines [42]. We used two databases to retrieve scientific publications: Web of Science (WoS) (www.webofknowledge.com, accessed on 21 January 2021) and Scopus (www.scopus.com, accessed on 25 January 2021). A comprehensive search on the bioactivity of complex lipids from seaweeds was performed based on a query by topic (title, abstract and keywords) of the terms: ((alga* OR seaweed* OR macroalga*) AND ("complex lipid*" OR lipid* OR glycolipid* OR phospholipid*) AND (bioactiv* OR activ*)); spanning over the years 2000 to January 2021. The search query resulted in 3114 papers that were subsequently reviewed by the authors, of which 270 were considered eligible for the present work. From those publications, 29 were included in a more in-depth analysis according to criteria described below ( Figure 2).

Selection of Eligibility and Exclusion Criteria
The eligibility and exclusion criteria ( Figure 2) were as follows: publication type (1); matrices studied (2); and extraction method using organic solvents (3). In line with the eligibility criteria selected, only journal articles with empirical data were considered (1); only studies reporting bioactivity assays using seaweeds were considered, and studies using seaweeds and mixed were also considered (2); and studies reporting assays with extracts obtained using organic solvents (e.g., n-hexane, diethyl ether, dichloromethane, n-butanol, chloroform, ethyl acetate, acetone, ethanol, and methanol) were considered (3). The following studies were excluded: reviews, book chapters, proceeding papers, conference papers, and notes (1); studies reporting bioactivity from organisms other than seaweeds (2); and studies using water extracts (3). A total of 270 publications were considered eligible, with these subsequently being screened using the following sub-criteria: only studies identifying an isolated complex lipid group, classes, or species, or reaching a mo-

Methods
This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA-P) guidelines [42]. We used two databases to retrieve scientific publications: Web of Science (WoS) (www.webofknowledge.com, accessed on 21 January 2021) and Scopus (www.scopus.com, accessed on 25 January 2021). A comprehensive search on the bioactivity of complex lipids from seaweeds was performed based on a query by topic (title, abstract and keywords) of the terms: ((alga* OR seaweed* OR macroalga*) AND ("complex lipid*" OR lipid* OR glycolipid* OR phospholipid*) AND (bioactiv* OR activ*)); spanning over the years 2000 to January 2021. The search query resulted in 3114 papers that were subsequently reviewed by the authors, of which 270 were considered eligible for the present work. From those publications, 29 were included in a more in-depth analysis according to criteria described below ( Figure 2).

Selection of Eligibility and Exclusion Criteria
The eligibility and exclusion criteria ( Figure 2) were as follows: publication type (1); matrices studied (2); and extraction method using organic solvents (3). In line with the eligibility criteria selected, only journal articles with empirical data were considered (1); only studies reporting bioactivity assays using seaweeds were considered, and studies using seaweeds and mixed were also considered (2); and studies reporting assays with extracts obtained using organic solvents (e.g., n-hexane, diethyl ether, dichloromethane, n-butanol, chloroform, ethyl acetate, acetone, ethanol, and methanol) were considered (3). The following studies were excluded: reviews, book chapters, proceeding papers, conference papers, and notes (1); studies reporting bioactivity from organisms other than seaweeds (2); and studies using water extracts (3). A total of 270 publications were considered eligible, with these subsequently being screened using the following sub-criteria: only studies identifying an isolated complex lipid group, classes, or species, or reaching a molecular structure were considered for a more in-depth analysis to assess a structure-function relationship. After applying these sub criteria, 29 publications were selected, with these being discussed in detail in Section 3.1.
Mar. Drugs 2021, 19, x 4 of 25 lecular structure were considered for a more in-depth analysis to assess a structure-function relationship. After applying these sub criteria, 29 publications were selected, with these being discussed in detail in Section 3.1.

Results and Discussion
After applying the eligibility criteria adopted in the present work, 270 publications were considered for further analysis. These publications were evaluated taking in account the methodological approaches employed to perform bioassays, namely in vitro versus in vivo studies. Data analysis revealed that 178 publications referred to in vitro experiments, 73 to in vivo assays, and 19 included both in vitro and in vivo assays ( Figure 3). It was also possible to record those in vivo assays included experimental work usually framed within two different approaches: (i) raw seaweed biomass; or (ii) organic extracts administrated intragastrical or in the diet as additives or feed supplements ( Figure 3). Papers that described in vitro assays aimed to evaluate bioactive properties of organic extracts, and in some papers, complex lipids were identified or isolated. The papers that describe both in vitro and in vivo results, evaluated bioactive activities of organic extracts using in vitro assays and also the biological effects after oral administration performed mainly in animal models.

Results and Discussion
After applying the eligibility criteria adopted in the present work, 270 publications were considered for further analysis. These publications were evaluated taking in account the methodological approaches employed to perform bioassays, namely in vitro versus in vivo studies. Data analysis revealed that 178 publications referred to in vitro experiments, 73 to in vivo assays, and 19 included both in vitro and in vivo assays ( Figure 3). It was also possible to record those in vivo assays included experimental work usually framed within two different approaches: (i) raw seaweed biomass; or (ii) organic extracts administrated intragastrical or in the diet as additives or feed supplements ( Figure 3). Papers that described in vitro assays aimed to evaluate bioactive properties of organic extracts, and in some papers, complex lipids were identified or isolated. The papers that describe both in vitro and in vivo results, evaluated bioactive activities of organic extracts using in vitro assays and also the biological effects after oral administration performed mainly in animal models.
To assess the biological effects reported in eligible studies, data was plotted considering the most frequently prospected bioactivities in the 270 eligible publications ( Figure 5). Antioxidant activity (138 studies) was the most reported bioactivity, followed by antimicrobial (61 studies), antitumor (30 studies), anti-inflammatory (19 studies) activities, fat reduction (12 studies), and growth performance (7 studies). Other bioactivities included a wide range of different actions, which was not possible to group within a specific clas-Mar. Drugs 2021, 19, 686 5 of 24 sification. It was also possible to record that most bioactivities reported were related to antioxidant or anti-inflammatory activities; the accurate bioactivity or bioactivities reported on each of these studies are summarized in Table S1. Data (270 publications) were plotted in a word cloud ( Figure 4) featuring seaweed genus. This representation highlighted genera Sargassum, Fucus, Dictyota, and Padina (Ochrophyta; brown seaweeds), genera Ulva and Codium (Chlorophyta; green seaweeds), and genera Gracilaria (Rhodophyta; red seaweeds) as the most reported seaweeds with known bioactivities. . Word cloud assembled using the genera of seaweed species reported in the 270 eligible publications that reported bioactivity on raw seaweed biomass or seaweeds organic extracts. Genera featured with a larger size in the word cloud indicate that species within those genera were the ones mostly reported. Words in brown, green and red refer to genus within phylum Ochrophyta, Chlorophyta, and Rhodophyta, respectively (brown, green, and red seaweeds, respectively).
To assess the biological effects reported in eligible studies, data was plotted considering the most frequently prospected bioactivities in the 270 eligible publications ( Figure  5). Antioxidant activity (138 studies) was the most reported bioactivity, followed by antimicrobial (61 studies), antitumor (30 studies), anti-inflammatory (19 studies) activities, fat reduction (12 studies), and growth performance (7 studies). Other bioactivities included a wide range of different actions, which was not possible to group within a specific classification. It was also possible to record that most bioactivities reported were related to antioxidant or anti-inflammatory activities; the accurate bioactivity or bioactivities reported on each of these studies are summarized in Table S1.  Data (270 publications) were plotted in a word cloud ( Figure 4) featuring seaweed genus. This representation highlighted genera Sargassum, Fucus, Dictyota, and Padina (Ochrophyta; brown seaweeds), genera Ulva and Codium (Chlorophyta; green seaweeds), and genera Gracilaria (Rhodophyta; red seaweeds) as the most reported seaweeds with known bioactivities. . Word cloud assembled using the genera of seaweed species reported in the 270 eligible publications that reported bioactivity on raw seaweed biomass or seaweeds organic extracts. Genera featured with a larger size in the word cloud indicate that species within those genera were the ones mostly reported. Words in brown, green and red refer to genus within phylum Ochrophyta, Chlorophyta, and Rhodophyta, respectively (brown, green, and red seaweeds, respectively).
To assess the biological effects reported in eligible studies, data was plotted considering the most frequently prospected bioactivities in the 270 eligible publications ( Figure  5). Antioxidant activity (138 studies) was the most reported bioactivity, followed by antimicrobial (61 studies), antitumor (30 studies), anti-inflammatory (19 studies) activities, fat reduction (12 studies), and growth performance (7 studies). Other bioactivities included a wide range of different actions, which was not possible to group within a specific classification. It was also possible to record that most bioactivities reported were related to antioxidant or anti-inflammatory activities; the accurate bioactivity or bioactivities reported on each of these studies are summarized in Table S1. . Word cloud assembled using the genera of seaweed species reported in the 270 eligible publications that reported bioactivity on raw seaweed biomass or seaweeds organic extracts. Genera featured with a larger size in the word cloud indicate that species within those genera were the ones mostly reported. Words in brown, green and red refer to genus within phylum Ochrophyta, Chlorophyta, and Rhodophyta, respectively (brown, green, and red seaweeds, respectively). Data (270 publications) was also ranked based on biomass of various seaweeds, or their extracts used in the bioassays performed, being grouped in five categories: studies using seaweed as raw seaweed biomass (category 1); studies using organic extracts (category 2); studies using organic extracts with identified complex lipids (category 3); studies of extracts enriched in isolated groups or classes of complex lipids (category 4); and studies of isolated complex lipid molecular species (category 5).
In some of the selected categories (e.g., category 1 and 2) most studies did not highlight the identification of lipids, neither attributed the bioactivity reported to lipids. However, to our knowledge, the role of complex lipids in the observed bioactivity cannot be excluded. The distribution of eligible studies by category 1-5 and bioactivity assayed is summarized in Figure 6. Most studies were classified according to category 2 (177 studies), followed by category 1 (39 studies) and 3 (25 studies). Category 4 and 5 displayed a smaller number of studies (18 and 11, respectively). Category 1 included studies addressing the improvement of growth and/or immune system/health status, fat reduction, including reduction in hyperlipidemia/cholesterolemia/triglycerides, anti-obesity/anti-adipogenic effects; antioxidant and other activities (Table S1). Studies related with categories 2 to 5 pinpoint antioxidant, antitumor, anti-inflammatory, and antimicrobial (including antibacterial, antiviral, anti-protozoal, anti-microalgal, and anti-fouling) bioactivities. It is important to highlight that several studies reported more than one single bioactivity. Data (270 publications) was also ranked based on biomass of various seaweeds, or their extracts used in the bioassays performed, being grouped in five categories: studies using seaweed as raw seaweed biomass (category 1); studies using organic extracts (category 2); studies using organic extracts with identified complex lipids (category 3); studies of extracts enriched in isolated groups or classes of complex lipids (category 4); and studies of isolated complex lipid molecular species (category 5).
In some of the selected categories (e.g., category 1 and 2) most studies did not highlight the identification of lipids, neither attributed the bioactivity reported to lipids. However, to our knowledge, the role of complex lipids in the observed bioactivity cannot be excluded. The distribution of eligible studies by category 1-5 and bioactivity assayed is summarized in Figure 6. Most studies were classified according to category 2 (177 studies), followed by category 1 (39 studies) and 3 (25 studies). Category 4 and 5 displayed a smaller number of studies (18 and 11, respectively). Category 1 included studies addressing the improvement of growth and/or immune system/health status, fat reduction, including reduction in hyperlipidemia/cholesterolemia/triglycerides, anti-obesity/anti-adipogenic effects; antioxidant and other activities (Table S1). Studies related with categories 2 to 5 pinpoint antioxidant, antitumor, anti-inflammatory, and antimicrobial (including antibacterial, antiviral, anti-protozoal, anti-microalgal, and anti-fouling) bioactivities. It is important to highlight that several studies reported more than one single bioactivity. Antioxidant activity was most studied in categories 1 (13 studies out of 39), 2 (113 studies out of 177), and 3 (11 studies out of 25). In category 1, most studies that evaluated the antioxidant activity tested the inclusion of the raw seaweed biomass on diet, with no specification of the bioactive compound. In category 2, most studies tested organic extracts and were oriented towards phenolic compounds, which were recognized by their antioxidant properties. In category 3, the antioxidant activity was evaluated testing organic extracts with identified complex lipids, assigning the bioactivity to the whole extract and the synergic effect between molecules. The in vitro assays of antioxidant evaluation using free radical scavenging activities were one of the bioactivities more intensively investigated, likely because of well-established and easy-to-use methodologies. However, these in chemico assays have limited biological relevance considering the effect in the mod- Antioxidant activity was most studied in categories 1 (13 studies out of 39), 2 (113 studies out of 177), and 3 (11 studies out of 25). In category 1, most studies that evaluated the antioxidant activity tested the inclusion of the raw seaweed biomass on diet, with no specification of the bioactive compound. In category 2, most studies tested organic extracts and were oriented towards phenolic compounds, which were recognized by their antioxidant properties. In category 3, the antioxidant activity was evaluated testing organic extracts with identified complex lipids, assigning the bioactivity to the whole extract Mar. Drugs 2021, 19, 686 7 of 24 and the synergic effect between molecules. The in vitro assays of antioxidant evaluation using free radical scavenging activities were one of the bioactivities more intensively investigated, likely because of well-established and easy-to-use methodologies. However, these in chemico assays have limited biological relevance considering the effect in the modulation of redox homeostasis of in vivo organisms. Therefore, additional studies are still needed using in vivo models, and measuring biologically relevant biomarkers of redox homeostasis, such as catalase, and superoxide dismutase enzymes, or addressing the proper value of seaweeds lipid antioxidant bioactivities.
Antimicrobial and antitumor activities were mostly studied on categories 4 (11 studies out of 18) and 5 (5 studies out of 11), respectively. Several studies reported the antimicrobial properties of lipid extracts from seaweeds. However, the majority of the studies reported only the estimation of inhibition of bacterial growth, lacking information on the identification of the bioactive lipids promoting such response and/or elucidating the mechanism of antimicrobial action. Interestingly, some studies reported antibacterial and antiviral activity of lipid extract from specific seaweeds and activities seem to be dependent on their origin. As society urgently needs new antibiotics to overcome the current scenario of antibiotic resistance, along with powerful new antiviral drugs to face future pandemics [7], it is urgent to further explore these bioactivities in seaweeds. Concerning antitumor activity, information is also scarce and lacks key information on putative structure function relationship.
To unravel the most studied phyla of seaweeds, data (270 publications) were ranked considering how reported bioactivities were distributed over the phyla Ochrophyta, Chlorophyta, and Rhodophyta ( Figure 7). Seaweed species belonging to the Ochrophyta were the most reported on antioxidant, antimicrobial, antitumor, and anti-inflammatory activities, followed by species within the Rhodophyta. Species within the Chlorophyta were the less studied. the most reported on antioxidant, antimicrobial, antitumor, and anti-inflammatory activities, followed by species within the Rhodophyta. Species within the Chlorophyta were the less studied. Bioactivity distributed by phylum combined with the five categories selected in the present study is plotted in Figure 8. Seaweeds within the Ochrophyta were the most screened to evaluate antioxidant, antimicrobial, antitumor, and anti-inflammatory bioactivities on category 2-4. On the other hand, seaweeds from the Rhodophyta were the most investigated to screen for growth performance, fat reduction, and antioxidant activity over criteria 1. Although with a lower number of studies on category 5, seaweed species within the Chlorophyta and Ochrophyta appeared as the most screened for antitumor activity. Seaweed species within the Rhodophyta were the most studied for anti-inflammatory activity under category 5. Bioactivity distributed by phylum combined with the five categories selected in the present study is plotted in Figure 8. Seaweeds within the Ochrophyta were the most screened to evaluate antioxidant, antimicrobial, antitumor, and anti-inflammatory bioactivities on category 2-4. On the other hand, seaweeds from the Rhodophyta were the most investigated to screen for growth performance, fat reduction, and antioxidant activity over criteria 1. Although with a lower number of studies on category 5, seaweed species within the Chlorophyta and Ochrophyta appeared as the most screened for antitumor activity. Seaweed species within the Rhodophyta were the most studied for anti-inflammatory activity under category 5. screened to evaluate antioxidant, antimicrobial, antitumor, and anti-inflammatory bioactivities on category 2-4. On the other hand, seaweeds from the Rhodophyta were the most investigated to screen for growth performance, fat reduction, and antioxidant activity over criteria 1. Although with a lower number of studies on category 5, seaweed species within the Chlorophyta and Ochrophyta appeared as the most screened for antitumor activity. Seaweed species within the Rhodophyta were the most studied for anti-inflammatory activity under category 5.  In most studies (Table S1), the bioactivity reported for seaweed lipids was often associated with the most abundant molecules identified in organic extracts, or with other molecules detected by the methodology used for structural characterization (e.g., fatty acid identification by Gas Chromatography-Mass Spectrometry (GC-MS)). PUFA have been frequently identified as bioactive lipids in many studies because FA identification was the only approach used for extract characterization on those publications [31,[43][44][45][46]. Nevertheless, this is an inadequate approach since FA commonly exist in low amounts as free FA and they are mostly esterified in complex lipids. Other studies tested extracts obtained with organic solvents, which also extract complex lipids. However, these studies only focused on the identification of well-known phytochemicals, which are present at a lower abundance in seaweeds, such as phenolic compounds, excluding the putative role of lipids and/or the synergic effect of other lipid-soluble compounds [47][48][49].
Knowledge progression of natural bioactive products and their application depends on the isolation of pure molecules to achieve a possible structure-function relationship [50,51]. While this is a very laborious and time-consuming task, it is also essential to understand specific biological effects of these biomolecules. Moreover, this task will also provide a new perspective to plan chemical synthesis and subsequent applications on different fields, such as in the pharmaceutical industry, aiming to add-value to seaweeds as natural sources of bioactive compounds. To date, few studies have tried to overcome this drawback. New studies being performed on bioassays using specific groups or class of seaweed lipids are scarce; although, they are paramount to isolate molecules to address a proper clarification of structure-bioactivity relationship. These studies are detailed bellow.

The Complex Lipids of Seaweeds as Derived Bioactive Phytochemicals
Studies addressing extracts enriched in isolated groups or classes of complexes lipids (category 4) and studies of isolated complex lipid species (category 5) are a minority. However, they provide a greater level of confidence concerning the bioactivity reported on complex lipids. These studies were selected for inclusion criteria following PRISMA-P workflow. Herein, they were ranked based on the bioactivities they evaluated.
Screening of antiproliferative activity is the most common approach to evaluate antitumor potential of complex lipids. Several cancer cell lines have been used including hepato [55,56], cervix [57], breast [56,58], leukemia [58,59], colon [58,60], lung [58,61], melanoma [62], and prostate and ovarian cancer [58]. The majority of these studies used lipid fractions enriched in a specific lipid group or class, obtained by using silica gel columns and solvents with different polarities. This approach was performed, for example, to evaluate the PLs fraction of the brown seaweed Sargassum marginatum inhibiting promyelocytic cells (HL-60) [59]. There is a huge variety of classes within PLs group that can contribute for bioactivity of these fractions; thus, the analysis of enriched lipid fractions solely provides a partial interpretation of results. Fractions enriched in GLs classes were isolated, allowing the identification of inhibitory activity against several cancer cells lines in digalactosyldiacylglycerol (DGDG) [60] and sulfoquinovosyldiacylglycero (SQDG) [55][56][57]60] enriched fractions (Table 1).
Few works have evaluated bioactivities of isolated lipid classes. The monogalactosyldiacylglycerol (MGDG) (MGDG 14:0_16:1) from the red seaweed Solieria chordalis and DGDG (14:0_18:3) from the green seaweed Ulva armoricana were found to have activity against NSCLC-N6 cancer cells [61]. However, to the best of our knowledge, the authors only identified the most abundant lipid species in the fraction, undervaluing other unidentified lipid species. Therefore, the antiproliferative activity of previous GLs molecular species could be incorrectly attributed.
There are very few studies that achieved the isolation and identification of pure compounds, such as  Figure 9B) from the red seaweed Hydrolithon reinboldii, which demonstrated inhibitory activity against a range of 12 cancer cell lines [58].
The species of GL isolated from the green seaweed Avrainvillea nigricans, named Nigricanoside A (Figure 9E), showed the capacity to arrest MCF-7 cells in mitosis, stimulating the polymerization of pure tubulin in vitro and thus inhibiting the proliferation of MCF-7 and HCT-116 cells [66]. The potent antimitotic activity of Nigricanoside A was seen without precedent among previously known GL.

Anti-Inflammatory Activity
Inflammation is a multifactorial condition ubiquitously present in most diseases and particularly in non-communicable diseases. It involves a large number of identified mediators, comprising leukocyte cells that release specialized substances such as pro-inflammatory cytokines [67] and high levels of nitric oxide (NO) in response to the inflammatory process [68].
NO is a potent pro-inflammatory mediator in over inflammation conditions [69]. On a small scale, and for research purposes, inhibition of NO, represents a protective effect of several anti-inflammatory compounds. The reduction in NO production from immune cells is assessed as a first step in the anti-inflammatory potential of natural products. Using this approach, several studies evaluated the anti-inflammatory activity of isolated and characterized seaweed lipid molecules (Table 2) [71], which showed significant NO inhibition through down-regulation of inducible Nitric Oxide Synthase (iNOS). PUFA side chains, mainly EPA and arachidonic acid (AA), esterified to polar lipid structure seem to be relevant for their potent NO inhibition. Curiously, isolated PUFA, such as EPA, AA, and DHA, showed less NO inhibitory activity when compared to their esterified forms in polar lipid [70,71].  Figure 9B) from the red seaweed Hydrolithon reinboldii, which demonstrated inhibitory activity against a range of 12 cancer cell lines [58].
The species of GL isolated from the green seaweed Avrainvillea nigricans, named Nigricanoside A ( Figure 9E), showed the capacity to arrest MCF-7 cells in mitosis, stimulating the polymerization of pure tubulin in vitro and thus inhibiting the proliferation of MCF-7 and HCT-116 cells [66]. The potent antimitotic activity of Nigricanoside A was seen without precedent among previously known GL.    The capacity to inhibit phospholipase A2 (PLA2) has been linked to the efficacy for the treatment of inflammatory processes, since PLA2 hydrolyze membrane phospholipids releasing AA, the precursor of the pro-inflammatory mediators prostaglandins, thromboxanes, and leukotrienes [72,73]. Inhibition of PLA2 is the pharmacological mechanism of action of corticosteroids, a group of drugs with potent anti-inflammatory properties. The 7-methoxy-9-methylhexadeca-4,8-dienoic acid (MMHDA) ( Figure 10C) isolated from the brown seaweed Ishige okamurae was tested in vitro for inhibition of PLA2 activity, and in vivo on edema and erythema induced in rat models. In both models, it demonstrated potent inhibitor of PLA2 activity and inflammation, with IC 50 concentrations lower than the ones reported for rutin, a flavonoid model [74].

Antimicrobial Activity
The emergence of antibiotic resistance of human pathogenic microorganisms and the need for new antiviral drugs has been a key driver for searching new antimicrobial compounds [75]. Complex lipids from seaweeds could play an active role in this field. In this section we describe the lipids from seaweeds with reported antibacterial, antiviral, anti-algal, anti-fouling, antifungal and anti-protozoal activities (Table 3). In spite of the range of antimicrobial activities tested, there is still opportunity to gain a more in-depth knowledge on this bioactive property of seaweed lipids, namely by testing against other strains of bacteria and virus that are major drivers of infection diseases The GLs classes MGDG, DGDG, and SQDG from some species of Laminaria genus [76,77]; the brown seaweeds Fucus evanescens [78], Alaria fistulosa [76], Saccharina cichorioides [79]; and the red seaweed Chondria armata [80], demonstrated activity against a range of bacteria, yeast, and fungus. Likewise, sulfolipids classes from several seaweed species proved antibacterial activity [56]. In addition to antibacterial and antifungal activity, an isolated mixture of SQDG species from the brown seaweed Lobophora variegata showed anti-protozoal activity [81]. Isolated sub-fractions enriched in GL from the green seaweed Ulva prolifera [82] and the brown seaweed Sargassum vulgare [83] showed anti-algal and anti-fouling activities, respectively.
The studies surveyed pinpoint the evaluation of the complex lipid antiviral activity on Herpes simplex virus (HSV). The SQDG class from the red seaweed Osmundaria obtusiloba, the brown seaweed Sargassum vulgare and several species within genus Laminaria (brown seaweeds), were highlighted by its antiviral activity against HSV-1 [56,84,85] and HSV-2 [84,85]. The role of palmitic acid and sulfonate group on SQDG molecular structure was considered as relevant on activity against HSV virus and on cellular receptors [85].
Prospecting new antimicrobial compounds should follow a systemic protocol once the goal is to design solutions for human protection. Tested compounds must also show low toxicity against erythrocytes, which was evaluated in parallel in some studies that revealed hemolytic activity [76][77][78]. Table 3. Lipid species extracted from seaweeds with antimicrobial activities. Extraction and characterization methodologies and cell lines used in bioassays are reported. Data is reported by phylum (Ochrophyta, Rhodophyta, Chlorophyta, or mixed phyla) and ranked by alphabetical order of seaweed species name within each phylum (or mixed phyla).

Other Bioactivities Attribute to Seaweed Lipids
Complex lipids from seaweeds have showed a broad spectrum of bioactivates (Table 4), including antioxidant activity associated with GL and PL groups from the red seaweed Solieria chordalis and the brown seaweed Sargassum muticum, and evidenced through in vitro free radical scavenging activity [86]. However, the study did not characterize the compounds in the isolated fractions, which raises doubts about their purity and possible interference of other compounds.
Fractionated lipid classes, such as MGDG, were suggested to play an important role in the design of optimized nanoparticulate tubular immune-stimulating complexes. Sanina et al. (2021) found different degrees of effectiveness on anti-porin response, porin conformation, and cytokine profile of MGDG from different phyla with different FA composition [87].
A study that bio-prospected and isolated bioactive molecular species from the green seaweed  Figure 11B) from the brown seaweed Sargassum horneri was also reported in 3T3-L1 adipocytes [89]. These two MGDG species have in common the presence of linoleic acid (LA) (18:2 n-6) on sn-2 FA chain position, and when compared to other isolated MGDG species they were the most effective. Thus, this study suggested that LA on the sn-2 position of MGDG species played an important role on the inhibition of triglyceride accumulation in this biological model. Complex lipids from seaweeds have showed a broad spectrum of bioactivates (Table  4), including antioxidant activity associated with GL and PL groups from the red seaweed Solieria chordalis and the brown seaweed Sargassum muticum, and evidenced through in vitro free radical scavenging activity [86]. However, the study did not characterize the compounds in the isolated fractions, which raises doubts about their purity and possible interference of other compounds.
Fractionated lipid classes, such as MGDG, were suggested to play an important role in the design of optimized nanoparticulate tubular immune-stimulating complexes. Sanina et al. (2021) found different degrees of effectiveness on anti-porin response, porin conformation, and cytokine profile of MGDG from different phyla with different FA composition [87].
A study that bio-prospected and isolated bioactive molecular species from the green seaweed  Figure 11B) from the brown seaweed Sargassum horneri was also reported in 3T3-L1 adipocytes [89]. These two MGDG species have in common the presence of linoleic acid (LA) (18:2 n-6) on sn-2 FA chain position, and when compared to other isolated MGDG species they were the most effective. Thus, this study suggested that LA on the sn-2 position of MGDG species played an important role on the inhibition of triglyceride accumulation in this biological model.
A human sperm motility stimulating activity was achieved by an isolated sulfonoglycolipid (named by S-ACT-1) from the red seaweed Gelidiella acerosa, whose molecular structure was not evidenced [90]   A human sperm motility stimulating activity was achieved by an isolated sulfonoglycolipid (named by S-ACT-1) from the red seaweed Gelidiella acerosa, whose molecular structure was not evidenced [90]

Concluding Remarks and Future Prospects
Seaweeds remain largely untapped reservoirs of natural bioactive molecules [10]. In fact, more than 11,300 species of seaweeds are reported on Algabase, of which only 42 species were surveyed on category 4 (studies of extracts enriched in isolated groups or classes of complex lipids) and category 5 (studies of isolated complex lipid molecular species), most of them within the Ochrophyta phylum. This reveals that the bioprospecting potential of seaweed lipids remains largely untapped.
Complex lipids from seaweeds are emerging as bioactive molecules with hidden potential; however, their exploitation is far from being optimized and their action mechanisms are still poorly understood. This figure is likely to change as more seaweeds have their bioactive complex lipids characterized and more mechanism-oriented studies are performed.
To date, not only do most studies lack a systematic research approach, but most of the lipid bioactivities already identified refer to total lipid extracts. Indeed, only a few studies have achieved molecular isolation and characterization of bioactive lipids. Interestingly, complex lipids isolated from seaweed species with reported bioactivity have been classified mainly as GLs species. This systematic analysis pinpoints the promising results of naturally occurring GLs in seaweeds, with emphasis to their antitumor and antiinflammatory potential. The advances of emerging food/feed, nutraceutical, cosmeceutical, pharmaceutical, and complementary medicine research fields [91][92][93], as well as biological and experimental sciences, will contribute to boost structural characterization of complex lipids and to link lipid structure and bioactivity through different mechanisms of action.
Regardless of their polyphyletic nature, it is unquestionable that seaweeds as a whole, remain an important reservoir of lipid phytochemicals. Despite the low abundance of these biomolecules in seaweeds, they remain largely uncharacterized and unexplored. Complex lipids from seaweeds offer an unmatched chemical diversity and structural complexity when compared to terrestrial phytochemicals. It seems that seaweeds species or genera feature unique lipidomes, which likely enhances the potential number of target applications. Lipidomic characterization strategies using high-resolution apparatus, such as mass spectrometry, can be paramount to unleash the true potential of these biomolecules. The species-specific lipidome for each seaweed could be applied to the production of target bioactive lipids. Otherwise, isolated bioactive complex lipids can be used as a largescale synthesis model. While some of their natural chemotherapy diversity has already been studied, resulting in open access and proprietary compound libraries, there is still a multitude of lipids from algal origin that have hardly been characterized. The potential of these biomolecules to develop new products and processes is certainly far from being exhausted. It is expected that the bioprospecting of seaweed extracts enriched in active lipids for the formulation of high-end products can foster the added value of seaweed biomass production.
Under this scope it will be possible to put forward innovative processes for the production of farmed seaweeds biomass under controlled conditions, as these will allow to target new markets and consumers under a circular and sustainable blue bioeconomy framework.

Conflicts of Interest:
The authors declare no conflict of interest.