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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	n.a.	n.a.	[65]
Nostoc flagelliforme	Glc, Xyl, GlcA, Gal	—	n.a.	Antiviral	[66,67]
Phormidium autumnale	Rha, Rib, Man, Glc, Fuc, Gal, Ara, GalA, GlcA	Sulf	n.a.	n.a.	[58]
Synechocystis aquatilis	Fuc, Glc, Rha, Xyl, Man, GlcN, GalA, GlcA	Sulf	n.a.	n.a.	[58]
Chlorophyta
Chlamydomonas reinhardtii	GalA, Rib, Ara, Xyl, Glu, Gal, Rha	Pyr	2.3 × 105	Antioxidant	[68]
Chlorella stigmatophora	Glc, GlcA, Xyl, Rib/Fuc	Sulf, UA	22 × 103	Anti‐inflammatory, immunomodulatory (immunosuppressant)	[69]
Cyanobacterium aponinum	GalA, Fuc, 3‐OMe‐GalA, Glc, Ara, Gal, Man, Rha, 4‐OMe‐GlcA	—	1.06 × 106	Immunomodulatory	[70]
Dictyosphaerium chlorelloides	Gal, Me‐Gal, Rha, Man, Me‐hexose, Glc, Me‐Glc, Xyl, Me‐Xyl, Ara	Protein	9.6 × 105	Antiproliferative immunostimulation of pro‐ and anti‐inflammatory cytokines	[71]
Dunaliella salina	Glc, Gal, Fru, Xyl	—	n.a.	n.a.	[72]
Dunaliella tertiolecta	Glc	—	n.a.	n.a.	[73]
Graesiella sp.	Fuc, Gal, Ara, Glc, Man, Xyl, Rib, Rha	Sulf, UA, protein	n.a.	Antioxidant, antiproliferative	[74]
Haematococcus pluvialis	Rib, Ara, Man, Glc	Acetyl and amino groups	23 × 106	Antiaging, immunomodulatory	[75]
Parachlorella kessleri	Ara, Rha, Xyl, Man, Gal	—	65 × 103	Antitumor, immunomodulatory	[76]
Chromophyta (diatoms)
Navicula directa	Fuc, Xyl, Gal, Man, Rha, Glc, GlcA	Protein, sulf	2.2 × 105	Antiviral	[77]
Phaeodactylum tricornutum	Glc, GlcA, Man	Sulf, UA	27 to 449 × 103	Anti‐inflammatory, immunomodulatory (immunostimulatory)	[69]

n.a., not available; Ara, arabinose; Fuc, fucose; Fru, fructose; Gal, galactose; GalA, galacturonic acid; GalN, galactosamine; Glc, glucose; GlcA, glucuronic acid; GlcN, glucosamine; Man, mannose; Me, methyl derivatives; Pyr, pyruvate; Rha, rhamnose; Rib, ribose; Sulf, sulfate; UA, uronic acids; Xyl, xylose.

There are few studies where the glycosidic bonds and polysaccharide structure were evaluated. Examples include (i) spirulan, the polysaccharide produced by A. platensis, which is composed of two disaccharide repeating units (→3)‐α‐L‐Rha‐(1→2)‐α‐L‐Aco‐(1→, where Aco (acofriose) was a sulfated 3‐O‐methyl‐Rha, and O‐hexuronosyl‐Rha (aldobiuronic acid) [185]), and (ii) nostoflan, a polysaccharide from Nostoc flagelliforme composed of →4)‐β‐D‐Glc‐(1→4)‐D‐Xyl‐(1 and →4)‐[β‐D‐GlcA‐(1→6)‐]‐β‐D‐Glc‐(1→4)‐D‐Gal‐(1→ [66].

It should be noted that the chemical composition, type of linkage, sulfate content, and position might be significantly different depending on the species, the cultivation and extraction conditions, and the analytical methods employed [182,183,186,187]. As so, their biological activity can be significantly different. For example, the molecular weight of the EPS produced by Porphyridium cruentum influenced the immunomodulatory activity, with the polymer with lower molecular weight (Mw) having the strongest immunoenhancing effect [188].