Extraction-driven structural modulation of fucoidans from Sargassum assimile enhances antioxidant and immune-stimulating activities

Fucoidans are multifunctional marine polysaccharides with promising therapeutic potential, yet their structure–function relationships remain insufficiently understood. This study investigates the structure–function relationship of fucoidans extracted from Sargassum assimile using different extraction conditions and subsequent molecular weight fractionation. Fucoidans were extracted using distilled water and 0.025 M hydrochloric acid (2, 4 and 8 h), followed by ultrafiltration to fractionate polysaccharides (2, 10 and 30 kDa). Acidic extraction significantly increased yield, sulfate content and fucose levels compared to aqueous extraction. Prolonged acid exposure reduced molecular weight and altered conformation. Fractionation demonstrated that lower molecular weight fractions (2–10 kDa) exhibited superior antioxidant activity, while medium fractions (10–30 kDa) enhanced immunostimulatory effects, inducing nitric oxide production in RAW264.7 macrophages. Structural analysis highlighted that sulfate content and molecular weight are important factors influencing these biological properties. Overall, these findings suggest that controlled extraction and fractionation strategies can be applied to tailor the biological functions of fucoidans, thereby paving the way for the sustainable development of high-value functional ingredients from S. assimile within the emerging blue bioeconomy. • Acidic extraction boosts fucoidan yield (6.9 %) and sulfate content (17.4 %). • Low-MW fucoidans (2–10 kDa) show highest antioxidant activity (61.19 %). • Medium-MW fucoidans (10–30 kDa) enhance macrophage NO production (26.54 μM). • Controlled HCl extraction reduces MW while preserving bioactivity. • Ultrafiltration enables targeted fractionation by molecular weight.