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medium; thus less wastewater is generated at the end of the process. Furthermore, algae cultivation is possible with non‐potable water and using the sunlight, which contributes to a more ecological and economical process [68].
Currently, there are more than 30 000 known species of microalgae [175] that are present in different evolutionary lines and have contrasting ecological requirements, meaning this group has an enormous metabolic diversity and great potential for biotechnology. In fact, microalgae production is an emergent market with an expected yearly growth of 10% [6]. Besides the use of biomass as feed in aquaculture and livestock production [176,177], microalgae are the source of high‐value products, such as natural pigments (e.g. β‐carotene and astaxanthin), polyunsaturated fatty acids, proteins, and antioxidants, that are commercialized mainly for the nutraceutical [178] and skin care [179] industries. Moreover, microalgae are increasingly investigated as a new sustainable and environmentally friendly alternative to fossil fuel resources, since they accumulate high amounts of lipids and carbohydrates that can be used as feedstock for biofuel production [180,181]. However, almost all commercial microalgae products are obtained from the biomass, and only recently their relevance as producers of valuable polysaccharides has started to be considered.
2.6.1 Polysaccharide‐Producing Microalgae
Microalgae are a large group of photosynthetic unicellular or multicellular organisms, which includes both prokaryotic (Cyanophyta, i.e. cyanobacteria/blue‐green algae) and eukaryotic organisms, belonging to the phyla Chlorophyta (green algae), Rhodophyta (red algae), Chrysophyta (diatoms), and Pyrrophyta (dinoflagellates). Polysaccharide‐producing microalgae are found in all microalgae phyllo (Table 2.3), and their EPS are characterized by complex chemical structures generally with a high diversity of sugar monomers in the same macromolecule, including rare sugars such as fucose, rhamnose, and ribose, which are known to confer the biopolymers' biological activity [6]. Examples include the EPS secreted by Arthrospira platensis [43], Porphyridium marinum [40], and Rhodella sp. [46]. Fructose was also found in EPS secreted by Dunaliella salina [72]. Of notice is the presence of methyl‐derivate sugars (e.g. Dictyosphaerium chlorelloides) [71] and uronic acids, mainly glucuronic and galacturonic acid [182,184]. As can be seen in Table 2.3, sulfate groups are also found within algal EPS (sEPS), which further contributes to their unique properties [6].
Table 2.3 Polysaccharide‐producing microalgae and polysaccharide characterization.
| Organism | Sugar composition | Other | Mw (Da) | Bioactivity and applications | References |
|---|---|---|---|---|---|
| Rhodophyta | |||||
| Porphyridium sp. | Xyl, Gal, Glc, GlcA | Sulf | 2.4 × 105 to 1.8 × 106 | Anti‐inflammatory, antioxidant, hypocholesterolemic, biolubricant | |
| Porphyridium purpureum | Gal, Xyl, Glc, GlcA, Fuc | Sulf | n.a. | Antiviral, antimicrosporidian activity | [42,43] |
| Porphyridium cruentum | Gal, Glc, Ara, Man, Fuc, Xyl, Rha | Protein, sulf, UA | n.a. | Antibacterial, antiviral, antiglycemic | [44,45] |
| Rhodella sp. | Xyl, Gal, Glc, Rha, Ara, GlcA | Sulf | n.a. | n.a. | [46] |
| Rhodella maculate | Gal, Xyl, GlcA, Rha, Ara, Glc | Protein, sulf | n.a. | n.a. | [43] |
| Rhodella reticulata | n.a. | n.a. | n.a. | Antioxidant | [47] |
| Rhodella violacea | Gal, Xyl, Glc, GlcA, Rha, Ara | Protein, sulf | n.a. | n.a. | [43] |
| Pyrrophyta (dinoflagellates) | |||||
| Cochlodinium polykrikoides | Man, Gal, Glc | UA, sulf | n.a. | Antiviral | [48] |
| Gymnodinium sp. | Gal | Sulf, lactic acid | 1.3 × 106 | Antitumor activity | [49,50] |
| Gyrodinium impudicum | Gal | Sulf, UA | 1.9 × 107 | Immunomodulatory and antitumor activity | |
| Cyanophyta (cyanobacteria) | |||||
| Aphanothece halophytica | Glc, Fuc, Man, Ara, GlcA | — | 2.0 × 106 | Adjuvant activity, antiviral, anticancer | |
| Anabaena augstmalis | Glc, Gal, Man, Xyl, Fuc, Rha, GalN, GlcN, GalA, GlcA | Sulf | n.a. | n.a. | [58] |
| Arthrospira platensis | Rha, Gal, Glc, Fuc, Xyl | Sulf | n.a. | Antiviral, antibacterial, antioxidant, anticoagulant, skin repair | [42, 59–63] |
| Microcoleus vaginatus | Glc, Gal, Ara, Xyl, Man, Rha, Fuc, GalA, GluA | Protein | n.a. | [64] | |
| Nostoc sp. | Glc, Gal, Xyl, Rha, Man, GalA, GlcA |
Protein
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