Brown seaweeds are marine bioresources rich in bioactive compounds such as carbohydrates, proteins, pigments, fatty acids, polyphenols, vitamins, and minerals. Among these substances, brown algae-derived polysaccharides (alginate, fucoidan, and laminarin) have promising industrial prospects owing to their distinctive structural features and diverse biological activities.
Introduction
Phaeophyceae, commonly known as brown algae, are a large and diverse group of multicellular, photosynthetic organisms that belong to the kingdom Chromista. Brown seaweed, comprising approximately 250 genera and 2000 species, is a significant part of intertidal zone biomass, notably from predominant genera such as Sargassum, Ascophyllum, Laminaria, and Macrocystis. As a foundational group, brown algae species hold immense ecological value, providing essential habitats, nursery grounds, and shelter for numerous marine species while also playing a key role in carbon cycling. Their name is derived from their distinctive greenish-brown color, which results from the fucoxanthin pigment in their chloroplasts and from specific tannins found in their cell walls that mask residual pigments.
For centuries, marine brown algae have been an integral part of oriental culture, especially in the Asian-Pacific region. Their widespread use is owing to their dietary, medicinal, and functional food properties. Today, seaweeds have become a globally important economic resource, especially in East and South Asia, where they are extensively cultivated. Much attention has been paid to seaweeds as a valuable source of structurally novel and biologically active metabolites with a significant industrial potential, particularly polysaccharides. As the principal structural polysaccharides in brown seaweed, fucoidan, alginate, and laminarin exhibit notable heterogeneity in their physicochemical properties and structural characteristics that directly influence their biological activities. This diversity depends on several factors such as the species of algae, growth environment, harvesting season, and extraction process. Consequently, such variability poses substantial challenges in terms of pretreatment, extraction, characterization, and evaluation of bioactivity.
Global Seaweed Production
Seaweed production originated approximately 1700 years ago in China for domestic purposes such as food, feed, and medicine prior to its adoption for industrial applications. Over the years, the exponential growth of the global seaweed industry led to the cultivation and harvesting of a wide variety of seaweed species. This diversification is a direct response to market pressures seeking novel value-added products. However, the rapid expansion and transition from artisanal collection to large-scale commercial farming raise critical questions regarding its environmental and socio-economic sustainability, particularly for coastal countries in Asia.
According to statistics from the Food and Agriculture Organization, the global market share of seaweed farming production increased by approximately 203% between 2000 and 2019, reaching over 35 million tons per year. Recent studies (2020) have indicated that the algae industry, primarily dominated by Asia (97% of the total production), is expected to grow by 137% by 2027, increasing its value from 40 billion to 95 billion dollars. The leading Asian producers are China, Indonesia, Korea, and the Philippines, while recent growth has also been observed in Europe (0.8%, e.g., Ireland, Norway, and France) and the Americas (e.g., Chile, Canada, and Mexico). However, increasing production sustainably requires the optimization of cultivation systems. The expansion of the seaweed sector is largely attributed to the increasing global demand for brown seaweed biomass and its bio-based derivatives. Statistical records indicate an average annual increase of around 11%, with production volumes expanding from 13,000 tons in 1950 to 17.6 million tons in 2021. Today, 96.5% of the total global production of brown seaweed comes from aquaculture. This expanding biomass is crucial to respond to the market needs for valuable polysaccharides. For instance, the bioactivities of fucoidan have positioned it as a compound of notable commercial value, with a global market valuation of USD 30 million in 2022. Similarly, the alginate industry was valued at USD 728.4 million in 2020, with an expected annual growth rate of 5% until 2028. Microalgae can be farmed offshore in artificial ponds or wildly harvested from natural beds (Table 1). The choice of cultivation technique is largely determined by the biological requirements of the target species and its optimal growth environment.
| Cultivation Method | Advantages | Disadvantages | Species |
| Offshore seaweed farming | High environmental control (nutrients, temperature, light, pH, and pathogens) Stable productivity (yield, growth cycles) Lower physical risks (storms, currents) | Substantial operational costs (water, energy, and ponds) External input dependency (fertilizers, treatments) Limited scalability (available area, cost) | Sargassum horneri Cystoseira barbata Himantothallus grandifolius |
| Onshore seaweed farming | Lower production costs Natural resources (nutrients) Unlimited space | High environmental risks (storms, waves, and grazing) Yield variability Logistical challenges (monitoring, harvesting, and maintenance) | Sargassum muticum Dictyota menstrualis Turbinaria ornata |
Sample Processing
The postharvest handling of seaweed biomass is a critical factor influencing all subsequent valorization pathways. As illustrated in Figure 1, the standard processing chain involves key unit operations such as washing, drying, and size reduction. The choice of processing method primarily depends on the algal material, the intended end product, and the socio-economic objectives of production.
Washing
Unwashed algal biomass contains a wide range of natural contaminants including sediment (e.g., mud, sand), epibionts (e.g., small invertebrates), and associated macro algal species. Washing with sea or tap waters is an important pretreatment step strongly recommended to reduce the salt content and eliminate adhering impurities. The washing conditions for each macro algal species should be optimized to preserve color and texture regardless of whether freshwater or seawater was used.
Drying
Seaweeds, with a moisture content exceeding 90-95%, are considered highly perishable material. Drying is a major operation in the seaweed industry, as it extends the shelf life and improves storage efficiency, transport, and distribution, as well. Substantial progress has recently been made in drying technologies to satisfy the growing demands of the industry. Currently, two main drying technologies are commercially emerging: (i) natural sun drying, which is a relatively simple and low-cost technique, but its dependence on weather conditions affects both the quality and sanitary standards of the final product, hindering its application in some industrial sectors; and (ii) mechanical drying using industrial thermal drying systems, which, despite their fast processing rates, are constrained by limited throughput, high energy consumption, and degradation of key nutrients. As a promising alternative, effective freezing ensures good preservation of the organoleptic and nutritional quality of seaweed, particularly for high-value applications that require fresh-like attributes.
Milling or Size Reduction
For extraction purposes, dried seaweeds are typically crushed or manually ground using a mechanical mill or blender to obtain a homogeneous mass and increase the surface-to-volume ratio. Milling methods involve the production of particles with a wide range of sizes from millimeters to microns. Finally, sieving is applied to separate algal particles into different size fractions and obtain the desired particle size. However, the requirement for this mechanical pretreatment is highly species-specific. In taxa having fragile cell wall structures, the extraction medium led to effective cell disruption, thereby requiring less mechanical energy or potentially removing the necessity for prior milling.
Genus and Species Delimitation
Accurate algal species identification is crucial for ecological studies, biodiversity conservation, and sustainable management of marine resources, as well as for ensuring effective traceability and quality control in the commercialization of seaweed-based products. The challenges associated with the morphological species concept revealed significant limitations. In algae, phenotypic plasticity, polymorphism, and convergent evolution are common phenomena, creating marked discrepancies between the morphologically defined species and those delimited by phylogenetic or reproductive isolation criteria. Moreover, cryptic species are frequently observed in diverse lineages. In addition, morphological diagnostic keys may vary depending on the life stage or sex of the algae. Ultimately, the reliable application of morphological keys requires advanced taxonomic expertise for accurate identification.
To remove such constraints, integrative approaches that combine morphological characteristics with DNA data (nuclear, mitochondrial, and/or plastid) have been increasingly adopted in taxonomic and ecological research to complement classical morphological identification methods.
DNA-based tools, such as DNA barcoding, have proven to be a practical method for marine flora studies, making simple, rapid, and cost-effective identification of novel species. As illustrated in Figure 2, this approach involves the sequencing of a target gene from accurately authenticated specimens to create a reference database and facilitate the identification of unknown species via a comparative sequence analysis with/of this library. With current advances in molecular technologies, multiple validated DNA barcodes are available in accessible sequence databases. DNA barcoding markers are derived from distinct cellular compartments: (i) nuclear genomes, providing ribosomal RNA genes (18S/28S) and the ribosomal internal transcribed spacer region (ITS1-5.8SrDNA-ITS2); (ii) chloroplast genomes, featuring the widely adopted rbcL gene (ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit) and psaA/psbA (photosystem I P700 chlorophyll Apo protein A1); and (iii) mitochondrial genomes, particularly the COI-5P (cytochrome c oxidase subunit I 5′ region) and the cox2-3 spacer (cytochrome c oxidase subunit II-III intergenic spacer).
Marine Brown Algae Polysaccharides: Biodiversity and Chemical Structure
Brown seaweeds are characterized by a high content of polysaccharides that serve as structural cell wall constituents and energy-reserve storage compounds. Specifically, the brown algal intercellular matrix and cell walls are predominantly composed of alginate and fucoidan, which are both critical for thallus support in water. The primary storage of the polysaccharide laminarin is a direct product of photosynthesis and is sequestered within plastids as a carbon reserve for core metabolism. As major components, brown seaweed polysaccharides, including sodium alginate, fucoidans, and laminarin, exhibit significant structural and chemical heterogeneity. This heterogeneity is influenced by many factors such as species-specific traits, geographic location, harvesting season and extraction methods. The substantial variability among these polysaccharides presents great industrial challenges for valorization and innovation.
Alginate
Alginate, which accounts for approximately 40% of the dry weight, is the primary polysaccharide in the cell wall and intercellular matrix of brown algae. It naturally occurs as an insoluble mixed salt of alginic acid complexed with various cations, mainly Ca2+, Na+, Mg2+, and K+, along with other ions normally existing in seawater. Structurally, alginate is a linear and unbranched polysaccharide with a molecular mass larger than 50 kDa. Its structure consists of β-D-mannuronic acid (M) and α-L-guluronic acid (G), linked through (1,4)-glycosidic linkages. These monomers are arranged in homogeneous (MM or GG) or heterogeneous (MG) blocks (Figure 3). The monomeric ratios (M/G) of sodium alginate vary in different species of brown algae, with reported values of 0.52 for Sargassum dentifolium, 0.77 for Cystoseira compressa, 1.12 for Laminaria digitata, and 1.5 for Padina gymnospora. The relative proportions of M and G, as well as block distributions (MM, GG, MG, and GM), influence the physicochemical properties of alginate. For example, alginate with high-M blocks showed higher viscosity, whereas high-G alginate shares better gelling properties. Alginate’s gelling ability is intrinsically linked to its anionic polyelectrolyte nature. The process is initiated by the exchange of sodium ions from guluronic acid residues with divalent cations, leading to the formation of cross-linked junctions described by the «egg-box» model. As a direct result, its rheological behavior in an aqueous solution, including flow and viscosity, is highly dependent on pH and ionic strength.
Fucoidan
Fucoidan is a sulfated polysaccharide commonly found in brown seaweeds and some lower plants and typically yields up to 30% of dry weight. As an alginate, fucoidan is essentially a structural component of the intercellular matrix where it plays a crucial role in maintaining cellular integrity and providing protection against environmental stresses. The relative molecular weight of fucoidan generally varies from about 7 kDa to 2379 kDa. The fucoidan backbone is highly variable among species primarily composed of fucose with additional monosaccharide residues (such as mannose, galactose, arabinose, glucose, xylose, and uronic acids). Generally, α-(1,3) and α-(1,4) glycosidic bonds constitute the main chain of the macromolecule (Figure 4). A major structural feature of fucoidan is its specific substitution pattern characterized by sulfate groups, particularly those attached at the C-2, C-4, and/or C-3 and the O-acetylation. These functionalities confer a strong polyanionic character and enhanced chain stiffness, which profoundly influence the molecule’s physicochemical behavior, particularly its conformation in aqueous media and its interactions with water and other polymers. From a techno-functional standpoint, the polyanionic nature of fucoidan allows for interactions with counter-ions and cationic species (e.g., proteins, polysaccharides, or mineral ions), thereby modulating the rheology, stability, and texture of dispersions, emulsions, and gels, as well. Rheologically, fucoidan solutions and blends typically exhibit non-Newtonian, shear-thinning (pseudoplastic) behavior, where the apparent viscosity decreases as the shear rate increases.
Laminarin
Laminarin, a water-soluble β-glucan, is another major storage polysaccharide found in brown algae, particularly in Laminaria, Saccharina, Ascophyllum, Fucus, and Alaria species. Laminarin is mainly composed of β-(1,3)-β-D-glucan backbone with some 6-O-branching, glucose, and β-(1,6)-intrachain link. The ratio of β-(1,3) to β-(1,6) linkages vary depending on the type of algae. Additionally, laminarin is structurally classified into two forms based on the type of sugar at the reducing end: M-chains, featuring a terminal D-mannitol (1-O-substituted), and G-chains, which end with glucose (Figure 5). The average molecular weight of laminarin is approximately 5 kDa, depending on the degree of polymerization. Furthermore, Laminarin represents around 22-49% of algal dry matter.
Conclusions and Future Perspectives
Brown algae represent an approachable and abundant source of functional polysaccharides. Their biocompatible, biodegradable, and non-toxic properties have garnered significant industrial interest. Unlocking this potential requires the development of optimized and integrated processing value chains that simultaneously ensure product quality, cost-effectiveness, and environmental sustainability. This critical requirement has driven considerable innovation, resulting in advanced extraction, purification, and characterization techniques. Now, sustainable technologies (e.g., UAE, MAE, and SFE) and green solvents offer promising alternatives to conventional processing methods. Addressing these challenges through interdisciplinary collaboration represents a critical opportunity to unlock the full potential of brown algal polysaccharides for sustainable industrial development.
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