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Sulfated galactans is matrix polysaccharides with sulfate group and found in algae cell wall, especially, in the cell wall of green and red algae. It is a polymer that composed of sulfated α-D(L)-galactopyranosyl β-D-sand subunits (Figure 2.2B). Sulfated galactans in the cell wall of green algae have both types of hetero and homopolysaccharides and it could be varying among the species. The red algae cell wall contains unbranched, alternative linked (1–3) β and (1–4) α-D-galactans. There are also some functional moieties been attached to algal polysaccharides such as, sulfate hemiesters, pyruvic acid and methyl ester. Methyl and sulfate moieties occasionally attached to the position O-4 and O-2 of β (1–3) galactans and pyruvic acid attached to the β (1–3) D-Galactans at position C-6. In algal polysaccharides, there are good balance between hydrophilic moiety such as, sulfate and hydrophobic like, methoxyl group. The algal enzymes have these abilities to equilibrate the proportion between these two functional groups prior to their biological deigning [36, 37].

Figure 2.2 Figure illustrates different polysaccharides molecules in the algal cell wall. (A) is alginate, having β (1–4) linked D-mannuronic acid subunits with epimerically attached α-L-guluronic acid at C-5 position. (B) Fucoidan is sulfated α (1–3) (1–4)-linked un sulfated and 2-sulfated-α-L-fucose residues. (C) Sulfated galactans which composed of 3-sulfated with 4-linked α-D(L)-galactopyranosyl β-D-sand subunits [62, 63].

Red algae glycans are structurally different among the species depends upon harvesting season and location. Functionally, the algae produce sulfated polysaccharides for the adaptation to high salinity marine environment and it’s also found in the other marine organisms like, angiosperms and invertebrates. During high saline condition it for 3D hydrogel network that can retain a large amount of water, thus protect the macroalgal from desiccation. Moreover, the attached sulfated group have natural tendency to retain the divalent metals such as, Ca²⁺ and Mg²⁺ [33].