Abstract: In order to explore the mechanism by which Lactobacillus fermentation enhances the bioactivity of Saccharina japonica, this study examined changes in the physicochemical properties, structural characteristics, and bioactivities of fucoidan (Fuc) from S. japonica during fermentation with Lactobacillus and analyzed its structure-activity relationship. The results showed that Lactobacillus fermentation significantly increased the total sugar content of Fuc and changed the contents of sulfate and uronic acid as well as the monosaccharide composition of Fuc. Fuc fermented for three days (F3) contained more fucose (38.07%) compared to 31.06% for unfermented Fuc (NF). The antioxidant, hypolipidemic, and immunoregulatory activity of Fuc were all increased after fermentation. Fuc fermented for one day (F1) had the highest antioxidant activity, with scavenging capacity of (44.89 ± 0.60)% and (77.48 ± 0.46)% against 2,2’-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) cation radical and 1,1-diphenyl-2-picrylhydrazyl radical compared to (42.80 ± 0.83)% and (71.76 ± 1.68)% for NF, respectively. F3 had the highest hypolipidemic and immunoregulatory activities, with cholesterol binding capacity of (71.32 ± 1.53)% compared to (47.67 ± 2.80)% for NF, respectively. The immunoregulatory activity of F3 (200 μg/mL) was 1.15 times as high as that of NF at the same concentration. The structure-activity relationship analysis showed that the contents of fucose and uronic acid were significantly positively correlated with the bioactivity of Fuc (r > 0.75). This study provides a theoretical basis for the development of health-promoting fermented S. japonica products.

Key words: fucoidan; fermentation; Lactobacillus; structural characteristics; bioactivities; structure-activity relationship