Resource Recovery from Green Tide Biomass: Sustainable Cascading Biorefinery Strategies for Ulva spp.

Abstract

This review examines sustainable cascading biorefinery strategies for the green alga Ulva, which is globally prevalent in eutrophic marine waters and often forms extensive “green tides.” These blooms cause substantial environmental and economic damage to coastal communities. The primary target products within an Ulva biorefinery typically encompass salts, lipids, proteins, cellulose, and ulvan. Each of these components possesses unique properties and diverse applications, contributing to the economic robustness of the biorefinery. Salts can be repurposed for agricultural or even human consumption. Lipids offer high-value applications in nutraceuticals and animal feed. Proteins present significant potential as plant-based nutritional supplements. Cellulose can be transformed into various advanced materials. Finally, ulvan, a polyanionic oligosaccharide unique to Ulva, holds promise due to its distinct properties, particularly in the biomedical field. Furthermore, state-of-the-art chemical modifications of ulvan are presented with the aim of tailoring its properties and broadening its potential applications. Future research should prioritize optimizing these integrated extraction and fractionation processes. Furthermore, a multi-product biorefining approach, integrated with robust Life Cycle Assessment studies, is vital for transforming this environmental challenge into a significant opportunity for sustainable resource valorization and economic growth.

1. Addressing Green Tides and Valorizing Biomass

“Green tides,” characterized by extensive accumulations of unattached green macroalgae, are increasingly prevalent in eutrophic marine environments worldwide. The rapid and excessive proliferation of macroalgae, particularly Ulva spp., poses significant environmental and economic threats to coastal communities, adversely affecting fisheries, aquaculture, and tourism [1,2]. Ulva is globally distributed in marine environments and exhibits an increased frequency and intensity of bloom formation, driven by factors such as nutrient enrichment and climate change [3]. However, these marine organisms represent a potential resource for biorefineries, yielding various biochemicals, including polysaccharides, proteins, and lipids, as well as essential minerals [4,5,6].

Ulva lactuca, commonly known as sea lettuce, is an Ulva spp. consumed in various countries globally, including Denmark, Indonesia, Ireland, Italy, Japan, Scandinavia, and Scotland, typically in culinary applications such as soups and salads. Consequently, its cultivation is undertaken within controlled systems, either inland or offshore, to ensure its safety for human consumption. Apart from being used for food, the large amount of Ulva biomass creates a big management problem, requiring the creation of sustainable methods to reduce its harmful effects and find ways to use it as a valuable resource.

The commercial exploitation of macroalgal biomass, including collection, transport, and processing, can be cost-prohibitive, often hindering the economic feasibility of conventional applications. The primary economic constraint lies in the costs associated with biomass collection and transport, which vary regionally. For instance, the cost for the removal of Sargassum sp. biomass from Caribbean beaches has been reported to be substantial (at least USD 120 million), and in Europe, the collection and disposal of macroalgae from beaches can range from EUR 6 to 120 per ton [7,8]. However, techno-economic analysis suggests that Ulva cultivation and biochar production for CO₂ sequestration may be financially viable, indicating that utilizing seaweed as a raw material in industrial processes could offset the economic costs of harmful seaweed removal [9]. Furthermore, repurposing beached Ulva biomass for carbon sequestration is being explored as a strategy to mitigate global climate change [10]. Research has investigated converting shoreline Ulva biomass into biochar via pyrolysis within biorefineries, highlighting its capacity to sequester approximately 3.85 million tons of CO₂ equivalent (CO₂e), with the potential to stabilize roughly 1.93 million tons of CO₂e through biochar conversion [10]. A life cycle assessment of various disposal options for Ulva prolifera blooms in the Yellow Sea has evaluated the environmental impact and cost of managing this biomass, considering both current and potential alternative strategies involving its energetic valorization [11].

Biorefining is a sustainable approach to resource recovery that involves the integrated processing of biomass to produce a spectrum of marketable products and bioenergy [12]. A key strategy within biorefining is the “cascading biorefinery” concept. This approach prioritizes the sequential extraction of various valuable components from biomass, where the residual material from one extraction step becomes the feedstock for the next. Developing integrated biorefinery solutions to obtain multiple products from Ulva could enable the full utilization of seaweed biomass. This strategy should prioritize economically feasible processes to extract both valuable compounds present in low amounts (e.g., fatty acids, proteins, phenolic compounds, and pigments) and less valuable products available in higher amounts (e.g., biofuels and soil biostimulants) [13]. The integrated approach, combining chemical feedstock recovery and residue energetic valorization, has the potential to significantly improve the sustainability and economic viability of marine macroalgal biomass utilization. Transforming waste into a valuable resource can reduce the environmental burden and enhance the profitability of managing adverse seaweed accumulations.

In the literature, the most studied seaweed biorefinery framework is focused on recovering a plethora of added-value compounds and the energetic valorization of residue through different technologies [13,14,15,16,17,18,19,20]. Within a macroalgal biorefinery framework based on Ulva, products can be categorized as ulvan, a polysaccharide with significant market potential, co-products, and residual materials. A sequential extraction strategy that prioritizes ulvan recovery while valorizing other biomass components is highly attractive. Ulva-derived co-products, including salts, pigments, lipids, and proteins, can be strategically extracted to mitigate impurities during ulvan isolation, thereby improving its yield. The primary objective of this review is to advance holistic and targeted biomass valorization through comprehensive biomass utilization, a perspective often less emphasized or lacking in broader reviews. While fuel valorization is deliberately omitted as an already well-established research domain, the focus of this review is to prioritize the valorization of ulvan, a polyanionic oligosaccharide with unique properties. To expand its range of applications, the final section provides an overview of potential chemical modifications of ulvan.

2. Sustainable Biorefinery Strategies

2.1. Cascade Strategy

The economic viability of biorefineries is significantly influenced by the variability in biomass composition, as well as the yields and purities of extracted products, as shown in

Table 1. Variability in biochemical composition among Ulva species.

Species	Location	Month	Lipid	Protein	Carbohydrates	Ash	Ref.
U. lactuca	SE of India	/	1.6%	3.2%	25.0%	/	[22]
	India	/	3.3%	8.4%	35.3%	/
	Hong Kong	Dec	1.6%	7.0%	14.6% 1	21.3%
	Philippines	May	0.7%	4.0%	55.0%	17.3% 2
	The Netherlands	Dec	0.3–5.0%	24.6%	20.0%	15.9%	[23]
	Tunisia	Jul	7.9%	8.5%	54.0%	19.5%	[24]
Ulva rigida	Mexico	/	/	8.7%	55.0%	30.1%	[25]
Ulva spp.	Denmark	May	1.0%	20.6%	/	41.2%	[26]
		Jun	1.7%	6.5%	/	36.9%
		Aug		10.7%	/	35.5%
	The Netherlands	May	0.4–1.3%	8.0–22.0%	35.0–45.0%	20.0–15.0%	[27]

¹ Dietary Fibers; ² Calculated by difference.

. Such variability introduces substantial uncertainty in production costs and output volumes, directly impacting profitability. For instance, the starch content in green macroalga Ulva sp. has been observed to fluctuate tenfold depending on cultivation conditions, consequently leading to a broad range in potential total revenue (USD 1.56 to USD 3.93 kg⁻¹ Ulva DW) [21]. Therefore, it is evident that relying on a single component presents significant risks. Consequently, developing integrated marine biorefinery strategies is essential to optimize the utilization of Ulva sp., a widely distributed seaweed. Co-producing a diverse range of valuable biochemicals from a single seaweed is crucial for the efficient valorization of marine biomass, as it helps mitigate economic fluctuations and will fulfill the general concept of a sustainable process. Therefore, the inherent variability of biomass feedstocks, coupled with fluctuating market demands, necessitates a multi-product biorefinery approach. This strategy allows the efficient valorization of a diverse array of value-added biochemicals, such as proteins, lipids, and carbohydrates. Essentially, the initial biochemical extracted profoundly influences the entire process design and operational efficiency, highlighting the interaction of product selection and overall biorefinery viability.

2.2. Full Exploitation of the Biomass

Depending on the target compounds, the integrated process, comprising several pathways and technologies, can be designed differently. For instance, the recovery yields of bioactive compounds are solvent-dependent, aligning with compound polarity and location. Aqueous solvents are optimal for (poly)saccharide isolation, while organic solvents are preferred for phenolics and carotenoids. Organic solvent extracts typically demonstrate higher bioactivity, whereas polar solvent extracts exhibit enhanced antibacterial activity [28]. For example, the number of phenolic acids in the extract directly correlated with increasing solvent polarity in Euphrasia brevipila herb extracts. Specifically, petroleum ether yielded one compound, while ethyl acetate extracted two. Trichloromethane, butanol, ethanol, and water each extracted three compounds, which were identified as caffeic acid, ferulic acid, and chlorogenic acid through comparison with authentic samples. Furthermore, these extracts exhibited varying antimicrobial activities against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, with activity generally increasing with extractant polarity (i.e., petroleum ether and trichloromethane > butanol > ethanol > water) [29]. The substantial biomass yield, elevated protein concentration, significant sugar content, and diverse array of essential amino acids open opportunities to tap into algal potential through biorefining to obtain a spectrum of bioproducts. Current algal biorefinery models predominantly employ single-product extraction, co-extraction, or cascading fractionation methodologies. Despite Ulva sp. representing the most prevalent group within algal blooms, their bioprocessing remains comparatively under-investigated, highlighting a significant opportunity for the valorization of these ecologically problematic blooms within a circular economy framework.

From a biorefinery point of view, the genus Ulva stands out as being of particular interest due to its unique polysaccharide component, ulvan, which has considerable application potential in various fields. Like classical lignocellulosic biomass, different processes have been developed to extract the various components, using different strategies. Since all the fractions require proper valorization, following the idea of cascade biorefinery, a general scheme comprising all the main steps is reported in Figure 1.

It is well known that algae can undergo natural desiccation due to tidal conditions, seasonal droughts, or osmotic stress. Typically, desiccation or de-watering is performed either naturally under sunlight or through forced drying in an oven after biomass washing. While this process protects the algae from rotting, it can induce ultrastructural changes, which may have significant implications for the biorefinery process. These modifications commonly include alterations to the cell wall, variations in cytoplasmic density, chloroplast changes, and modifications to membrane integrity [30,31,32,33].

3. Ulva Biomass Components and Extraction

3.1. Salt Recovery

Given Ulva’s marine origin, its inherent high salt content can impede extraction efficiency. One of the first steps is washing the biomass to remove salts and impurities. Salt can be removed through osmotic shock by suspending the biomass in deionized water with a high liquid-to-solid ratio (LSR) from 10 to 100. The biomass is then collected, and the extraction process is generally repeated [5,22,34]. As shown in

Table 2. Yield of the desalting process among Ulva sp.

Species	Yield [w/w]	Reference
Ulva ohnoi	45%	[9]
	29%	[22]
Ulva tepida	36%	[22]
Ulva fasciata	35%	[34]
	26%	[35]

the desalting step can result in a considerable fraction of the initial biomass, with the wash solution accounting for 26% to 45% of the initial biomass dry weight, depending on the specific Ulva spp.

Distilled water can be effectively employed to enhance the desalting process, enabling the removal of salts from the biomass. This yields a wash solution that contains macro- and microelements summarized in

Table 3. List of the macro- and microelements present in Ulva sp. [5,22,34,35,36].

Macroelements	Microelements	Heavy Metals 1
Na	B	Ni
K	Al	Cu
Ca	Cr	Zn
Mg	Mn	Mo
P	Fe	Ni
	Co

¹ In trace.

, where potassium, magnesium, and sodium are the most representative elements.

The amount and variety of these elements depend on the species, as well as the place and conditions of harvesting. The utilization of this salt mixture ranges from agricultural applications as a fertilizer to its use in the human diet and functional foods. Algal extracts hold a complex biochemical profile that facilitates their utilization in agriculture, enhancing plant nutrition, stress tolerance, and crop quality. Indeed, algae extracts are an expanding category of plant-health products both in soil and as a foliar spray, holding a market share exceeding 33% of the global biostimulant sector [37,38].

The recovery of natural mineral salt from U. ohnoi and U. tepida was investigated across a spectrum of time and temperature parameters. Subsequently, the resulting marine macroalgal salts were characterized. It was demonstrated that washing at a higher temperature or for a longer period can further remove as much as 60% of the ash in both macroalgae. Furthermore, the removal of salt eventually increased organic matter, particularly the protein fraction (25–27% DW), enabling the utilization of the processed biomass in animal feed applications. The results not only suggest the potential of processed macroalgae as fertilizer, animal feed, or fuel, but also point out the possibility of utilizing the recovered salt for human consumption [22]. Apart from its mineral content, Ulva sp. holds interesting polysaccharides (starch, ulvan, and cellulose), among which ulvan has unique characteristics related to its structure and composition.

To access valuable intracellular products from macroalgae, cell disintegration is essential. Apart from a rigid cell wall, macroalgae have a large macrostructure that acts as an additional barrier. While this macrostructure simplifies harvesting and lowers de-watering costs, it also complicates continuous liquid flow processes for cell disruption [39]. As detailed in Figure 1, the resulting liquid fraction after cell disintegration and filtration can be further separated into a mineral-rich liquid extract and starch. This fractionation is feasible because cell wall disruption via homogenization facilitates the release of starch granules. To achieve this separation, the washed homogenized biomass undergoes sequential filtration with progressively smaller pore sizes. Subsequently, starch granules can be isolated from the filtrate using centrifugation and purified by washing with ethanol to remove pigment and lipid contaminants [9,21]. While it is crucial to acknowledge that algal starch properties and large-scale applications are still limited, preliminary findings indicate promising potential. The composition and characteristics of algal starch suggest they could be valuable in various industries, including food, biochemicals, paper, textiles, animal feed, and biofuels. In particular, the high amylose content is a significant attribute for food applications, possibly indicating the presence of resistant or slow-digesting starch, which has nutritional implications for low-calorie food formulations [40,41].

Hence, the sequential filtration and centrifugation process enables the separation of starch from macroalgae biomass, yielding a potentially valuable resource for diverse industrial applications. However, a portion of ulvan may be co-extracted, potentially decreasing its recovery. Thus, optimizing the extraction process for starch necessitates a compromise in the recovery of other marketable compounds, as the simultaneous attainment of high yields for all components is not feasible.

3.2. Lipids Recovery

Another important class of components is lipids, which also include glycolipids, phospholipids, and fatty acids, such as essential unsaturated fatty acids, which have significant market value. The amount of the lipid, including the fatty acid fraction, varies depending on the species, seasonality, location, and growth conditions;

Table 4. Fat and fatty acid composition of some macroalgae, Ulva spp.

	Fat (% DW)	Fatty Acids (% DW)	1 SFA (% DW)	2 MUFA (% DW)	3 PUFA (% DW)	Reference
Ulva armoricana	2.6		46.5	24.3	29.2	[42]
Ulva clathrata	2–4	74–77	29–51	10–12	13–34	[43]
Ulva lobata	2–3		22–29	13–15	55–64	[44]
U. lactuca	7–13		27–69	16–24	7–43	[24,45]
U. rigida	1–6		29–37	21–22	33–34	[46,47]

¹ SFA, saturated fatty acid; ² MUFA, monounsaturated fatty acid; ³ PUFA, polyunsaturated fatty acid.

.

Although these compounds can potentially be recovered in biorefinery processes, they are predominantly employed in their natural form as a dietary supplement for animals, particularly fish, as well as for human consumption. They serve as an excellent alternative source of polyunsaturated fatty acids, including oleic, linoleic, and linolenic acids. However, in the biorefinery concept for lipid recovery (Figure 1), the strategy mainly exploits the starch flow, as demonstrated for U. ohnoi, and involves a simple extraction with ethanol, recovering almost 14% of lipids based on the initial dry biomass [9].

Lipids are also obtained after biomass crushing and water washing, from the solid fraction after extraction with a solvent; see Figure 1. Unfortunately, the solvent employed is generally chloroform, and there is a general lack of utilization and reuse of green solvents [36].

To this end, a conceptual downstream process for obtaining pure chlorophylls and xanthophylls was designed, incorporating optimized extraction and purification steps with solvent reuse. This process integrates the following three main stages: (i) ultrasound-assisted solid–liquid extraction of pigments using ethanol, (ii) separation of chlorophylls and xanthophylls using a liquid–liquid extraction system (50:30:20 mixture of ethanolic extract, hexane, and water), and (iii) solvent recovery and reuse using a low-pressure vacuum dryer at low temperature (≈35 °C) [48]. Hence, the investigation demonstrates that spray drying and evaporation techniques are efficient methods for pigment purification and solvent recovery and are industrially scalable.

In summary, while Ulva spp. represent a source of valuable lipids, including essential unsaturated fatty acids, their efficient recovery within a biorefinery context presents challenges. Current strategies often prioritize starch extraction, potentially limiting lipid yields. Furthermore, established lipid extraction protocols continue to rely on solvents such as chloroform, posing significant environmental concerns and underscoring the critical need for greener alternatives that offer comparable dissolving power to traditional methods [49]. Future efforts should optimize lipid recovery yields while minimizing environmental impact to enhance the sustainability of Ulva-based biorefineries.

3.3. Protein Recovery

Despite the high seasonal fluctuations, interest in Ulva spp. for protein extraction is justified by their notable plant-based protein content and the favorable composition of essential amino acids, particularly for vegetarian and vegan diets [50,51,52]. Various extraction methods and processing techniques for obtaining seaweed proteins are available in the literature, including aqueous, alkaline, and enzyme-assisted extraction [53,54]. A biorefinery model for marine macroalgae was demonstrated by integrating crude protein extraction with the recovery of valuable by-products [36]. The study optimized the extraction process to recover proteins, minerals, lipids, ulvan, and cellulose from U. lactuca biomass. The extracted protein showed high digestibility and a favorable amino acid profile, suggesting its potential as a nutritional supplement [36]. For instance, a recent study compared different methods for extracting water-soluble proteins and carbohydrates from fresh U. lactuca macroalgae [39]. The goal was to find efficient, less energy-intensive ways to disrupt the algal cell structure and release valuable intracellular components by osmotic shock, enzymatic incubation, pulsed electric field (PEF), and high shear homogenization (HSH). The latter was the most efficient method for extracting proteins and carbohydrates from fresh U. lactuca. This method showed the best yields and the largest potential for energy savings. The study also highlighted the importance of using fresh biomass for extraction and provided optimal conditions for other extraction methods like osmotic shock and PEF. These findings provide valuable insights for developing sustainable and efficient biorefinery processes for macroalgae utilization. A comparison of methodologies, based on the percentage of total biomass protein recovered, revealed that HSH yielded the highest percentage at approximately 39%. This value represents one of the highest values reported in the current literature for Ulva spp., followed by enzymatic degradation (~25%), osmotic shock (~20%), and PEF (~12%). Similarly, carbohydrate yields were definitively higher for all treatments, with percentages of total carbohydrates ranging from 51% (HSH) to 15% (PEF). Subsequent processing techniques like ultrafiltration, precipitation, and drying are crucial for concentrating and purifying the extracted proteins. Further developments are hence needed for effective fractionation protocols to unlock the potential of Ulva spp.

Recent advancements have optimized the extraction and purification of the entire protein content, including both water-soluble and insoluble fractions, from Ulva spp., with a focus on methods suitable for food applications [53]. This study investigated protein recovery following traditional mechanical pressing, comparing acid precipitation (pH 2) and heat denaturation (90 °C) prior to microfiltration. As detailed in Table 5, acid precipitation yielded the highest total protein (5.5%) in comparison with heat denaturation (4.1%). Acid precipitation was also more effective, precipitating approximately 81% of the solubilized protein compared to 43% for heat denaturation. While microfiltration successfully retained chlorophyll in the retentate, the formation of large protein complexes hinders protein recovery during filtration, demonstrating a need for further optimization of pre-treatment methods. The complex nature of Ulva spp. biomass necessitates the development of more effective fractionation protocols to maximize protein extraction and purification for food applications [53].

For instance, a pilot-scale protein recovery platform from Ulva sp. has recently been assessed. Ulva spp. biomass was initially pre-processed through rinsing and shredding. The pretreated biomass was then mechanically fractionated via a twin-screw press, resulting in pulp and juice fractions [55]. The juice fraction was subjected to two downstream processing pathways, as follows: direct processing or clarification via decanter centrifugation to yield a precipitate pellet and a cleared juice supernatant. Proteins were subsequently recovered from the cleared juice through acid-induced precipitation (HCl, pH 2) followed by centrifugation. Furthermore, a portion of the pulp fraction was subjected to secondary processing through homogenization with water (1:5 ratio), alkaline treatment (NaOH pH 8.5), and subsequent screw pressing to extract residual proteins. The resulting secondary juice was then processed analogously to the primary juice, either with or without clarification. The results showed the highest protein yield when the clearing step was omitted, ranging from 9.5–11.2% DW, with no significant differences among whole biomass or pulp. On the contrary, lipid content was observed to be marginally higher in the pulp fraction when compared to the whole biomass. This slight increase was hypothesized to stem from the specific cellular localization of structural lipids within the fiber component, which is likely more prevalent in the analyzed pulp. Furthermore, the inclusion of a clearing step in the juice processing increased the protein content but did not significantly affect nitrogen digestibility. However, resulting extracts showed a higher concentration of heavy metals (i.e., arsenic), which may limit the use of the extracted protein in food applications. While demonstrating promising scalability and highlighting the necessity of a preliminary desalination step, the successful industrial development of Ulva protein extraction for the food industry necessitates a rigorous assessment at all stages of potential toxic element concentrations, such as arsenic. This is particularly vital when biomass is derived from uncontrolled environments (e.g., marine sources) [55].

Another methodology that sensibly differs from Figure 1 applies subcritical water hydrolysis (SWH) as a key step for the simultaneous co-production of multiple valuable compounds, including protein, hydrochar, monosaccharides, 5-hydroxymethylfurfural (5-HMF), and free amino acids [56]. Ulva sp. biomass was treated with seawater under subcritical water conditions (180 °C, 10.5 bar, 40 min, 8% w/w solid load), resulting in the generation of a solid fraction (hydrochar) and a liquid fraction containing various dissolved compounds. The liquid fraction analysis revealed the presence of significant amounts of total monosaccharides, 5-HMF, protein, and free amino acids. Notably, the treatment facilitated a high protein extraction yield of 84.9% of the total protein content in Ulva. As shown in Table 5, the methodology for protein quantification was through the Lowry assay; the reaction is susceptible to various interfering molecules, including polysaccharides and 5-HMF [57]. Therefore, although promising, the liquid fraction coming from the subcritical water process is a complex mixture of several compounds (e.g., polysaccharides, protein, phenols, 5-HMF), which may interfere with the protein determination assay. Moreover, this study demonstrates the efficacy of subcritical water hydrolysis as a promising additional pretreatment technology for the comprehensive valorization of Ulva spp. biomass in a biorefinery context.

In conclusion, the extraction of proteins from Ulva holds significant promise due to their substantial protein content and favorable amino acid profiles, particularly for vegetarian and vegan diets. Various extraction methodologies, including aqueous, alkaline, enzyme-assisted extraction, pulsed electric fields, and high shear homogenization, have been explored, each with varying degrees of efficiency and energy requirements. While methods like high shear homogenization and subcritical water hydrolysis demonstrate high protein yields, challenges remain in optimizing extraction and purification processes for food applications, specifically concerning the removal of toxic elements and the effective fractionation of protein complexes.

Table 5. Ulva spp. protein extraction methodologies and quantification methods in the literature.

Species	Protein Content [%DW]	Total Protein Yield [%]	Extraction Methodology	Protein Quantification Methodology	Reference
Ulva spp.	9.5–11.2	3–10	Acid precipitation pH 2, homogenization for 1 h with tap water at pH 8.5, and then screw-pressed	N to protein conversion factor of total amino acid content after protein hydrolysis	[55]
	16.0	5.5	Acid precipitation pH 2	Bradford assay	[53]
		4.1	Heat denaturation at 90 °C
	18.0	6.8	Alkaline extraction pH 8.5 + 12, acid precipitation pH 2	Nitrogen analyzer for solids and modified Lowry assay for liquid	[58]
		5.0	Mechanical pressing, acid precipitation, pH 2
		23.9	Mechanical pressing, acid precipitation pH 2 + alkaline extraction pH 8.5 + 12, acid precipitation pH 2
	6.9	84.9	Subcritical water hydrolysis at 180 °C, 10.5 bar for 40 min, 8% w/w solid load	N to protein ratio for solids and Lowry assay for liquid	[56]
U. lactuca	12.3–19.8	19.5	Osmotic shock at 30 °C for 24 h	Commercial Lowry assay of protein hydrolysate	[39]
		26.1	Enzymatic incubation at 30 °C for 4 h with 2% PMC-R10
		15.1	PEF 1 at 7.5 kv cm−1, 2 pulses of 0.05 ms
		39.0	HSH 2 2 phase set-up “21 → 16” m s−1
	14.4	84.0	Alkaline extraction at 80 °C, then neutralization	Amino acid PITC 3 derivatization after protein hydrolysis and HPLC analysis	[36]
Ulva fenestrata	18.0	9.0	Alkaline extraction pH 8.5	Nitrogen analyzer for solids and modified Lowry assay for liquid	[54]
		5.3	Alkaline extraction pH 8.5 + 12
		6.9	Mechanical pressing

¹ Pulsed electric field; ² high shear homogenization; ³ phenyl isothiocyanate.

Table 5. Ulva spp. protein extraction methodologies and quantification methods in the literature.

Species	Protein Content [%DW]	Total Protein Yield [%]	Extraction Methodology	Protein Quantification Methodology	Reference
Ulva spp.	9.5–11.2	3–10	Acid precipitation pH 2, homogenization for 1 h with tap water at pH 8.5, and then screw-pressed	N to protein conversion factor of total amino acid content after protein hydrolysis	[55]
	16.0	5.5	Acid precipitation pH 2	Bradford assay	[53]
		4.1	Heat denaturation at 90 °C
	18.0	6.8	Alkaline extraction pH 8.5 + 12, acid precipitation pH 2	Nitrogen analyzer for solids and modified Lowry assay for liquid	[58]
		5.0	Mechanical pressing, acid precipitation, pH 2
		23.9	Mechanical pressing, acid precipitation pH 2 + alkaline extraction pH 8.5 + 12, acid precipitation pH 2
	6.9	84.9	Subcritical water hydrolysis at 180 °C, 10.5 bar for 40 min, 8% w/w solid load	N to protein ratio for solids and Lowry assay for liquid	[56]
U. lactuca	12.3–19.8	19.5	Osmotic shock at 30 °C for 24 h	Commercial Lowry assay of protein hydrolysate	[39]
		26.1	Enzymatic incubation at 30 °C for 4 h with 2% PMC-R10
		15.1	PEF 1 at 7.5 kv cm−1, 2 pulses of 0.05 ms
		39.0	HSH 2 2 phase set-up “21 → 16” m s−1
	14.4	84.0	Alkaline extraction at 80 °C, then neutralization	Amino acid PITC 3 derivatization after protein hydrolysis and HPLC analysis	[36]
Ulva fenestrata	18.0	9.0	Alkaline extraction pH 8.5	Nitrogen analyzer for solids and modified Lowry assay for liquid	[54]
		5.3	Alkaline extraction pH 8.5 + 12
		6.9	Mechanical pressing

¹ Pulsed electric field; ² high shear homogenization; ³ phenyl isothiocyanate.

3.4. Cellulose Recovery

The solid residue generated after protein recovery represented in Figure 1, is mainly characterized by a complex matrix of cellulose and other minor components. Cellulose isolated from Ulva spp. holds distinct properties, making it amenable to diverse industrial applications such as composite materials, polymers, foams, and in the biomedical field [59,60]. Consequently, fractionating protein and cellulose produces valuable coproducts, enhancing both the economic viability and environmental sustainability of the overall process.

A sequential extraction process was specifically adapted for isolating cellulose from U. lactuca. The process involved an initial removal of lipids and extractives, a subsequent extraction of ulvan and pigments, and, eventually, cellulose was obtained through alkali and acid treatments, followed by bleaching [61]. The cellulose extraction procedure began with the removal of extractives and lipids from the algal biomass with ethanol as the solvent. Subsequently, ulvan was eliminated by incubating the material in a solution containing ammonium oxalate at 100 °C for 1 h. The biomass was then repeatedly bleached in a solution containing acetic acid and sodium chlorite until the residual material turned white. Finally, after several acid-neutral washings, the resulting extracted cellulose was thoroughly washed to achieve neutrality and subsequently oven-dried with a final yield of 12.4% (w/w). A key advantage in the cellulose purification stage was attributed to the specific biochemical composition of U. lactuca. Indeed, complete pigment removal was achieved efficiently with only a single bleaching cycle, significantly reducing the processing time and chemical consumption typically required. The developed protocol shows promise for industrial scalability, suggesting that producing cellulose from U. lactuca could be economically and environmentally comparable to sourcing it from traditional woody biomass. Another study enhanced the environmental profile of the cellulose extraction process from U. lactuca by utilizing a sequential treatment protocol involving ethanol, hydrogen peroxide, followed by alkaline and acidic hydrolysis, a cellulose-rich insoluble fraction was obtained in a 2.2% DW yield [59]. As previously mentioned, the chemical composition of U. lactuca varies considerably depending on location and age. Age is a significant factor, as early harvesting results in lower cellulose content, which probably explains the observed differences in cellulose levels along with the different processing methods used, making it difficult to compare the various proposed processes.

Cellulose-based materials are increasingly used due to rising demand, driven by their low cost, density, non-toxicity, biodegradability, recyclability, and strong mechanical properties comparable to commercial polymers. For example, Ulva sp. is a viable, sustainable source for nanocellulose production, yielding a nanomaterial with significant antimicrobial properties suitable for development into advanced medical materials [60,62]. Therefore, the resulting algal cellulose, as a co-product, possesses distinct properties, such as a high degree of crystallinity, which may enable novel applications. For instance, the combination of excellent mechanical performance and environmental benefits makes cellulose nanofibrils (CNFs) suitable for diverse applications, such as composite materials, polymer films, foams, and the biomedical field [60]. Furthermore, Ulva spp. cellulose is an alternative, non-terrestrial source for producing platform biochemicals such as carboxymethyl cellulose (CMC). Thanks to its versatile physicochemical properties, CMC finds extensive application across a multitude of industrial domains. Its utility stems primarily from its efficacy as a viscosity modifier, suspending agent, stabilizer, film-former, and water-retention agent. CMC from U. fasciata was synthesized and characterized by its film properties [63]. The results indicate that algal CMC not only has film properties comparable to those of commercially available CMC, but it also has valuable antimicrobial and biodegradable properties, making it a strong candidate for various industrial applications.

In summary, while challenges remain in standardizing cellulose extraction from Ulva spp. due to variability in biomass composition and processing methods, the resulting cellulose offers significant potential. The development of efficient sequential extraction processes allows for the recovery of this valuable co-product alongside other compounds like protein. Given its favorable properties, including biodegradability, mechanical strength, and suitability for modification, algal cellulose represents a compelling alternative to traditional sources, warranting further research and development for broader industrial applications.

3.5. Ulvan Recovery

Ulvan is a complex sulfated heteropolysaccharide with polyanionic behavior found in the cell walls of green algae, and it is primarily composed of rhamnose, uronic acids (glucuronic and iduronic), and xylose, with repeating disaccharide units. The ulvan backbone is mainly composed of α- and β-(1,4)-linked monosaccharides, with characteristic repeating disaccharide units called ulvanobiuronic acids (Figure 2). Its chemical structure is like some human glycosaminoglycans (i.e., hyaluronic acid, chondroitin sulfate, heparin), and, due to this, it is gaining attention in the biomedical and cosmetic fields. Quality can be assessed by molecular weight, degree of sulfation, and sugar composition, and it is highly variable depending on the species, eco-physiological factors, and processing procedures. This structural variability significantly influences its physicochemical properties and biological activities [64,65,66,67].

A simple sequential extraction procedure yielded consistent outputs from the residual biomass after each successive extraction step [35]. The process involved an initial water extraction to obtain a mineral-rich water extract, followed by solvent extraction for lipids. Then, ulvan water extraction and ultimately, the cellulose fraction, were subjected to enzymatic hydrolysis and fermentation to produce ethanol (Figure 1). The first water extraction allowed the removal of intra- and extracellular minerals, yielding an extract with a promising composition as a liquid fertilizer. This simple pretreatment process can potentially offset the costs associated with seaweed dewatering. Furthermore, the sequential extraction strategy reduced reagent consumption compared to direct extraction methods, enhancing the overall efficiency of biomass processing [35].

Downstream yields are directly influenced by the specific pretreatment and extraction methodologies employed, which, in turn, affect the cascading process. For instance, the preliminary salt removal can increase ulvan yields from 3.7% to 8.2% of Ulva dry weight [35]. In contrast, another study showed that a pre-washing stage to isolate a mineral-rich fraction negatively impacted ulvan extraction yields [14]. The reason for this contradiction can be attributed to the different process methodologies. The former extraction procedure involved autoclaving the residual biomass at 90 °C for 2 h, followed by filtration of the solution and subsequent precipitation with isopropyl alcohol for 24 h at −40 °C. In contrast, the latter strategy employed non-isothermal autohydrolysis utilizing hot pressurized water, reaching temperatures up to 160 °C, to specifically recover low molecular weight ulvan enriched in phenolic compounds. Therefore, the initial desalting process could increase or decrease the recovery of ulvan according to the methodology adopted. Finally, the remaining solid by-product was anaerobically digested for biogas production. Biomethane potential (BMP) of the U. lactuca residue was also assessed after the individual and sequential recovery of algal extract, ulvan, and protein [68,69]. Indeed, the highest methane yield was observed in residues obtained after the removal of minerals and ulvan in comparison to untreated U. lactuca. The authors concluded that the sequential extraction of value-added products prior to the anaerobic digestion process not only improves biomass availability for digestion and the corresponding methane yields but also enhances the overall efficiency and viability of the process.

In contrast to the initial salt recovery aimed to simultaneously enhance ulvan yield and quality, a defatting step is inefficient due to low yields, the requirement for prior biomass drying, and the negligible impact on ulvan recovery [14,70]. For example, a cascading biorefinery process for U. ohnoi was developed and evaluated to optimize ulvan yield and purity [70]. A factorial design was utilized to examine the effects of biomass pre-washing, pigment pre-extraction, and ulvan extraction methodologies. The optimal sequential process involved a pretreatment for salt removal, followed by ulvan extraction using hot dilute hydrochloric acid. Under the optimal conditions examined, the resulting extracts demonstrated the lowest protein (0.4% w/w) and ash (22.5% w/w) content. Concurrently, these extracts also yielded about 8% DW of ulvan with the highest concentrations of uronic acid (23.8% of extract) and sulfate (12.4% w/w) across all experimental conditions [70]. Therefore, the results demonstrated that further treatment to remove pigments did not significantly affect ulvan yield or quality and was unnecessary. Finally, even if these compounds possess significant applications in cosmetics, food additives, and pharmaceuticals, the recovery of a lipophilic fraction is not worthwhile in a seaweed-integrated biorefinery due to the higher environmental impact associated with this unnecessary process.

To summarize, ulvan commonly constitutes the primary target compound, a trend prevalent in most integrated processes reported in the literature. Notably, the structural and chemical characteristics of ulvan, and thus its resulting properties, are dependent on the extraction conditions utilized. Current approaches frequently limit the use of residual output from ulvan extraction to biofuel production, without exploring alternative valorization strategies. Consequently, Figure 3 illustrates a biorefinery design developed for the recovery of ulvan and the full utilization of side streams.

The primary challenge in recovering ulvan is preserving its integrity, particularly its molecular weight and sulfur content, which define its distinctive and desirable characteristics. Preserving the sulfate group is also important, as its structural integrity and charge characteristics are maintained through a combination of covalent bonding and electrostatic interactions. Under physiological pH conditions, these sulfate ester groups are typically deprotonated, resulting in a negatively charged sulfate polyanion. These negative charges are stabilized by the surrounding aqueous environment through ion-dipole interactions with water molecules [71,72]. Maintaining the structural integrity of sulfate groups during the extraction necessitates carefully controlled conditions to prevent their hydrolysis or modification. Generally, aqueous extraction at temperatures around 80 °C and acidic pH is preferred to minimize desulfation, which is accelerated under strongly acidic or alkaline conditions and elevated temperatures [64]. Furthermore, the use of specific buffers during extraction and subsequent purification steps can help stabilize the pH and ionic environment, thereby contributing to the retention of the native sulfation patterns of the polysaccharides (Figure 2).

Extensive research has focused on elucidating the gelling mechanisms of ulvan polysaccharides [73,74,75]. The ulvan gelation mechanism is influenced by several factors, including the extraction method employed, pH, temperature, ulvan structure, concentration, and the presence of specific ions, as evidenced in various studies [76,77]. Generally, higher molecular weight ulvan typically exhibits enhanced gel formation, increased viscosity, and improved gel stability due to increased chain entanglement. These mechanisms are intrinsically linked to structural features where sulfate groups within ulvan play a crucial role, facilitating key intermolecular interactions, including hydrogen bonds and electrostatic interactions, which drive gelation. Furthermore, studies have demonstrated the capacity of ulvan to form gels in the presence of calcium or borate ions [74]. Ulvan chain entanglement also contributes to gel formation by trapping water molecules [65]. Nevertheless, a comprehensive understanding of these complex molecular interactions is crucial for optimizing their applications from therapeutic to material science [78].

Ulvan polysaccharides have demonstrated a range of potential therapeutic activities, including antibacterial, immunostimulatory, antitumor, antioxidant, antihyperlipidemic, antiviral, and anticoagulant effects [64,79]. However, the complex structural heterogeneity of ulvan makes it challenging to establish definitive structure-activity relationships, necessitating further research to fully elucidate the connection between structural characteristics and biological activities (Figure 2).

4. Life Cycle Assessment

Life cycle assessment (LCA) is an increasingly important methodology for evaluating the environmental impacts associated with different seaweed biorefinery designs. Comparative LCAs assess the environmental performance of seaweed-derived products against their fossil-based counterparts, aiming to provide insights into the sustainability and viability of seaweed. While the literature on LCA of Ulva biorefining remains scarce, most studies have focused on the environmental impact of Ulva sp. as a feedstock for biofuel and bioplastic production [80,81,82,83].

Generally, macroalgae-based chemical production facilities have been comprehensively investigated to displace fossil resources and mitigate climate change impacts [84]. The authors concluded that the production cost for integrated marine biorefinery products, projected at USD 2010 ton⁻¹, would be significantly lower than their respective market prices. This economic advantage makes an integrated marine biorefinery more advantageous in comparison with lignocellulosic terrestrial biorefinery systems. Furthermore, social life cycle assessment indicates that the displacement of animal-based protein, sugars, and minerals would result in the most significant avoided social impacts.

In the literature, one of the earliest assessments involved a hybrid solar-seaweed biorefinery system on a pilot scale (10-ton DW per hour capacity) based on Ulva cultivation [85]. The aim is to obtain multiple products such as protein, hydrochar, ethanol, distilled water, and electricity using both solar thermal energy and marine macroalgae as inputs. Although the estimated ethanol production from this system was limited, the scenario achieved 32% overall efficiency with a potential increase to 40% if waste streams were recycled. From an economic perspective, seaweed cost (about 90%) and transportation (more than 10%) are the main operating costs, while electricity generation (about 47%) and protein production (41%) are the key revenue drivers. The authors concluded that, under the selected baseline conditions, the hybrid biorefinery system is unlikely to be profitable due to feedstock production costs and product market prices.

To address the prohibitive cost of cultivated seaweed, another study recently assessed the environmental performance of converting excess beach-cast Ulva spp. through two thermochemical technologies, namely pyrolysis and hydrothermal liquefaction (HTL) [86]. The aim was to identify a promising path for processing beach-cast macroalgae while evaluating the environmental performance of thermal methods of bio-oil production compared to traditional fossil fuel production systems. The study found that both pyrolysis and HTL produce biofuels with lower greenhouse gas emissions compared to traditional fossil oil production. Furthermore, a sensitivity analysis revealed that the type of electricity used significantly impacts the environmental and climatic performance of both technologies. The authors concluded that changing the electricity source to renewable options further enhances the eco-friendliness of HTL, making it a more promising technology for sustainable biofuel production from beach-cast Ulva spp.

Another LCA study investigated the impacts of various disposal methods for managing the U. prolifera green tide [11]. The options analyzed included incineration, biological fast decomposition, stabilization through anaerobic digestion, and recycling to obtain oligosaccharides and fertilizers. Moreover, two potential disposal options for biofuel production (anaerobic digestion to produce biogas and fermentative distillation to produce ethanol) were assessed. Results from the scenario analysis indicate that recycling outperformed other options in terms of toxicity potential, highlighting the overall environmental advantage in terms of conserving metal resources and reducing greenhouse gas emissions. Despite these benefits, recycling requires more fossil fuels and fresh water, while leading to higher emissions of volatile organic compounds, acid gases, and particulate matter. Whereas, considering the alternatives to obtain biofuels, biogas production from U. prolifera offered significant environmental benefits in comparison with bioethanol production. Moreover, the estimated yields for bioethanol production were particularly low (i.e., 0.136 g bioethanol/g Ulva DW).

To conclude, while comprehensive LCA studies on Ulva biorefining are somewhat limited, existing literature consistently highlights a significant economic hurdle associated with cultivating macroalgae for industrial-scale applications. Consequently, the strategic utilization of beach-cast seaweed biomass overcomes the prohibitive costs associated with dedicated cultivation and harvesting operations. Such an approach not only mitigates coastal environmental issues but also positions marine biorefineries favorably against more established lignocellulosic terrestrial biorefinery systems in terms of economic advantage. Furthermore, within these integrated biorefinery frameworks, the recovery of high-value co-products is paramount for achieving robust economic returns and broader societal benefits. While current research has mainly focused on biofuel production, oligosaccharides are emerging as key candidates that may hold the highest market value among various seaweed-derived products. Therefore, the development and implementation of improved, more efficient, and environmentally benign methodologies for oligosaccharide recovery is necessary.

5. Chemical Modifications of Ulvan

Due to its distinctive chemical and molecular characteristics (Figure 2), optimizing ulvan utilization via chemo-enzymatic modifications is crucial for maximizing its application potential. As shown in Figure 4, specific functional groups on the ulvan saccharide backbone allow for the potential design of biomaterials through interesting chemical or biochemical modifications, in addition to the possibility of modifying the saccharide structure.

Specifically, the acidic nature of the -COOH group on the glucuronic acid unit and the -SO₃H group on the rhamnose moiety can be exploited to pursue various transformation strategies summarized in

Table 6. Main reactions for the modification of ulvan.

Reaction	Involved Moiety	Reactant	Reference
Oxidation	Saccharide unit	NaIO4; NaClO2	[88]
Condensation	-OH	Acetic anhydride	[89]
	-OH	Methacrylic anhydride	[90]
Covalent cross-linking	Oxidized saccharides	Chitosan	[91]
	Oxidized saccharides	Gelatine	[92]
	Oxidized saccharides	Chitosan	[93]
	-COOH; -OH	1,4-Butanediol diglycidyl ether	[94]
	Functionalized -COOH	Tyramine	[95]
	Functionalized -COOH	kappa-carrabiose disaccharide	[96]
Ionic cross-linking	-COO−; -SO3−	Boric acid	[97]
	-SO3−	Phospholipids	[98]
	-COO−; -SO3−	Chitosan	[99]
		Soy protein	[100]

The physicochemical and/or biological properties of ulvan polysaccharides are altered by such modifications. Consequently, this enables the development of both self-assembled products and complex structures in conjunction with other macromolecules, extending its application scope [87].

To prepare materials with enhanced crosslinking properties, one of the proposed strategies is based on the oxidation of saccharides to introduce aldehydic groups. The oxidation reaction can be performed with NaIO₄ according to the procedure described by de Carvalho [88], eventually followed by further oxidation with NaClO₂ to obtain carboxylic groups. Periodate oxidation resulted in the oxidation of up to 23.0% of the total carbohydrates, accompanied by a reduction in uronic acids from 22.4% to 12.5% and a decrease in xylosyl units. Subsequent overoxidation with chlorite yielded di-/tri-carboxylated units within the polysaccharide backbone. The resulting polycarboxylic ulvan polysaccharides were assessed for anticoagulant activity, which was found to be 7.7–17.9 times higher than that of native ulvan.

A second strategy is to take advantage of the aldehydic groups along the ulvan chain, which paves the way for the interaction with amino groups from different substrates and polymers. This was found to be the case in the formation of Schiff base hydrogels obtained by the reaction of dialdehyde algal polysaccharides with gelatin or chitosan [91,92]. The former example shows the possibility of increasing the mechanical properties of gelatin hydrogels by using dialdehyde polysaccharides as crosslinkers instead of the traditional chemical ones. The same application was explored by using chitosan and functionalized polysaccharides as substitutes for glutaraldehyde and genipin [91]. Functional composite hydrogels were also prepared using a Schiff base formed from the reaction of chitosan with ulvan dialdehyde, in the presence of dopamine to facilitate the in-situ reduction and generation of silver nanoparticles, and were loaded with human umbilical cord mesenchymal stem cells to enhance chronic wound healing [93]. The resulting materials demonstrated the capacity to promote wound healing in chronic diabetes, attributed to the combined action of the ulvan derivative-driven functional hydrogels and the active factors released by the human umbilical cord mesenchymal stem cells.

Another strategy involves an initial amidation reaction to graft tyramine moieties onto the pendant carboxyl groups of ulvan. The resulting tyramine-functionalized polysaccharide chains were then utilized to prepare hydrogels in the presence of H₂O₂ and horseradish peroxidase as a catalyst to facilitate the coupling of hydroxyphenyl compounds. The covalent bonds can be formed between the carbon atoms at the ortho position to the hydroxyl group or the carbon atom at the ortho position to the oxygen atom of the phenol group. The enzymatically crosslinked gels that were produced demonstrated the capacity to function as a vehicle for viable cells, indicating their potential suitability for application as injectable cell delivery systems [95]. The same approach has been employed in the functionalization of ulvan with alkylamines such as 1,3-diaminopropane and 1,6-diaminohexane, selectively inserted into ulvan carboxylic units [96]. Through reductive amination, the resulting amides reacted with kappa-carrabiose, generating hybrid ulvan–kappa-carrabiose polysaccharides. These modified polysaccharides displayed good biocompatibility in vitro and are considered promising candidates for the development of tailored scaffolds in biomedical applications.

Hydrogels can also be prepared by exploiting cross-linking reactions on ulvan themselves, without prior modifications. A series of novel hybrid scaffolds based on crosslinked ulvan and gelatin was prepared to design biomaterials for tissue engineering by using 1,4-butanediol diglycidyl ether in alkaline and acidic environments [94]. This result was obtained by exploiting the crosslinkers’ ability to interact with the hydroxyl and carboxyl groups of the ulvan backbone. Furthermore, ionic cross-linking strategies have been pursued to obtain hydrogel films of ulvan interacting with boric acid using glycerol as a plasticizer [97]. Complex films were also successfully prepared based on chitosan and ulvan through ionic crosslinking and demonstrated to possess antioxidant and whitening abilities [99]. Ulvan polysaccharides have also been sought to enhance the properties of soy protein-based films by employing high-pressure homogenization to obtain ionic cross-linked materials possessing distinctive bioactivities [100]. Ionic interaction also lies behind the preparation of liposomes starting from phospholipids and ulvan [98]. Ulvan-containing charged liposomes readily occurred upon the addition of the polysaccharide solutions to lipid films containing charged phospholipids. The resulting nanometer-scaled liposomes enabled the entrapment of fusidic acid, leading to improved antibacterial activity relative to unencapsulated acid.

Methacrylated ulvan was used as an additive to traditional gelatin methacryloyl-based bioink for 3D printing, employed in skin tissue engineering for manufacturing living constructs [90]. The inclusion of functionalized ulvan in the bioink facilitated the extrusion printing process by reducing yield stress. On the other hand, acetylated ulvan polysaccharide exhibits the capacity for self-assembly into nanogels, which, due to their amphiphilic nature, can function as self-organizing assemblies for the conveyance and delivery of water-insoluble bioactive compounds [90].

The chemical versatility of ulvan polysaccharides, stemming from their distinct molecular characteristics and functional groups, enables a wide array of chemo-enzymatic modifications. Strategies such as oxidation, amidation, and cross-linking have proven effective in altering the physicochemical and biological attributes of ulvan, leading to the development of self-assembled structures, hydrogels, and complex composites with other macromolecules. Further research is required to refine and optimize these modification techniques, thereby enhancing ulvan’s potential for advanced materials design.

6. Conclusions

The successful implementation of Ulva-based biorefineries hinges on effectively addressing the inherent variability in biomass composition, which significantly impacts economic viability through fluctuations in product yields and purities. The development of integrated, multi-product biorefinery strategies is therefore paramount to ensure efficient biomass valorization, mitigate economic risks, and align with circular economy principles, ultimately fostering a more sustainable and economically robust bio-based industry. The inherent versatility of Ulva biomass lends itself exceptionally well to a cascading biorefinery approach, wherein various fractions are sequentially extracted and valorized. Ulva spp. are of particular interest due to their unique polysaccharide component, ulvan, alongside the significant potential to extract other valuable compounds such as proteins, lipids, starch, and a diverse array of macro- and microelements. Achieving the efficient extraction of these components necessitates a comprehensive, multi-stage approach. Current research indicates that industrially promising pathways for valorization involve initial mild aqueous extraction for soluble components like salts and ulvan, followed by protein recovery, and subsequent green solvent extraction for lipids. Techniques such as membrane filtration also show high potential for subsequent purification steps, ensuring the recovery of high-purity fractions. Ulvan, a unique polyanionic oligosaccharide and a major polysaccharide component of Ulva spp., presents a particularly compelling target for biorefinery development due to its diverse application potential across fields such as biomedicine, food, and biomaterials. Specifically, promising chemical modifications include sulfation to enhance anticoagulant properties, periodate oxidation for hydrogel formation, and targeted conjugation with other biopolymers to develop advanced composite materials, thereby expanding their range of applications. Future research should prioritize the optimization of these integrated extraction and fractionation protocols, focusing on minimizing energy consumption and waste generation through the adoption of more sustainable approaches like pressurized liquid extraction or the more expensive enzyme-assisted extraction. The integration of life cycle assessment (LCA) early in the development of these biorefinery processes is crucial to ensure their comprehensive environmental and economic sustainability. Additionally, continued exploration of novel chemical and enzymatic modification strategies will be vital for tailoring ulvan’s properties to transform environmental challenges into economic opportunities.