Ulvan in Agriculture: An Eco-Friendly Approach to Plant Disease Management

Abstract

Plant pathogens can result in massive crop destruction globally, thereby increasing starvation, while conventional or synthetic pesticides are harmful to the environment and human health. The urgent need for sustainable and eco-friendly disease management strategies has driven interest in natural biocontrol agents. Ulva sp. produce a sulfated polysaccharide named ulvan, which serves as a multifunctional biostimulant with pronounced antibacterial, antiviral, and antifungal properties against a broad spectrum of phytopathogens. Its complex anionic structure plays a dual role by directly inhibiting pathogen growth through cell membrane disruption and biofilm suppression, while simultaneously inducing plant defense mechanisms through reactive oxygen species (ROS) signaling and activation of pathogenesis-related (PR) proteins. Recent advances in ulvan extraction, purification, structural analysis, and inhibitory mechanisms of phytopathogens are discussed in this review. Furthermore, the biodegradability and biocompatibility of ulvan highlight its potential applications beyond agriculture, including biomedical and sustainable biomaterial development. By comprehensively analyzing the bioactivity spectrum and mechanistic pathways of ulvan, this review proposes strategic approaches for integrating ulvan into environmentally friendly plant disease management systems, supporting its role in advancing a circular bioeconomy.

1. Introduction

The growing global demand for food, driven by a population exceeding 8.2 billion, has intensified the need for sustainable agricultural practices that balance productivity with environmental stewardship [1]. Currently, for attaining sustainable practices, we need to overcome three more challenges, other than the exceeding population: 1. Versatile plant disease management with respect to the increased agrochemical resistance in the pathogen, 2. global change in the climate, 3. overdependence on nonrenewable We resources and the increased monetary and environmental costs of these resources [2,3,4]. To address these issues, researchers worldwide are conducting groundbreaking research on the development and implementation of sustainable agriculture through innovative solutions [5,6]. Plant disease management is one of the critical issues we are facing while optimizing sustainable techniques in agriculture [7]. The increase in climate change patterns has impacted the interaction of pathogens, their growth cycles, and pathogenicity. Similarly, massive usage of multiple agrochemicals also affected the same, as well as the soil dynamics, and an overall increase in pathogen resistance is also a significant change in the agriculture industry [8]. Conventional plant disease management strategies rely heavily on synthetic pesticides and fungicides; however, these inputs are often costly, environmentally persistent, and pose significant risks to human health and ecosystems. Moreover, their prolonged use contributes to pathogen resistance, soil degradation, and contamination of agroecosystems, disproportionately affecting small and low-income farmers [9]. These challenges underscore the urgent need for eco-friendly, cost-effective, and sustainable alternatives for crop protection. Therefore, it is imperative to look for a solution that not only addresses these issues but also fits into the economic viability criteria [10].

Furthermore, as a sustainable approach, natural bioresources have gained considerable attention, particularly marine macroalgae (seaweeds), which offer promising solutions for sustainable agriculture [11,12]. Among them, green seaweeds belonging to the order Ulvales, especially species of the genus Ulva, have emerged as valuable resources due to their high levels of polysaccharide elicitors, rich biochemical composition, and multifunctional properties. Ulva species are widely distributed, fast-growing, and environmentally adaptable, making them a sustainable and scalable biomass source for agricultural applications [13]. Ulva species rich in ulvan, a water-soluble sulphated polysaccharide (SP), serve as natural allies in sustainable agriculture. Ulvan has been reported to reinforce plant defense pathways, suppress infection, and promote growth under various experimental conditions [14]. As a biodegradable and eco-friendly biopolymer, it offers residue-free crop protection and stimulation, making it particularly valuable in organic farming [15]. Owing to its polysaccharide nature, ulvan is readily decomposed by soil microbes, leaving minimal environmental persistence and posing no toxicity to non-target organisms [16]. By activating plant immune responses without harmful residues, ulvan is organically certified and recognized for its broad antiviral, antibacterial, and antifungal properties [17,18]. Beyond defense, it functions as a potent biostimulant, enhancing biodiversity, improving soil quality, supporting integrated pest management, and ultimately boosting plant growth, yield, and resilience to stress, positioning ulvan as a cornerstone of sustainable crop production [19]. Structurally, ulvan consists of repeating disaccharide units primarily composed of rhamnose sulfate linked with glucuronic acid, iduronic acid, or xylose, forming characteristic ulvanobiuronic acid units (A3S and B3S) (Figure 1a). This unique chemical composition, including sulfate groups and uronic acids, imparts significant biological activity and functional versatility [20,21]. Ulvan can be efficiently extracted through enzymatic and acid-based methods, and its biodegradable and non-toxic nature makes it particularly suitable for sustainable agricultural use (Figure 1b).

Recent studies have demonstrated the effectiveness of ulvan in managing a wide range of plant diseases, particularly those caused by fungal pathogens, which are major contributors to postharvest losses in fruits and vegetables [12]. Ulvan treatments have shown significant reductions in infections caused by Botrytis cinerea, Fusarium oxysporum, and Colletotrichum species while also enhancing plant resistance against diseases such as powdery mildew, rust, and anthracnose [22,23,24]. Studies have reported that foliar application of crude ulvan is an effective strategy for managing plant diseases caused by fungal pathogens such as Colletotrichum trifolii and Colletotrichum lindemuthianum, which are responsible for alfalfa anthracnose in Medicago truncatula and bean anthracnose in Phaseolus vulgaris, respectively [25,26]. Another study demonstrated that a 1000 mg/L ulvan extract from Ulva rigida reduced grape spoilage by 41% and Botrytis cinerea infection by 43%. Notably, effective concentrations reported in the literature vary widely, ranging from 100 mg/L to 1000 mg/L, reflecting differences in purity, formulation, and application methods, which complicates direct comparison across studies. Ulvan application does not adversely affect fruit quality and has been associated with enhanced defense biomarkers, including reactive oxygen species (ROS) (1.4-fold), catalase (4-fold), superoxide dismutase (SOD) (2-fold), and chitinase (1.4-fold) [22,23]. Ulvan extracts have also been reported to substantially reduce postharvest spoilage in grapes while strengthening plant defense systems without compromising crop quality [24,25]. Similarly, ulvan treatments have been shown to mitigate the effects of rust, powdery mildew, and anthracnose in bean plants [26,27]. In addition, ulvan inhibits Fusarium oxysporum f. sp. phaseoli (Fop), the causal agent of bean wilt, and improves seedling emergence [28,29]. Furthermore, purified ulvan fractions have exhibited enhanced efficacy against diseases in economically important crops such as wheat and barley, highlighting their potential as next-generation plant protection agents [24]. Studies indicate that purified forms of ulvan compounds achieve optimal results against powdery mildew in wheat and barley [30]. Ulvan has also been shown to enhance resistance in wheat against Zymoseptoria tritici without significantly altering the leaf metabolome, indicating its suitability as a natural crop protectant in agricultural systems [31,32,33].

Comparative analysis of existing studies reveals significant variation in ulvan effectiveness across different plantpathogen systems, with disease suppression ranging from approximately 30% to over 70%, depending on crop type, ulvan concentration, and method of application. For instance, moderate ulvan concentrations are typically associated with around a 40% reduction in postharvest systems, whereas higher efficacy is often observed under controlled laboratory conditions and with purified ulvan fractions. Beyond disease control, ulvan contributes to broader agricultural sustainability by supporting integrated pest management strategies, promoting soil health, and enhancing plant tolerance to abiotic stresses [34]. Its multifunctional role positions it as a viable alternative to synthetic agrochemicals, aligning with global efforts toward residue-free and environmentally sustainable agriculture [35] (Figure 2). Ulvan activates plant innate immunity through signaling pathways involving salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), thereby inducing both systemic acquired resistance (SAR) and induced systemic resistance (ISR), unlike conventional pesticides that act directly on pathogens [17,36]. Additionally, ulvan may stimulate oxalate oxidase (OXO), leading to the production of hydrogen peroxide (H₂O₂), which plays dual roles in signal transduction and antimicrobial defense [37]. By inducing lipoxygenase (LOX) and allene oxide synthase (AOS) genes, ulvan activates the octadecanoid pathway, enhancing JA biosynthesis and strengthening resistance against insect herbivores and necrotrophic pathogens [36]. Since ulvan is suggested to mimic methyl jasmonate (MeJA)induced transcriptomic responses, it is likely involved in JA-mediated signaling and the activation of protease inhibitors and other systemic defense mechanisms [37,38]. Additional benefits of ulvan include increased β-1,3-glucanase activity, which facilitates degradation of fungal cell walls, and elevated peroxidase activity, which promotes lignin synthesis and strengthens cell walls [39]. The bioactivity of ulvan is largely attributed to its structural composition, particularly sulfated rhamnose, xylose, and uronic acid residues. Sulfate groups play a crucial role in interacting with microbial membranes and potentially disrupting pathogen viability [40]. However, it is important to note that some of these mechanisms are based on limited studies or inferred from analogies with other sulfated polysaccharides, and their effectiveness may vary depending on plant species and environmental conditions.

Despite significant advancements in ulvan extraction, characterization, and application across agricultural, biomedical, and food sectors, further research is required to optimize its large-scale production, standardization, and field-level efficacy [41]. Nevertheless, the integration of ulvan-based formulations into modern agriculture represents a promising pathway toward sustainable crop protection, enhanced food security, and reduced environmental impact [42]. Most existing reviews on ulvan and sulfated polysaccharides (SPs) have primarily focused on extraction, purification, or their applications in the food and biomedical sectors [21]. However, there remains a critical need for a comprehensive synthesis that addresses plant disease mitigation mechanisms, structural implications, and the diverse biological activities of SPs, including their anticoagulant, anti-inflammatory, antiviral, antioxidant, and immunomodulatory properties, along with their broader health benefits [43]. Equally important is a detailed exploration of extraction procedures, characterization techniques, and the commercial potential of SPs derived from brown, red, and green seaweeds, highlighting their relevance to human well-being [44]. This review aims to fill these gaps by providing an in-depth discussion of ulvan compounds derived from green seaweeds, particularly those belonging to the genus Ulva. It incorporates case studies demonstrating optimized extraction methods, biochemical profiling, and nutritional characterization.

Furthermore, the review examines formulation and application strategies validated through field-based efficacy studies, with a focus on biostimulants and plant disease management. The novelty of this work lies in its systematic analysis of ulvan’s role in plant disease mitigation and management. By integrating evidence on antimicrobial activity, bioremediation potential, and agricultural relevance, it highlights ulvan’s unique chemical diversity and its expanding importance across industrial applications. Beyond current practices, the review outlines future prospects, including the development of standardized ulvan formulations, synergistic applications with other biostimulants, and potential pathways for regulatory approval. Overall, this work provides a comprehensive and critically evaluated perspective on ulvan as a key component of sustainable agriculture and crop protection. However, several critical challenges remain, including the lack of standardized ulvan composition, insufficient dose–response studies, and limited field-scale evaluations, all of which are essential for its effective agricultural application. By bridging scientific understanding with practical implementation and market potential, this review offers valuable insights for both researchers and policymakers to advance ulvan-based solutions for global food security and environmental sustainability [17,28,29].

2. Biosynthesis of Ulvan in Ulva

Ulvan is synthesized through a regulated process, in which precursor sugars like rhamnose, xylose, glucose, and glucuronic acid are generated via core metabolic pathways such as the Calvin cycle and glycolysis, although the pathway is not yet fully elucidated (Figure 3). Glycosyltransferases are assumed to convert these precursor sugars into nucleotide sugars (e.g., UDP-glucose, UDP-rhamnose, and UDP-glucuronic acid), which are proposed to be progressively polymerized into ulvan by sequential addition to the growing polysaccharide chain [38,45]. Glucuronic acid is believed to be epimerized to iduronic acid at the polymer level by parietal epimerase action. The resulting ulvan primarily consists of repeating ulvanobiuronic acid disaccharide units (types A and B), with distinct branching and sulfation patterns. Sulfation, likely catalyzed by sulfotransferases in the Golgi apparatus, is considered essential for the bioactivity of ulvan, influencing its antioxidant, anticoagulant, and immunomodulatory properties [46]. Nucleotide sugar precursors are synthesized in the cytoplasm and polymerized in the endomembrane system before being transported to the cell wall. Glucuronic acid may undergo polymer-level epimerization to form iduronic acid, akin to glycosaminoglycan biosynthesis in animals. Branching and sulfation can occur concurrently or sequentially during chain elongation [46,47]. Post-synthetic modifications, such as further sulfation or desulfation, may take place in the apoplast and are suggested to enhance ulvan’s functional diversity, similar to processes seen in other algae and animal systems [48]. Nevertheless, the biosynthesis of ulvan still remains poorly investigated, and a great deal of information in this field is based on the studies of analogous systems and indirect experimental data. The existence of various structural forms of ulvan described in the literature proves that biosynthesis depends not only on the species but also on other factors.

3. Physicochemical Characteristics of Ulvan

Ulvan is characterized by structural sophistication due to glycosidic bonds, a diverse monosaccharide composition, uronidation, and sulfation. Species, season, environment, and extraction method influence its yield and composition. Table 1 shows that the ulvan extracted from various Ulva species can have a large range in the variation in their yield, carbohydrate, protein, sulfate, and ash content. Such variability reflects differences in species, environmental conditions, and extraction methods. For instance, yield values range from as low as 1.1% in Ulva conglobata to over 43% in Ulva lactuca (formally Ulva fasciata). Further, the carbohydrate content varies considerably, ranging from approximately 24.3% in Ulva lactuca to as high as 79.6% in Ulva compressa (formerly Ulva gigantea). Again, the number of other compounds, including protein, sulfate, and ash, can also have a large variation from one Ulva species to another. This significant variation in the content of different Ulva species indicates their structural and material content significance for extracting any of the major quantities for any specific requirements. This variability also makes direct comparison between studies challenging. Ulvan yield is almost always calculated on a dry weight (DW) basis in the literature [49,50,51].

$$\begin{gathered} Ulvan Yield \left(\%\right)={\displaystyle \frac{Weight of extracted ulvan \left(dry\right)}{Weight of dried Ulva biomass}}\times 100 \\ Yield (\%)={\displaystyle \frac{W_2}{W_1}}\times 100 \end{gathered}$$

Table 1. Ulvan yield and composition of different Ulva species.

Ulva Species	Yield (%)	Carbohydrate (%)	Protein (%)	Sulfate (%)	Ash (%)	Reference
Ulva arasakii	-	54.9	-	0.4	-	[51]
Ulva conglobata	1.1	-	3.9	23.2	16.3	[52]
Ulva compressa	9.0	42.0	12	6.7	19.2	[53]
Ulva lacinulata (Formerly Ulva armoricana)	40.0	48.7	7.3	14.4	17.0	[54]
Ulva lactuca (Formerly Ulva rotundata)	8.0	35.9	12.6	11.4	-	[54]
Ulva rigida	10.9	59.5	10.0	19.8	18.1	[55]
Ulva clathrata	6.8	31.3	8.2	10.6	19.0	[56]
Ulva linza	9.8	51.0	1.9	17.7	-	[57]
Ulva lactuca	16.6	37.7	2.9	14.3	15.7	[58]
Ulva compressa (formerly Ulva gigantea)		79.6	5.8	11.9	-	[59]
Ulva ohnoi	6.3	54.7	2.8	14.1	25.9	[60]
Ulva meridionalis	18.7	-	6.8	10.0	12.9	[61]
Ulva prolifera	20.3	56.1	1.9	16.8	-	[62,63]
Ulva intestinalis	12.0	58.7 ± 0.41	10.4 ± 0.55	18.4 ± 0.14	-	[64]
Ulva lactuca (formerly Ulva fasciata)	13.0	48.8	4.2	18.6	14.9	[65]
Ulva reticulata	4.5	35.7	10.1	14.6	19.5	[66,67]
Ulva australis (formerly Ulva pertusa)	26.7	41.9–51.3	2.4–3.3	-	21.2–24.9	[49]
Ulva lactuca (formerly Ulva fasciata)	43.66	31.5	12.66	20.45 ± 1.98	33.57	[68]
Ulva lactuca	36.5	24.27	9.67	-	39.29	[69]

Table 1. Ulvan yield and composition of different Ulva species.

Ulva Species	Yield (%)	Carbohydrate (%)	Protein (%)	Sulfate (%)	Ash (%)	Reference
Ulva arasakii	-	54.9	-	0.4	-	[51]
Ulva conglobata	1.1	-	3.9	23.2	16.3	[52]
Ulva compressa	9.0	42.0	12	6.7	19.2	[53]
Ulva lacinulata (Formerly Ulva armoricana)	40.0	48.7	7.3	14.4	17.0	[54]
Ulva lactuca (Formerly Ulva rotundata)	8.0	35.9	12.6	11.4	-	[54]
Ulva rigida	10.9	59.5	10.0	19.8	18.1	[55]
Ulva clathrata	6.8	31.3	8.2	10.6	19.0	[56]
Ulva linza	9.8	51.0	1.9	17.7	-	[57]
Ulva lactuca	16.6	37.7	2.9	14.3	15.7	[58]
Ulva compressa (formerly Ulva gigantea)		79.6	5.8	11.9	-	[59]
Ulva ohnoi	6.3	54.7	2.8	14.1	25.9	[60]
Ulva meridionalis	18.7	-	6.8	10.0	12.9	[61]
Ulva prolifera	20.3	56.1	1.9	16.8	-	[62,63]
Ulva intestinalis	12.0	58.7 ± 0.41	10.4 ± 0.55	18.4 ± 0.14	-	[64]
Ulva lactuca (formerly Ulva fasciata)	13.0	48.8	4.2	18.6	14.9	[65]
Ulva reticulata	4.5	35.7	10.1	14.6	19.5	[66,67]
Ulva australis (formerly Ulva pertusa)	26.7	41.9–51.3	2.4–3.3	-	21.2–24.9	[49]
Ulva lactuca (formerly Ulva fasciata)	43.66	31.5	12.66	20.45 ± 1.98	33.57	[68]
Ulva lactuca	36.5	24.27	9.67	-	39.29	[69]

Table 2 represents various Ulva species used for ulvan extraction, their source, and chemical compositions. Chemically, ulvan is mainly composed of uronic acids (such as glucuronic acid and iduronic acid), natural sugars (such as rhamnose, xylose, etc.), sulfate groups, and a small number of minor sugars (such as galactose, arabinose, and glucose). Among uronic acids, glucuronic acid is more common and mostly found in several Ulva species. However, iduronic acid is less common but found in some structural variations, such as Ulva lacinulata (formerly Ulva armoricana), Ulva lactuca, Ulva ohnoi, Ulva meridionalis, Ulva rigida, etc.

Conversion to molar amounts: The measured mass (mg) of each sugar is converted into moles using its molecular weight (MW): moles of sugar = mass of sugar (mg)/MW (g/mol).

Calculation of mole percentage: The mole percentage of each sugar is then calculated relative to the total moles of all sugars: mol% of sugar = moles of sugar/total moles of all sugars × 100).

Rhamnose is often found in the form of 3-sulfated rhamnose. In the presence of sulfate groups, the sulfation process mainly takes place on the rhamnose and sometimes on the other sugar residues, which gives rise to the ulvan’s anionic nature. Although the levels of glucuronic, iduronic, xylose, and galactose differ widely, the most prevalent sugar in ulvan is rhamnose, which is about 10 to 68%. This heterogeneity is substantiated by the complexity of the structure and the broad range of physicochemical and biological properties of ulvan [48,49,70].

Table 2. Ulva species used for ulvan extraction, their source, and chemical compositions.

Ulva Species	Source	Rhamnose (mol%)	Glucuronic Acid (mol%)	Iduronic Acid (mol%)	Xylose (mol%)	Galactose (mol%)	Ref.
Ulva arasakii	Blade	33.2	-	-	11.0	-	[51]
Ulva conglobata	Blade	67.8	6.0	-	1.5	2.4	[52]
Ulva lacinulata (Formely Ulva armoricana)	Blade	51.6	21.3	7.0	7.5	3.1	[54]
Ulva clathrata	Filamentous	10.7	4.0	-	-	4.8	[56]
Ulva linza	Blade	61.6	22.6	-	10.5	-	[57]
Ulva compressa (formerly Ulva gigantea)	Blade	42.1	15.4	3.6	28.8	2.3	[59]
Ulva lactuca (formerly Ulva fasciata)	Blade	57.4	14.7	-	26.1	1.9	[71]
Ulva lactuca	Blade	33.5	22.5	6.0	7.3	1.6	[58]
Ulva meridionalis	Filamentous	39.0	-	17	-	6.0	[61]
Ulva ohnoi	Blade	47.1	28.6	8.8	5.5	2.6	[18]
Ulva intestinalis	Filamentous	30.18	-	-	8.53	15.79	[64]
Ulva australis (formerly Ulva pertusa)	Blade	22.4–29.5	-	-	10.0–13.3	0.3–0.5	[49]
Ulva rigida	Blade	49	26	18	6	-	[71]
Ulva australis	Blade	51	18	7	22	-	[71]
Ulva flexuosa	Filamentous	56	21	6	15	-	[71]
Ulva sp.	Blade	47	20	10	19	-	[71]
Ulva compressa	Filamentous	47	24	7	17	-	[71]
Ulva prolifera	Filamentous	60	17	7	15	-	[71]
Ulva ralfsii	Cultivated	38	24	4	16	16	[71]
Ulva ralfsii	Wild	43	26	6	14	10	[71]
Ulva papenfussii	South China Sea	44.9	15.7	23.4	8.5	2.2	[72]

Table 2. Ulva species used for ulvan extraction, their source, and chemical compositions.

Ulva Species	Source	Rhamnose (mol%)	Glucuronic Acid (mol%)	Iduronic Acid (mol%)	Xylose (mol%)	Galactose (mol%)	Ref.
Ulva arasakii	Blade	33.2	-	-	11.0	-	[51]
Ulva conglobata	Blade	67.8	6.0	-	1.5	2.4	[52]
Ulva lacinulata (Formely Ulva armoricana)	Blade	51.6	21.3	7.0	7.5	3.1	[54]
Ulva clathrata	Filamentous	10.7	4.0	-	-	4.8	[56]
Ulva linza	Blade	61.6	22.6	-	10.5	-	[57]
Ulva compressa (formerly Ulva gigantea)	Blade	42.1	15.4	3.6	28.8	2.3	[59]
Ulva lactuca (formerly Ulva fasciata)	Blade	57.4	14.7	-	26.1	1.9	[71]
Ulva lactuca	Blade	33.5	22.5	6.0	7.3	1.6	[58]
Ulva meridionalis	Filamentous	39.0	-	17	-	6.0	[61]
Ulva ohnoi	Blade	47.1	28.6	8.8	5.5	2.6	[18]
Ulva intestinalis	Filamentous	30.18	-	-	8.53	15.79	[64]
Ulva australis (formerly Ulva pertusa)	Blade	22.4–29.5	-	-	10.0–13.3	0.3–0.5	[49]
Ulva rigida	Blade	49	26	18	6	-	[71]
Ulva australis	Blade	51	18	7	22	-	[71]
Ulva flexuosa	Filamentous	56	21	6	15	-	[71]
Ulva sp.	Blade	47	20	10	19	-	[71]
Ulva compressa	Filamentous	47	24	7	17	-	[71]
Ulva prolifera	Filamentous	60	17	7	15	-	[71]
Ulva ralfsii	Cultivated	38	24	4	16	16	[71]
Ulva ralfsii	Wild	43	26	6	14	10	[71]
Ulva papenfussii	South China Sea	44.9	15.7	23.4	8.5	2.2	[72]

The agricultural functionality and significance of ulvan are largely governed by its chemical composition, as summarized in

Table 3. Functional significance of chemical composition in agriculture.

Component	Function	Significance in Agriculture	References
Glucuronic Acid and Iduronic Acid	Anionic nature, metal chelation	Enhances nutrient bioavailability and soil remediation by binding heavy metals.	[18,47,50]
Rhamnose	Structural rigidity, bioactivity	Triggers plant defense pathways and SAR.	[18,38]
Xylose	Structural stability	Influences viscosity and film-forming ability in coatings for seed protection.	[73,74]
Sulfate Groups	Electrostatic interactions, antimicrobial properties	Disrupts pathogen membranes, binds to plant receptors, and activates immune responses.	[18,69]
Minor Sugars (Galactose, Arabinose, Glucose)	Polysaccharide integrity	Alters bioadhesive properties for foliar applications.	[38,47]

. Owing to their high metal-chelating potency and anionic character, glutaric and iduronic acids may enhance nutrient availability and remediate soil by sequestering heavy metals. Rhamnose is considered bioactive as well as structurally rigid, particularly in the initiation of plant defense responses and SAR. Xylose is considered beneficial in seed-coating technologies, as it promotes structural stability and affects viscosity and film-forming capacity. Due to antibacterial and electrostatic properties possessed by sulfate groups, ulvan may contribute to the disruption of pathogen membranes, bind to plant receptors, and stimulate the immune system. In addition, the effectiveness of ulvan in foliar applications is enhanced by minor sugars, such as galactose, arabinose, and glucose, which modify the bioadhesive properties and maintain polysaccharide integrity [7,42]. However, the relationship between ulvan functional properties and composition is not yet fully established.

4. Ulvan Extraction Methods

Ulvan is reported to constitute approximately 9–36% of the DW of green seaweeds belonging to the Ulva genus [18,75]. This polysaccharide has attracted attention due to its broad range of biological activities and potential applications [21,76]. Extraction parameters such as method, duration, and temperature significantly influence the yield, purity, and structural integrity of ulvan, as mentioned further in Table 4. However, direct comparison between reported extraction efficiencies is challenging because these vary considerably across studies. To address this, several ulvan extraction methods, each with distinct advantages and disadvantages as shown in Figure 4, are discussed in the subsequent sections.

4.1. Conventional Extraction Methods

The most common conventional methods used for ulvan extraction from Ulva species include hot water, acid, and alkaline extraction techniques. Hot water extraction (HWE) is generally regarded as a simple and efficient approach in which crushed and dehydrated Ulva biomass is suspended in hot water, typically at temperatures ranging from 75 °C to 100 °C, for 60–420 min [77]. The extraction yield varies depending on the Ulva species. For instance, Ulva lactuca yields approximately 11 ± 3% ulvan when treated at 90 °C for 180 min [60], whereas Ulva australis (formerly Ulva pertusa) produces about 17.8 ± 0.6% [49]. Although HWE does not require harsh chemicals and is considered environmentally friendly, the extraction yield is generally moderate compared to other methods.

Acid extraction, most commonly performed using hydrochloric acid (HCl), is another widely used technique for ulvan isolation. In this method, dried Ulva biomass is treated with a dilute HCl solution. Wahlström et al. [50] reported that extraction of ulvan from Ulva lactuca using 0.01 M HCl at 90 °C for 4 h resulted in a yield of 18 ± 2%. Compared with HWE, acid extraction generally provides higher ulvan yields; however, prolonged exposure to acidic conditions and low pH may induce structural modifications and depolymerization of ulvan. In addition, disposal of acid waste raises environmental concerns [78]. Thus, while acid extraction offers relatively higher yields, it may compromise the structural integrity of ulvan. In contrast, HWE better preserves the native structure of ulvan, although it usually results in moderate yields, as illustrated in Figure 4.

Another conventional approach for ulvan isolation is alkaline extraction using weak bases. This method produced a yield of approximately 4.3 ± 0.5% when applied to Ulva ohnoi at 85 °C for 60 min [79]. Although alkaline extraction is less commonly employed than acid extraction, it may be suitable for certain Ulva species depending on species-specific characteristics and environmental conditions [80,81]. Nevertheless, its comparatively low extraction yield limits its suitability for large-scale ulvan production.

4.2. Advanced Extraction Techniques

Advanced ulvan extraction methods try to increase the yield, preserve structural integrity, and reduce their adverse impacts on the environment compared to conventional methods. Below are the advanced extraction techniques used for the ulvan extraction.

4.2.1. Enzyme-Assisted Extraction (EAE)

Enzyme-assisted extraction (EAE) involves the use of specific enzymes to degrade the cell wall structure of Ulva species, thereby facilitating the release of ulvan into the extraction medium. Commonly used enzymes include cellulases, proteases, and α-amylases [47]. Performed under moderate temperatures ranging from 37 °C to 50 °C, this method generally achieves higher extraction yields compared with conventional extraction techniques. For example, Ulva australis (formerly Ulva pertusa) yielded 25.3 ± 1.3% ulvan following extraction at 50 °C for 180 min, whereas Ulva lacinulata (formerly Ulva armoricana) produced 35.3 ± 0.3% after enzymatic treatment at 50 °C for 195 min [49,54,82]. EAE enhances ulvan recovery by selectively solubilizing cell wall components while preserving the structural integrity of the polysaccharide. Furthermore, the efficiency of the process largely depends on the selection of appropriate enzymes and the optimization of reaction conditions, including temperature, pH, enzyme concentration, and extraction time.

4.2.2. Physical-Assisted Extraction

Ultrasound-assisted extraction (UAE) exploits sonic cavitation to rupture cell membranes and enhance mass transfer. Ulva australis (formerly Ulva pertusa) is treated for 180 min at 90 °C in UAE to enhance the yield to 20.6 ± 1.2% [49]. Under normal conditions, it takes approximately 40 min, with an amplitude of 80% and a frequency of 20 kHz. Using UAE, either separately or in combination with other methods of extraction, can further enhance its effectiveness [83,84]. Microwave-assisted extraction (MAE) can significantly reduce extraction time while enhancing or even doubling production and is considered an efficient and rapid means of isolating ulvan [85].

Microwave-assisted extraction (MAE) is considered a rapid and efficient technique for ulvan isolation, as it significantly reduces extraction time while enhancing, and in some cases doubling, the extraction yield [85]. For example, Ulva prolifera achieved a maximum ulvan yield of 36.38 ± 0.94% when extracted at 120 °C for approximately 15 min. Similarly, microwave treatment of Ulva ohnoi and Ulva meridionalis at 85 °C for 60 min resulted in ulvan yields of 36.5 ± 3.1% and 40.4 ± 3.2%, respectively [86,87]. The rapid and efficient cell disruption induced by microwave heating greatly improves the ulvan extraction process.

In recent years, deep eutectic solvents (DES) have emerged as environmentally friendly, safe, and biodegradable alternatives to conventional solvents for ulvan extraction [88,89]. DES are typically prepared by mixing hydrogen bond donors, such as glycerol, urea, or ethylene glycol, with hydrogen bond acceptors, such as choline chloride, commonly in a molar ratio of 1:2. DES extraction is generally conducted at a material-to-solvent ratio of 1:20 (w/v) for approximately 1 h at temperatures between 85 °C and 95 °C [90,91]. The extraction efficiency and physicochemical properties of ulvan largely depend on the composition of the DES system. For example, a 30% choline chloride–glycerol solution enhances sulfate content and viscosity, whereas a 30% choline chloride–urea DES provides maximum extraction yields. Additionally, pretreatment with 2% peracetic acid prior to DES extraction increased the ulvan yield to 37.41% [92]. Despite these technological advancements, challenges still remain in optimizing extraction conditions and maintaining consistent ulvan quality across different biomass sources.

Table 4. Extraction methods of different Ulva species.

Ulva Species	Extraction Method	Duration	Temp. (°C)	Yield (%)	Ref.
Ulva rigida	HWE	1 h	100	7.0	[93]
Ulva rigida	Pressurized hot water	30 min	130	24.2	[55]
Ulva lactuca	Enzymatic and chemical extraction	5 h	50	17.1	[94]
Ulva lacinulata (formerly Ulva armoricana)	EAE	3.25 h	50	35.3	[95]
Ulva ohnoi	MAE	1 h	85	36.5 ± 3.1	[86]
Ulva meridionalis	MAE	1 h	85	40.4	[86]
Ulva ohnoi	Acid extraction	1 h	85	8.1	[60]
Ulva ohnoi	Weak alkali extraction	1 h	85	4.3 ± 0.5	[60]
Ulva prolifera	MAE	15 min	120	36.4	[87]
Ulva lactuca	Maceration	24 h	4	42.5 ± 4.4	[96]
Ulva lactuca	EAE	3 h	50	58.4 ± 7.3	[96]
Ulva lactuca	Acid extraction	4 h	90	18	[50]
Ulva clathrata	HWE	2 h	100	11.2	[97]
Ulva lactuca	HWE	1–3 h	80–90	High (not specified)	[14]
Ulva lactuca	HWE	3 h	90	11 ± 3	[50]
Ulva Lactuca (Formerly Ulva fasciata)	Pressurized hot water	1 h	110	19.6	[13]
Ulva australis (formerly Ulva pertusa)	EAE	3 h	50	25.3	[49]
Ulva intestinalis	HWE	1.5 h	100	15.2	[98]
Ulva australis (formerly Ulva pertusa)	Ultrasound and EAE	3 h	50	26.7 ± 0.9	[49]
Ulva australis (formerly Ulva pertusa)	UAE	3 h	90	20.6	[49]
Ulva australis (formerly Ulva pertusa)	HWE	3 h	90	17.8	[49]
Ulva ohnoi	Acid extraction	3 h	37	3.5	[99]
Ulva tepida	Acid extraction	3 h	37	3.9	[99]
Ulva prolifera	Acid extraction	3 h	37	6.7	[99]
Ulva rigida	Aqueous extraction	30 min	40	Not specified	[100]
Ulva fenestrata	EAE (Cellulysin)	20 h	50 (estimated)	14.1	[88]
Ulva fenestrata	Combined enzymatic and ultrasound	17 h + 40 min	50 (estimated)	17.9	[88]
Ulva lactuca	EAE	2 h	90	Not specified	[88]
Ulva sp.	Enzymatic extraction (Viscozyme L)	20 h	50	~9	[88]
Ulva sp.	UAE	40 min	Not specified	~12	[88]
Ulva lactuca	HCl 0.01 M extraction	4 h	90	18	[23]

Table 4. Extraction methods of different Ulva species.

Ulva Species	Extraction Method	Duration	Temp. (°C)	Yield (%)	Ref.
Ulva rigida	HWE	1 h	100	7.0	[93]
Ulva rigida	Pressurized hot water	30 min	130	24.2	[55]
Ulva lactuca	Enzymatic and chemical extraction	5 h	50	17.1	[94]
Ulva lacinulata (formerly Ulva armoricana)	EAE	3.25 h	50	35.3	[95]
Ulva ohnoi	MAE	1 h	85	36.5 ± 3.1	[86]
Ulva meridionalis	MAE	1 h	85	40.4	[86]
Ulva ohnoi	Acid extraction	1 h	85	8.1	[60]
Ulva ohnoi	Weak alkali extraction	1 h	85	4.3 ± 0.5	[60]
Ulva prolifera	MAE	15 min	120	36.4	[87]
Ulva lactuca	Maceration	24 h	4	42.5 ± 4.4	[96]
Ulva lactuca	EAE	3 h	50	58.4 ± 7.3	[96]
Ulva lactuca	Acid extraction	4 h	90	18	[50]
Ulva clathrata	HWE	2 h	100	11.2	[97]
Ulva lactuca	HWE	1–3 h	80–90	High (not specified)	[14]
Ulva lactuca	HWE	3 h	90	11 ± 3	[50]
Ulva Lactuca (Formerly Ulva fasciata)	Pressurized hot water	1 h	110	19.6	[13]
Ulva australis (formerly Ulva pertusa)	EAE	3 h	50	25.3	[49]
Ulva intestinalis	HWE	1.5 h	100	15.2	[98]
Ulva australis (formerly Ulva pertusa)	Ultrasound and EAE	3 h	50	26.7 ± 0.9	[49]
Ulva australis (formerly Ulva pertusa)	UAE	3 h	90	20.6	[49]
Ulva australis (formerly Ulva pertusa)	HWE	3 h	90	17.8	[49]
Ulva ohnoi	Acid extraction	3 h	37	3.5	[99]
Ulva tepida	Acid extraction	3 h	37	3.9	[99]
Ulva prolifera	Acid extraction	3 h	37	6.7	[99]
Ulva rigida	Aqueous extraction	30 min	40	Not specified	[100]
Ulva fenestrata	EAE (Cellulysin)	20 h	50 (estimated)	14.1	[88]
Ulva fenestrata	Combined enzymatic and ultrasound	17 h + 40 min	50 (estimated)	17.9	[88]
Ulva lactuca	EAE	2 h	90	Not specified	[88]
Ulva sp.	Enzymatic extraction (Viscozyme L)	20 h	50	~9	[88]
Ulva sp.	UAE	40 min	Not specified	~12	[88]
Ulva lactuca	HCl 0.01 M extraction	4 h	90	18	[23]

5. Purification and Characterization

The purity, structural integrity, and physicochemical properties of ulvan are critically influenced by the purification methodology employed, which typically involves a multi-step process. To achieve maximum yield with minimal impurities, several purification techniques, including membrane separation, chromatography, precipitation, desalting, and isolation, are utilized to obtain high-quality ulvan suitable for structural and functional studies [60,68,101]. During the initial stages of extraction, centrifugation or filtration is commonly performed to separate the polysaccharide-rich extract from the residual biomass. Even though it facilitates ulvan recovery, ethanol precipitation, employed at 70–96%, often may bring in salt impurities. Therefore, lyophilization is essential to remove residual solvents and maximize yield accuracy [68]. The aqueous extract and ethanol are mixed in a ratio of 2:1, often with sodium acetate buffer, and centrifuged at 3300× g. Recovery can be enhanced using alternatives like longer cooling and isopropyl alcohol. Centrifugation parameters usually range between 3300 and 10,000× g for 5 to 15 min [50,66]. Ulvan purity is typically improved through desalting. Semi-permeable membranes with MW cut-offs (MWCO) are employed in dialysis for the effective removal of salts. One widely used and eco-friendly method is ultrafiltration, which integrates diafiltration and concentration phases and typically employs 10 kDa MWCO membranes to maintain high-molecular-weight fractions [60]. Low-molecular-weight molecules are also eliminated via membrane-based ultrafiltration, which concentrates ulvan. Membrane fouling may decrease efficiency, although 15 kDa Kerasep membranes run at 80 °C and 5 bar pressure, for example, can concentrate the retentate to about 4°Bx [102,103]. Purity can be further improved using Ion Exchange Chromatography (IEC), which eliminates charged impurities. Following demineralization, centrifugation, pH neutralization, and lyophilization, a two-column system employing Amberlite FPA 98 (OH^- form) and IR 120 Na (H⁺ form) resins promotes demineralization. Protein content can be decreased by IEC very well [41,104]. Purification is further enhanced by sophisticated chromatographic procedures. Size-exclusion chromatography (SEC) uses resins like Sephacryl S-400, Sephadex G-200, and Sepharose CL-6B to separate fractions by MW, whereas anion-exchange chromatography (AEC) uses DEAE or quaternary amine-functionalized media to remove proteins and neutral polysaccharides [105]. Although ethanol precipitation serves as the primary purification step, the production of high-purity ulvan suitable for downstream applications generally requires a combination of dialysis, ultrafiltration, and chromatographic techniques. Furthermore, trade-offs among yield, purity, and structural preservation are often encountered, necessitating careful optimization of the purification process based on the intended application.

Comprehensive characterization is necessary because ulvan has structural complexity with a range of monosaccharides, glycosidic linkages, and sulfation sites. Thermal stability is determined using thermogravimetric analysis (TGA), where evaporation of moisture is reported by weight loss below 160 °C [106,107]. Fourier transform infrared spectroscopy (FTIR), elemental analysis, HPLC, turbidimetric tests, and ion chromatography are widely employed techniques for the assessment of monosaccharide composition. FTIR typically reveals important characteristic peaks at 3412, 1645, 1251, 1080, and 852 cm⁻¹, indicative of glycosidic, uronic acid, hydroxyl, and sulfate functional groups [108]. In the absence of derivatization, HPAEC-PAD facilitates precise measurement of acidic, amino, and neutral sugars. GC, however, requires derivatization of sugars to alditol acetates or TMS ethers [109,110,111]. Glycosidic linkages are determined after uronic acid reduction, methylation, and conversion to partly methylated alditol acetates using GC–MS [94]. At ulvan signals, typically 65, 63, 76, and 73 ppm, two-dimensional NMR (¹H and ¹³C) offers detailed structural information [79]. SEC is commonly used to analyze MW and its distribution, often with UV or refractive index detectors. Polymer aggregation is measured using advanced techniques such as viscosity detectors and Multi-Angle Laser Light Scattering (MALLS). While ulvan heterogeneity can require direct detection methods, calibration most often employs standards such as dextrans or pullulans [54,112]. Crystallinity is analyzed using X-ray diffraction (XRD), wherein ulvan shows peaks at 14.6°, 16.5°, 22.9°, and 34.7°, reflecting a cellulose-like structure [113,114]. It is important to note that variations in analytical techniques and sample preparation may lead to discrepancies in reported structural parameters. In addition, rapid quantification methods are utilized. Sulfate esters are quantified using turbidimetric methods, while protein and carbohydrate content are assessed by means of colorimetric tests such as the Bradford method [115,116]. Enzymatic assays enable the specific identification of certain monosaccharides such as uronic acid, xylose, and rhamnose, although procedures such as the sulfuric acid or m-hydroxydiphenyl assays are not sufficiently specific for neutral sugars in every case. The integration of some complementary methods is ultimately required for a complete investigation into ulvan composition and structure. Irrespective of advanced analytical tools, establishing clear correlations between ulvan functional properties and structure remains challenging due to its inherent heterogeneity.

6. Mechanistic Insights: Ulvan-Induced Phytopathogen Inhibition

The SP ulvan extracted from Ulva spp. has been reported to act against phytopathogens at multiple biological levels, starting with biochemical and proceeding to molecular and physiological pathways. It is proposed to function through three main mechanisms in plant defenses: immune activation, antimicrobial activity, and pathogen adherence prevention. The underlying biochemical pathways primarily involve the following mechanisms, as explained in the subsections below.

6.1. Induction of Plant Defense Signaling Pathways

SP ulvan has been reported to induce multiple defense signaling pathways and is considered a potential elicitor of plant immunity. They are Pattern-Triggered Immunity (PTI), Effector-Triggered Immunity (ETI), SAR, and ISR. These pathways together regulate complex biochemical and molecular processes involving the mitogen-activated protein kinase (MAPK) cascades and plant hormones such as ET, SA, and JA. Several phytopathogens are resisted through increased resistance due to the induction of defense genes and transcription factors following these events. Pattern recognition receptors (PRRs) on plant cell plasma membranes are suggested to recognize ulvan or ulvan-derived fragments as non-self signals, initiating the first phase of plant immunity called PTI. These PRRs, such as LysM receptors CERK1 and wall-associated kinases (WAKs), recognize ulvan molecules as microbe-associated molecular patterns (MAMPs). To initiate MAPK signaling cascades (MPK3, MPK4, and MPK6), this recognition is supposed to trigger receptor-like cytoplasmic kinases (RLCKs) like BIK1 and PBL1. Superoxide radicals (O₂^•−) produced by the NADPH oxidases RBOH D and RBOH F dismutate to H₂O₂, which serves as a secondary messenger in defense signaling. Through stomatal closure, these oxidative signals exclude pathogens. Calcium ion flow through plasma membrane channels increases these reactions. MAPKs phosphorylate WRKY and MYB transcription factors, inducing the expression of PR proteins (e.g., chitinases, β-1,3-glucanases), callose synthase (CalS1) for callose deposition, and enzymes such as phenylalanine ammonia-lyase (PAL) for secondary metabolite biosynthesis. ETI, the second phase of immunity, can be elicited by inhibiting PTI, even though it provides basal broad-spectrum protection [18,117]. ETI is a stronger, long-lasting, and faster defense response, activated when pathogen effectors (secreted proteins) are detected by intracellular nucleotide-binding leucine-rich repeat (NLR) receptors. Effector-Triggered Intracellular NLR proteins detect pathogen effectors and initiate immunity. Ulvan may precondition plant cells for an accelerated and potent ETI response upon infection. Certain NLRs, like RPS2 and RPM1, directly interact with pathogen effectors, while others form multiprotein complexes to enhance signal amplification. This induces a second ROS burst and the triggering of the hypersensitive response (HR), which has localized programmed cell death that restricts pathogen transmission. ETI is associated with increased SA biosynthesis and signaling, leading to enhanced accumulation of NPR1 (Nonexpressor of PR genes 1), a key moderator of SA-mediated defense. NPR1 induces PR genes such as PR1 and PR5 and plays a role in structural protection through callose deposition and lignification. In addition, ETI induces the generation of systemic signals like azelaic acid (AzA) and methyl salicylate (MeSA), which move to far-away organs and induce SAR [18,48,118].

SAR is a prolonged acquired immunity that offers protection against a broad assortment of biotrophic diseases, beginning with HR or ETI responses. Ulvan enhances SAR via the isochorismate synthase 1 (ICS1) pathway of the chloroplast, which is responsible for SA production, as shown in Figure 5. Once converted to MeSA, the volatile signal that is transported to other tissues, SABP2 converts it into SA. Following redox-mediated monomerization, cytoplasmic NPR1 translocates into the nucleus and employs TGA transcription factors to induce the expression of defense genes like PR1, PR2, and PR5. Long-term immunity is also conferred to developing tissue by SAR, which also fortifies cell walls by the accumulation of lignin and callose and promotes the production of antimicrobial secondary metabolites, including phytoalexins and flavonoids [11,17,118]. In addition to SAR, ulvan also induces ISR, a defense process that relies on JA and ET signaling but is independent of SA. Plant growth-promoting rhizobacteria (PGPR) are typically associated with ISR; however, ulvan is suggested to mimic aspects of these microbial signals and induce the same defense processes. The α-linolenic acid-lipoxygenase (LOX) pathway within chloroplasts is how JA is biosynthesized. When JA binds to the COI1 (Coronatine-Insensitive 1) receptor, JAZ (Jasmonate ZIM-domain proteins) repressors are degraded, and MYC2 transcription factors, which regulate genes associated with ISR, are induced. EIN3/EIL1 transcription factors act on ET signaling simultaneously, and they coordinate with JA responses. ISR enhances the production of defense chemicals like proteinase inhibitors, phenolic acids, terpenoids, alkaloids, and phytoalexins. These compounds enhance resistance to bacterial pathogens, necrotrophic fungi, and some insect pests by disrupting pathogen metabolism [119]. Being a pleiotropic bio-elicitor, ulvan is considered capable of eliciting different immunological reactions in plants, such as PTI, ETI, SAR, and ISR. Ulvan enhances transcriptional reprogramming of defense genes and the synthesis of protection chemicals by regulating ROS, MAPKs, and hormone-mediated signaling pathways. This may contribute to enhancing plant immunity against diverse phytopathogens [120]. Nonetheless, it is critical to highlight that most of these signal transductions are deduced from a few experimental models or analogies with other polysaccharides, which might differ based on plant type, ulvan structure, and environmental factors.

6.2. Direct Antimicrobial Effects

Ulvan has been reported to exhibit antimicrobial activity against bacteria, fungi, and viruses, which acts to inhibit microbial virulence mechanisms and generate oxidative stress. It may induce ROS accumulation, mitochondrial dysfunction, ATP depletion, and cell death through inhibition of crucial antioxidant defenses such as GPX, CAT, and SOD and activation of excess ROS production by NADPH oxidases [101,103]. In addition, ulvan represses toxin-encoding genes, hydrolytic enzymes, and effector proteins, including bacterial Type III Secretion System (T3SS), and blocks enzymes that degrade the cell wall, for example, cutinases, cellulases, and pectinases [121]. The bacterial proteases, β-1,3-glucanases, and fungal chitinases required for pathogenicity and quorum sensing (QS) may also be affected. Ulvan reduces the infectiousness and virulence of pathogens by suppressing virulence factors and inducing oxidative damage simultaneously. In combination with plant immunity responses such as PTI, ETI, SAR, and ISR, these direct antimicrobial activities provide broad-spectrum protection against phytopathogens [71,122]. Nevertheless, it is worth pointing out that the antimicrobial activities of ulvan via direct mechanisms are yet to be well explained and may depend on various factors such as composition, molecular weight, and other aspects of the study.

6.3. Competitive Inhibition of Pathogen Adhesion

Ulvan has been proposed to interfere with microbial colonization through multiple mechanisms, including the primary ones being nutrient deprivation from microorganisms and inhibition of pathogen adhesion to plant surfaces. The mechanisms are molecular mimicry, steric blocking, electrostatic repulsion, inhibition of biofilms, metal ion chelation, and microbial adhesion molecule suppression. When merged, these traits inhibit infection and disrupt physiological processes crucial to the viability and virulence of microorganisms. Ulvan has structural homologies with key constituents of plant cell walls, such as different glycoproteins, rhamnogalacturonan I (RG-I), and homogalacturonan (HG). Its molecular composition, which comprises rhamnose, xylose, glucuronic acid, and sulfate groups, may enable it to mimic certain features of host cell wall polysaccharides [104,118]. By competitively binding to adhesion proteins of microbes, like lectins, fimbriae, pili, and flagella, on which pathogens base their identification and attachment to host surfaces, ulvan may act as a decoy molecule due to its structural resemblance. Gram-negative bacteria, including Pseudomonas species and Escherichia coli, adhere to host cell mannose and galactose groups through lectins (e.g., FimH, LecA, LecB) and Type I fimbriae. Microbial adherence efficiency is reduced by ulvan, which occupies such binding sites to reduce the initial phases of colonization [123].

Aside from molecular mimicry, ulvan’s high MW and negatively charged sulfate and carboxyl groups may form a wet, gelatinous barrier when deposited on plant surfaces [124]. The barrier confers electrostatic repulsion against charged microbial cell walls, such as bacterial lipopolysaccharides and fungal chitin, and steric hindrance to physically block microbial adhesion. The surface becomes increasingly hydrophilic, which inhibits the capacity of pathogens to adhere and initiate an infection, particularly those that rely on hydrophobic interactions, e.g., bacterial flagella and fungal spores. This, in effect, inhibits the germination, penetration, and early-stage biofilm development of pathogens [125]. Ulvan also interferes with microbial colonization through the prevention of biofilm formation, which is critical for disease persistence and resistance. Surface attachment at first, secretion of extracellular polymeric substance (EPS), and the development of structured microcolonies are all necessary events in biofilm formation [126]. Through EPS inhibition, interfering with the key biofilm-forming components such as Pel, Psl, and Alg polysaccharides in Pseudomonas aeruginosa and disrupting QS signaling systems (such as AHLs, AI-2, and DSF), ulvan inhibits these processes. Further, by inducing enzymes that degrade the biofilm matrix components, such as dispersin B, DNase I, and other proteases, ulvan induces the degradation of mature biofilms. On the surface of plants, this reduces the likelihood of chronic infection, microbial persistence, and the integrity of biofilms [124,127]. Nevertheless, the relative mechanism contributions are not consistently reported and may depend on ulvan structural variability and environmental conditions.

6.4. Nutrient Deprivation

Several essential nutrients, such as iron (Fe), calcium (Ca²⁺), magnesium (Mg²⁺), and trace metals (Zn²⁺, Mn²⁺, Cu²⁺, and Mo), are required for microbial pathogen proliferation, pathogenicity, and biofilm formation. By chelating these ions and reducing their bioavailability, ulvan may act as a chelator and inhibit microbial metabolism, QS, and regulation of oxidative stress. By its carboxyl, sulfate, and hydroxyl groups, ulvan may sequester iron, essential for respiratory and DNA replication enzymes, as well as for the control of oxidative stress. This may allow competition with microbial siderophores and deter Fe-dependent enzymes such as cytochromes, catalases, SOD, and ribonucleotide reductase [123,128]. The chelation of Ca²⁺, necessary for the synthesis of fungal chitin, structural stability of Gram-negative LPS, biofilm formation, and Ca²⁺-dependent pathogenicity enzymes, may also impact signaling as well as structural stability [129]. Microbial susceptibility is further enhanced by Mg²⁺ deficiency, which disrupts ribosome stability, nucleic acid metabolism, membrane charge balance, and stress response pathways (PhoPQ, PmrAB) [130]. Also, Zn²⁺, required for transcription factors, polymerases, and metalloproteases; Mn²⁺, required for Mn-SOD function and oxidative stress resistance; and Cu²⁺, crucial in cytochrome oxidase in fungal respiration, are bound by ulvan [131]. Ulvan enhances plant defense through the reduction in the virulence, stress tolerance, and structural coherence of pathogens by starving them of nutrients [132].

6.5. Effect of Ulvan on Phytopathogens

Ulvan has been reported to exhibit antimicrobial activity against diverse phytopathogens, including fungi, bacteria, and viruses. Fungal pathogens cause severe crop diseases by invading plant tissues through spore germination, appressorium formation, hyphal penetration, and toxin production. The fungal cell wall is primarily composed of chitin, β-glucans, and mannoproteins. Ulvan’s sulfate and carboxyl groups may interact with fungal cell wall polysaccharides, potentially leading to structural instability, reducing cell wall rigidity, and inhibition of chitin synthase enzymes, preventing proper wall formation [121]. Electrostatic interactions between ulvan’s anionic groups and fungal membrane phospholipids disrupt membrane permeability, causing ion leakage, osmotic imbalance, and cell death. Ulvan suppresses fungal spore germination by interfering with signaling pathways involved in dormancy breaking. It has been suggested to interfere with MAP kinase pathways (Hog1, Pmk1) in fungi like Magnaporthe oryzae, blocking the transition from spore to hyphal growth. ROS generation associated with ulvan treatment may induce oxidative stress in fungi, further limiting their growth [133]. Examples of fungal pathogens affected by ulvan are Botrytis cinerea (gray mold), Fusarium oxysporum (vascular wilt), Alternaria solani (early blight), and Rhizoctonia solani (root rot).

Bacterial pathogens cause plant diseases by penetrating through wounds, stomata, or hydathodes, often forming biofilms for protection. Gram-negative bacteria, including Pseudomonas syringae and Xanthomonas campestris, may experience disruption of outer membrane integrity by ulvan’s negatively charged sulfate and carboxyl groups. It has been reported to suppress biofilm production and virulence gene expression by interacting with bacterial QS molecules, such as acyl-homoserine lactones [18]. Outer membrane protein instability causes bacterial cell death, loss of cytoplasmic contents, and increased permeability. Ulvan may interfere with two-component regulatory systems such as PhoPQ in Xanthomonas, which bacteria use to perceive and respond to plant environments. Inhibiting bacterial Type III Secretion Systems (T3SSs) prevents effector proteins from being delivered into plant cells. Inhibiting the growth and pathogenicity of bacteria by depriving them of Fe^2+, required for enzymes and respiration, ulvan’s iron chelation is used [133]. Examples of bacterial pathogens affected by ulvan are Pseudomonas syringae (bacterial speck), Xanthomonas campestris (black rot), Erwinia amylovora (fire blight), and Ralstonia solanacearum (bacterial wilt).

Plant viruses cause devastating diseases by hijacking host cells for replication. Ulvan’s SP structure may mimic host cell surface receptors, competitively binding viral particles and preventing their attachment. It inhibits viral glycoproteins involved in cell entry, such as the coat protein of Tobacco mosaic virus (TMV). Ulvan has been reported to activate RNA interference (RNAi)-related responses, leading to dicer-like proteins (DCLs) processing viral RNA into small interfering RNAs (siRNAs) and Argonaute (AGO) proteins degrading viral RNA, suppressing replication. It may also trigger SA-mediated antiviral responses, increasing expression of PR-1 and PR-5 proteins that inhibit viral assembly. Ulvan induces SAR by increasing MeJA and ET levels. It enhances the synthesis of phenolic compounds and flavonoids, which interfere with viral replication [118]. Examples of viruses affected by ulvan are TMV, Tomato yellow leaf curl virus (TYLCV), Cucumber mosaic virus (CMV), and Potato virus Y (PVY). However, the extent and nature of these antimicrobial effects are not uniform and may be influenced by ulvan concentration, composition, and pathogen system.

Table 5 indicates the activity of ulvan isolated from different Ulva species against the defense strategies of various crop plants against fungal, bacterial, and viral diseases. Some investigations have demonstrated ulvan as a promising plant defense elicitor. For instance, foliar spray of Ulva sp. extract on cabbage (Brassica oleracea) has been effectively reported to minimize black rot disease by Xanthomonas campestris pv. campestris, which showed 65% inhibition of bacterial multiplication as validated by qPCR analysis. In a similar vein, Ulva rigida used on rice (Oryza sativa) via seed treatment and foliar spray suppressed rice blast (Magnaporthe oryzae) by about 60%, whereas Ulva linza suppressed bacterial leaf blight (Xanthomonas oryzae) in rice to the extent of 70% [18]. In soybean (Glycine max), foliar spraying with Ulva fasciata suppressed soybean rust (Phakopsora pachyrhizi) urediniospore germination by 70%, and Ulva intestinalis suppressed barley stripe rust (Puccinia striiformis) in barley (Hordeum vulgare) by 60%. Some of the other prominent examples are Ulva lactuca, which inhibited cotton wilt (Fusarium oxysporum) by inhibiting root colonization, and Ulva fasciata, which also had antiviral activity by decreasing TMV in tobacco and TYLCV in tomato plants. Also, ulvan extracts were successful against bacterial diseases like bacterial speck of tomato (Pseudomonas syringae), angular leaf spot of cucumber, and bacterial spot of pepper, predominantly by foliar spraying and seed priming, and achieved noteworthy declines in bacterial growth, biofilm formation, and disease severity. Fungal diseases like powdery mildew of grapevine, early blight of tomato, and downy mildew of lettuce were successfully inhibited by ulvan treatment. In addition, ulvan exhibited activity against severe viral complexes like maize lethal necrosis, induced by MCMV and SCMV, with significant disease reduction [133]. While these results reveal promising efficacy, differences in environmental conditions and experimental methods limit direct comparison across studies.

Table 5. Impact of ulvan on defense mechanisms of various plant species.

Ulvan Species	Crop Plants	Disease	(Fungi, Bact., Virus)	Application	Conc.	Assay	Response to Disease	Ref.
Ulva sp.	Cabbage (Brassica oleracea)	Black Rot	Xanthomonas campestris pv. campestris (Bacteria)	Foliar spray	2% (w/v)	qPCR detection of bacterial load	65% reduction in bacterial spread	[134]
Ulva rigida	Rice (Oryza sativa)	Rice Blast	Magnaporthe oryzae (Fungi)	Seed treatment and Foliar spray	1.5% (w/v)	Spore germination inhibition assay	60% reduction in fungal colonization	[27]
Ulva linza	Rice (Oryza sativa)	Bacterial Leaf Blight	Xanthomonas oryzae (Bacteria)	Foliar spray	2.5% (w/v)	qPCR detection	70% suppression of bacterial spread	[135]
Ulva lactuca (formerly Ulva fasciata)	Soybean (Glycine max)	Soybean Rust	Phakopsora pachyrhizi (Fungi)	Foliar spray	2% (w/v)	Spore germination assay	70% inhibition urediniospore germination	[136]
Ulva intestinalis	Barley (Hordeum vulgare)	Barley Stripe Rust	Puccinia striiformis (Fungi)	Foliar spray	1% (w/v)	Rust pustule count	60% decrease in pustule formation	[137]
Ulva lactuca	Cotton (Gossypium hirsutum)	Cotton Wilt	Fusarium oxysporum (Fungi)	Root dip	2% (w/v)	Wilting index	65% reduction in root colonization	[138]
Ulva fasciata	Tobacco (Nicotiana tabacum)	TMV	Virus	Foliar spray	2% (w/v)	ELISA viral titer assay	60% reduction in viral load	[11]
Ulva fasciata	Tomato (Solanum lycopersicum)	Bacterial Speck	Pseudomonas syringae (Bacteria)	Foliar spray	1% (w/v)	CFU count reduction assay	70% reduction in bacterial growth	[48]
Ulva lacinulata (formerly Ulva armoricana)	Cucumber (Cucumis sativus)	Angular Leaf Spot	Xanthomonas campestris (Bacteria)	Foliar spray	1.5% (w/v)	Biofilm inhibition assay	Disruption of bacterial biofilm formation	[11]
Ulva compressa (formerly Ulva compress)	Pepper (Capsicum annuum)	Bacterial Spot	Xanthomonas vesicatoria (Bacteria)	Seed priming and Foliar spray	2% (w/v)	Disease severity index	Reduction in necrotic lesions	[139]
Ulva prolifera	Potato (Solanum tuberosum)	Late Blight	Xanthomonas vesicatoria (Bacteria)	Foliar spray	2% (w/v)	Leaf disc assay	55% inhibition of sporangial germination	[140]
Ulva australis (formerly Ulva pertusa)	Tomato (Solanum lycopersicum)	TYLCV	Virus	Soil drench and Foliar spray	1.5% (w/v)	RT-PCR viral load assay	40% suppression of viral replication	[141]
Ulva rigida	Grape (Vitis vinifera)	Powdery Mildew	Botrytis cinerea (Fungi)	Foliar spray	1% (w/v)	Conidial germination assay	65% reduction in fungal growth	[142]
Ulva lactuca	Tomato (Solanum lycopersicum)	Early Blight	Alternaria solani (Fungi)	Foliar spray	2% (w/v)	Disease severity index	Reduced lesion size and fungal sporulation	[23]
Ulva reticulata	Citrus (Citrus spp.)	Citrus Canker	Xanthomonas citri (Bacteria)	Foliar spray	2% (w/v)	Symptom severity scale	50% reduction in lesion formation	[143]
Ulva intestinalis	Wheat (Triticum aestivum)	Fusarium Head Blight	Fusarium graminearum (Fungi)	Soil drench	2.5% (w/v)	PCR-based pathogen quantification	50% disease reduction	[144]
Ulva fasciata	Maize (Zea mays)	Maize Lethal Necrosis	MCMV and SCMV	Soil drench	1.5% (w/v)	Virus suppression index	55% reduction in disease severity	[145]
Ulva fasciata	Lettuce (Lactuca sativa)	Downy Mildew	Bremia lactucae (Oomycete)	Foliar spray	2.5% (w/v)	Disease severity scale	50% reduction in lesion expansion	[146]

Note: MCMV = Maize Chlorotic Mottle Virus, SCMV = Sugarcane Mosaic Virus.

Table 5. Impact of ulvan on defense mechanisms of various plant species.

Ulvan Species	Crop Plants	Disease	(Fungi, Bact., Virus)	Application	Conc.	Assay	Response to Disease	Ref.
Ulva sp.	Cabbage (Brassica oleracea)	Black Rot	Xanthomonas campestris pv. campestris (Bacteria)	Foliar spray	2% (w/v)	qPCR detection of bacterial load	65% reduction in bacterial spread	[134]
Ulva rigida	Rice (Oryza sativa)	Rice Blast	Magnaporthe oryzae (Fungi)	Seed treatment and Foliar spray	1.5% (w/v)	Spore germination inhibition assay	60% reduction in fungal colonization	[27]
Ulva linza	Rice (Oryza sativa)	Bacterial Leaf Blight	Xanthomonas oryzae (Bacteria)	Foliar spray	2.5% (w/v)	qPCR detection	70% suppression of bacterial spread	[135]
Ulva lactuca (formerly Ulva fasciata)	Soybean (Glycine max)	Soybean Rust	Phakopsora pachyrhizi (Fungi)	Foliar spray	2% (w/v)	Spore germination assay	70% inhibition urediniospore germination	[136]
Ulva intestinalis	Barley (Hordeum vulgare)	Barley Stripe Rust	Puccinia striiformis (Fungi)	Foliar spray	1% (w/v)	Rust pustule count	60% decrease in pustule formation	[137]
Ulva lactuca	Cotton (Gossypium hirsutum)	Cotton Wilt	Fusarium oxysporum (Fungi)	Root dip	2% (w/v)	Wilting index	65% reduction in root colonization	[138]
Ulva fasciata	Tobacco (Nicotiana tabacum)	TMV	Virus	Foliar spray	2% (w/v)	ELISA viral titer assay	60% reduction in viral load	[11]
Ulva fasciata	Tomato (Solanum lycopersicum)	Bacterial Speck	Pseudomonas syringae (Bacteria)	Foliar spray	1% (w/v)	CFU count reduction assay	70% reduction in bacterial growth	[48]
Ulva lacinulata (formerly Ulva armoricana)	Cucumber (Cucumis sativus)	Angular Leaf Spot	Xanthomonas campestris (Bacteria)	Foliar spray	1.5% (w/v)	Biofilm inhibition assay	Disruption of bacterial biofilm formation	[11]
Ulva compressa (formerly Ulva compress)	Pepper (Capsicum annuum)	Bacterial Spot	Xanthomonas vesicatoria (Bacteria)	Seed priming and Foliar spray	2% (w/v)	Disease severity index	Reduction in necrotic lesions	[139]
Ulva prolifera	Potato (Solanum tuberosum)	Late Blight	Xanthomonas vesicatoria (Bacteria)	Foliar spray	2% (w/v)	Leaf disc assay	55% inhibition of sporangial germination	[140]
Ulva australis (formerly Ulva pertusa)	Tomato (Solanum lycopersicum)	TYLCV	Virus	Soil drench and Foliar spray	1.5% (w/v)	RT-PCR viral load assay	40% suppression of viral replication	[141]
Ulva rigida	Grape (Vitis vinifera)	Powdery Mildew	Botrytis cinerea (Fungi)	Foliar spray	1% (w/v)	Conidial germination assay	65% reduction in fungal growth	[142]
Ulva lactuca	Tomato (Solanum lycopersicum)	Early Blight	Alternaria solani (Fungi)	Foliar spray	2% (w/v)	Disease severity index	Reduced lesion size and fungal sporulation	[23]
Ulva reticulata	Citrus (Citrus spp.)	Citrus Canker	Xanthomonas citri (Bacteria)	Foliar spray	2% (w/v)	Symptom severity scale	50% reduction in lesion formation	[143]
Ulva intestinalis	Wheat (Triticum aestivum)	Fusarium Head Blight	Fusarium graminearum (Fungi)	Soil drench	2.5% (w/v)	PCR-based pathogen quantification	50% disease reduction	[144]
Ulva fasciata	Maize (Zea mays)	Maize Lethal Necrosis	MCMV and SCMV	Soil drench	1.5% (w/v)	Virus suppression index	55% reduction in disease severity	[145]
Ulva fasciata	Lettuce (Lactuca sativa)	Downy Mildew	Bremia lactucae (Oomycete)	Foliar spray	2.5% (w/v)	Disease severity scale	50% reduction in lesion expansion	[146]

Note: MCMV = Maize Chlorotic Mottle Virus, SCMV = Sugarcane Mosaic Virus.

Overall, the results indicate that ulvan has a wide range of elicitor activity, inducing systemic resistance and preventing pathogen development by various modes of action, and thus is a potential biostimulant for environmentally safe crop protection. SCMV-with 55% disease severity reduction in the case of soil treatment using Ulva fasciata. In addition, ulvan-based treatments have successfully controlled a range of bacterial diseases, such as bacterial speck of tomato (Pseudomonas syringae), angular leaf spot of cucumber (Xanthomonas campestris), and bacterial spot of pepper (Xanthomonas vesicatoria) [48,131,139,140]. These effects were all brought about mostly by foliar spraying or seed priming and resulted in inhibited bacterial growth, interference with the formation of biofilms, and generally enhanced plant vigor [140]. The same success has been achieved in managing fungal diseases like powdery mildew of grapes (Botrytis cinerea), early blight of tomato (Alternaria solani), and downy mildew of lettuce (Bremia lactucae), where ulvan treatments reduced spore germination and lesion growth significantly [142]. Taken as a whole, the findings presented in Table 5 suggest that ulvan may function as a broad-spectrum elicitor, inducing innate plant defense mechanisms leading to the inhibition of various pathogens. The fact that this elicitation is phytotoxicity-free yet systematically induces resistance indicates that ulvan could be a promising candidate for the development of environmentally friendly biostimulants and plant protection products for sustainable agriculture [146]. Further standardized and field-scale studies are required to validate these effects and establish optimal application strategies.

7. Formulation Strategies and Application Methods of Ulvan

Ulvan is considered to act as a defense elicitor and biostimulant in sustainable agriculture [147,148]. To improve its efficacy as a foliar agent, it is commonly formulated as an aqueous solution (0.01–10 mg/mL) with 0.05% Tween 20 [149]. Although complex carriers are not required because of their polysaccharide nature, nano-formulations like ulvan-chitosan composites are being increasingly explored. At an optimal concentration of 0.07 mg/mL in strong light (~320 μmol m⁻² s⁻¹), ulvan shows dose- and light-dependent activity. At a concentration of 0.07 mg/mL in Arabidopsis thaliana, it induced photosynthetic and stress proteins, leading to an enhanced production of roots (174%) and shoots (54%). Inhibition of growth can be caused by a higher dose, thereby emphasizing the importance of optimization. The enzymatically produced ulvan oligosaccharides retain their elicitor properties and enhance bioavailability [147]. In particular for disease control, foliar spray is the primary method for ulvan application. However, the optimal concentration range varies across studies depending on plant species, growth conditions, and ulvan composition. The severity of Zymoseptoria tritici infection in wheat is decreased by about 45%, and the formation of pycnidia is decreased by about 50% when 10 mg/mL (30 mL per pot) is sprayed two days before inoculation. Through the activation of the PR-2 and PR-3 proteins, OXO, and enzymes of the JA pathway (LOX, AOS), this protection is the result of enhanced plant immunity rather than direct antifungal activity. This leads to oxidative burst and cell wall reinforcement. Ulva extract foliar sprays at concentrations of 0.5–1% in maize promote plant biomass and height, but higher concentrations (e.g., 5%) could be growth-inhibiting [148].

Ulvan (0.07 mg/mL) supplementation of the root or medium may enhance nutrient uptake, lateral root emergence, and root growth in agar or hydroponic culture. Proteins involved in growth and stress responses are regulated as a result of enhanced biomass. Increased surface area for microbial attachment and nutrient uptake could also be a benefit of improved root architecture and rhizosphere interactions [147]. Use of ulvan on seeds has been shown to improve the uniformity of germination, vigor of seedlings, and establishment by boosting the enzymes responsible for protein synthesis, energy metabolism, and antioxidant activities. Seed priming, a technique that partially imbibes seeds with ulvan solution and then dries them, accelerates and synchronizes germination by inducing pre-germinative metabolism without inducing radicle emergence. This technique enhances stress tolerance to oxidative, drought, and salt stress by virtue of faster activation of defense and developmental genes [148]. Despite these promising results, there is still a need for standardized protocols and field-scale validation for consistent agricultural performance.

8. Current Challenges and Limitations of the Ulvan Application

Despite its considerable promise as a next-generation, eco-friendly biostimulant and biocontrol agent for sustainable plant disease management, several challenges currently limit the widespread agricultural application of ulvan [39,150].

8.1. Variability Related to Species, Season, and Extraction Method

One of the primary constraints is the variability in its chemical composition, which depends on Ulva species, geographical origin, seasonal variation, and extraction methodology. Such variability leads to inconsistencies in MW distribution, sulfation degree, monosaccharide composition, and ultimately biological efficacy [151]. A major challenge arises from this inherent variability in ulvan from algal sources, ambient conditions, and processing techniques. Ulva species (formerly classified under Enteromorpha) are the primary sources of ulvan, which is a sulfated polysaccharide. Ulvan content varies significantly across different Ulva species. Moreover, there are differences in structural characteristics, such as monosaccharide composition [45]. Differences in the distribution of sulfate groups are also documented. There are significant differences in the MW of ulvan from various species, as well as differences in the branching degree. The bioactivity of ulvan from various species also differs. For instance, a formulation that is effective for a particular species may not be effective for another species [150]. The composition of ulvan is significantly affected by seasonal changes. The biosynthesis of ulvan is affected by various environmental factors, such as light intensity. Its structural characteristics are affected by changes in temperature. Ulvan accumulation is affected considerably by nutrient availability. Compositional differences are also affected by differences in salinity. There is considerable batch-to-batch variation due to these ecophysiological parameters. More differences arise due to the extraction and purification processes. Extraction efficiency may be enhanced using rigorous chemical treatments. However, these treatments may affect sulfate esters or degrade polymers. Gentle treatments increase costs and lower efficiency while maintaining bioactivity [39]. Reproducibility and scalability of processes are hindered by the absence of established processes. All these aspects combine to hinder the reliable and extensive application of ulvan in agriculture [18]. As a result, reported bioactivity is often difficult to compare directly.

8.2. Challenges in Standardization and Quality Control

The lack of quality control standards is an important barrier to making ulvan-based formulations reliable tools in agriculture. Variability in sugar composition, sulfate levels, molecular weight, and viscosity in batches is attributed to the considerable variation in composition of ulvan, which is attributed to differences in algae species, geographic location, seasonality, cultivation practices, and post-harvest management [152]. The production and integrity of ulvan also differ in response to extraction and purification strategies, thereby creating chemical differences in composition among studies. The reproducibility of bioactivity in controlling plant diseases is also restricted by the lack of quality control standards in sourcing, extraction, and purification [153]. The commercialization of ulvan-based formulations is also restricted by the lack of universally accepted reference standards for agricultural-grade ulvan. Therefore, to ensure consistent efficacy, safety, and reproducibility in agricultural applications, universally accepted standards with validated techniques must be developed [154]. This lack of harmonized standards remains a key challenge for commercialization.

8.3. Economic Feasibility and Industrial Scalability

Ulvan’s current production capacity and associated costs present significant barriers to its adoption in agriculture [155]. The production process is also energy-intensive, with some processes involving acids or solvents. However, economic viability in commercial activities demands that there be constant production in high quantities at affordable prices [156]. Estimating costs is also hindered by the fact that yields differ significantly depending on the type of algae, season, or conditions of growth. Ensuring high production, high yields, effective harvesting, and environmentally friendly fertilizer management is also a challenge associated with Ulva biomass [157]. The cost and quality of ulvan also depend on effective management of the post-harvesting process. Most of the processes used in ulvan production are not effective in supporting constant production in high quantities [158]. Compared to conventional agrochemicals, ulvan-based products currently remain less cost-competitive due to higher production and processing costs. Filtration, concentration, or drying also requires huge amounts of money and energy. Furthermore, the structural and functional properties of ulvan are highly dependent on processing conditions. This implies that it is imperative to observe stringent quality control measures. In effect, this increases costs. Therefore, it is difficult for existing processing systems to attain desired efficiencies in producing large quantities of agricultural products. Currently, production is limited and expensive without technology. In addition, integration with existing agro-industrial infrastructures is lacking. It is also worth noting that it is still imperative to optimize the process to enable increased production with low chemical requirements [39]. Currently, developing cost-effective biorefinery technologies is in progress. In addition, the stability of the supply chain is unknown. These limitations make it difficult to compete in the market. Therefore, ulvan-based products are only suitable in niche markets [156]. In effect, it is difficult to attain large-scale applications in farming. What is needed is technological innovation in agricultural production and processing to overcome such challenges. Until such technologies are developed, it remains challenging for ulvan to compete economically with conventional pesticides under current production conditions [158].

8.4. Regulatory Hurdles (Particularly in EU and US Contexts)

Stringent regulatory frameworks in certain areas have hindered the commercialization of the product. Fertilising Products Regulation (EU) 2019/1009 and Regulation (EC) No. 1107/2009 on plant protection products govern the approval of the product in the European Union [154,159]. These regulations require substantial evidence of the efficacy, reproducibility, and purity of the product [160]. Stringent residual limits as low as 0.01 ppm are also enforced. Standardization of the product has been hindered by the discrepancy in the biomass of the seaweed due to environmental conditions [161]. The FIFRA classifies products based on ulvan as biochemical pesticides in the USA, which requires thorough toxicological and environmental assessments. These requirements add to the time and cost due to field testing and the additional United States Department of Agriculture (USDA) regulations for biofertilizer properties [162,163]. Finally, while ulvan holds promise for sustainability, its overall use is limited by the unpredictability of raw materials, high compliance costs, and lack of harmonization in international regulations [161]. Overall, differences in regulatory frameworks further complicate global commercialization.

9. Future Prospects and Application

Addressing current challenges in ulvan utilization will require coordinated advances in standardization, mechanistic insight, formulation innovation, and large-scale validation. Future research should prioritize the optimization and harmonization of extraction and purification protocols, ensuring batch-to-batch consistency in key physicochemical attributes such as molecular weight, sulfation degree, monosaccharide composition, and conformational structure parameters that are considered to strongly influence biological activity [161]. Establishing robust structure–function relationships through advanced analytical platforms (NMR spectroscopy, FTIR, chromatographic profiling, SEC-MALLS) is expected to enable the rational design of ulvan preparations with predictable and reproducible efficacy. At the molecular level, integrative omics approaches (transcriptomics, proteomics, metabolomics, and gene expression profiling) are likely to play a key role in elucidating the signaling networks underlying ulvan-induced resistance, including ROS bursts and SA/JA pathway activation. A deeper understanding of pathway cross-talk will refine crop-specific application strategies [162,163]. From a technological perspective, innovation in stable and scalable formulations, such as nano-encapsulation, seed coatings, foliar sprays, and controlled-release matrices, will be crucial to enhance bioavailability, stability, and field-level efficacy. Exploring synergistic interactions between ulvan and beneficial rhizosphere microbes or other biostimulants may open new avenues for integrated pest and soil health management. Emerging biotechnological tools, including CRISPR/Cas9 genome editing, could further optimize Ulva growth rates, sulfation patterns, and ulvan yield, thereby improving biomass productivity, carbon sequestration potential, and integration into algal biorefinery systems [158,162].

Sustainable large-scale cultivation of Ulva sp. through aquaculture, wastewater-fed systems, or integrated multi-trophic aquaculture will simultaneously enhance biomass availability and contribute to nutrient recycling and environmental remediation. Coupling ulvan deployment with AI-assisted precision agriculture platforms may assist in optimizing dosing strategies, enabling real-time monitoring of crop resistance, and reducing reliance on synthetic pesticides. Integrating ulvan extraction into comprehensive algal biorefinery models allows the co-production of proteins, pigments, and bioenergy products, improving economic feasibility and supporting circular bioeconomy principles. To enable widespread agricultural adoption, regulatory approvals, standardized quality control frameworks, and farmer acceptance are critical. Large-scale, multi-location field trials under diverse agro-climatic conditions must validate agronomic performance, ecological safety, and commercial viability. Overall, through interdisciplinary innovation, regulatory alignment, and rigorous field validation, Ulvan has the potential to emerge as a biodegradable, multifunctional, and environmentally responsible alternative to synthetic pesticides. Its dual capacity to enhance plant resistance and integrate into circular bioeconomic systems positions ulvan as a promising component of resilient and sustainable agriculture.

10. Conclusions

Seaweed-derived natural compound, ulvan, represents a promising eco-friendly alternative in comparison to the conventional agrochemicals owing to its multifunctional biostimulant and biocontrol properties. Ulvan enhances plant defense mechanisms, improves stress tolerance, and suppresses phytopathogens, thereby contributing to sustainable disease management and improved crop productivity. Its mode of action involves the activation of hormonal signaling pathways, oxidative bursts, and stress-responsive genes, all of which strengthen plant immunity. In addition, ulvan exhibits antimicrobial activity through dual mechanisms by both modulating plant defense responses and directly inhibiting fungal, bacterial, and viral pathogens. These properties highlight its potential as an effective agent for integrated disease management. Beyond its agricultural applications, the broad spectrum of ulvan bioactivities also indicates promising potential in the development of biomaterials and human health-related products. However, the commercial-scale adoption of ulvan remains limited due to challenges associated with large-scale production, variable extraction efficiency (7–41%), and the need for extensive purification processes to remove impurities such as salts, proteins, and ash. Furthermore, inconsistencies in the efficacy reported across different studies underscore the necessity for standardized evaluation frameworks and more comprehensive comparative analyses. Species-specific differences, harvesting conditions, and processing methods also significantly influence product consistency and biological activity, thereby limiting reproducibility and industrial scalability. Addressing these limitations through standardized production protocols, advanced extraction technologies, and cost-effective purification strategies will be essential for achieving industrial scalability and economic viability. Future research should focus on elucidating structure–function relationships using advanced analytical tools and omics-based approaches, as well as developing stable formulations and efficient delivery systems to improve field performance and agronomic reliability. In addition, large-scale field validation studies, integration of ulvan production into circular bioeconomy and biorefinery frameworks, and interdisciplinary collaborations will play a crucial role in translating laboratory-scale innovations into commercially viable agricultural solutions. With continued technological advancements and cross-sector research efforts, ulvan holds considerable promise as a sustainable agricultural input, contributing to resilient crop production systems and reducing dependence on synthetic agrochemicals.