Insights into Structural and Molecular Variations of Alginates and Fucoidans Isolated from Padina australis Under Different Mechanical and Enzymatic Conditions

A series of extraction steps were carried out to evaluate the process of isolation of fucoidans and alginates from Padina australis in a single operation line under conventional, mechanical, and biological conditions. A broad range for amounts of polysaccharides were obtained for fucoidans and alginates (24.7–52.1%) with yield increasing by more than two-folds through enzymatic process. The color attributes, including L^* (lightness), a* (greenness-redness), and b* (blueness-yellowness), of polysaccharides isolated using enzyme assistance was favorably higher than conventional extractions. Fucoidans indicated a rather heterogeneous molecular weight distribution (251.4–1358.7 × 10³ g/mol) than alginate molecules (1450.1–1933.1 × 10³ g/mol). Galactose (32.7–44.70%) and fucose (21.5–38.3%) were predominant in fucoidan molecules negatively charged by varying levels of sulfates (10.0–16.5%) and carboxyl groups (0.5–6.3%). Isolated polysaccharides effectively and differently scavenged DPPH and ABTS free radicals and reduced Fe³⁺ to Fe²⁺. While both fucoidans and alginates induced proliferation in RAW264.7 cells, only fucoidans stimulated the macrophage cells to release nitric oxide. Polysaccharide chains of the most active fucoidan was formed of → 6)-Glcp-(1 → , → 2)-Fucp-(1 → , Galp-(1 → , → 4)-Glcp-(1 → , and → 3,4,6)-Galp-(1 → residues. Overall, the employment of enzymes in the extraction process improved extraction efficiency, enhanced color attributes, retained structural integrity, and resulted in significantly higher antioxidant and immunostimulation capacities. Greater efficacy of isolated polysaccharides could be correlated to collective effects of sulfates, molecular weights and sugar composition.