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[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].

2.6.2 Biological Activity and Potential Applications

Due to the structural and chemical diversity of microalgal EPS, they have been the subject of recent research on their biological activity and potential application in the fields of biomedicine, pharmaceuticals, cosmeceuticals, and therapeutics [189]. Reported bioactive properties are presented in Table 2.3 and include anti‐inflammatory, immunomodulatory, antioxidant, antiviral, antibacterial, antitumor, and antihyperlipidemic activity, as well as anticoagulant and/or antithrombotic properties [183,184,189,190].

2.6.2.1 Antiviral Activity

Generally, polysaccharides rich in sulfate groups have antiviral capacity. These molecules seemed to be able to block the binding of the virions to the host cell surfaces [77,183,191] and inhibit reverse transcriptase in human immunodeficiency virus (HIV), interfering with the replication and the production of new viral particles [183,191,192]. For example, sulfur‐containing EPS from Arthrospira presented antiviral activity against numerous viruses, including human cytomegalovirus (HCMV), antihuman immunodeficiency virus (anti‐HIV), herpes simplex virus type 1 (HSV‐1), human herpes virus type 6 (HHV‐6), measles virus, mumps virus, influenza A virus, ectromelia virus (ECTV), and vaccinia virus (VACV) [42]. P. cruentum secreted an EPS that had a higher antiviral activity against vesicular stomatitis virus than the chemical compounds ribavirin, brivudine, cidofovir, and ganciclovir [45].

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