The physicochemical properties, structural characteristics, antioxidant, radioprotective and lipid-lowering activities, as well as the underlying structure-activity relationships of polysaccharides extracted from Nostoc flagelliforme grown under normal (WL-EPS-1), salt stress (NaCl-EPS-1) and mixotrophic culture conditions (Glu-EPS-1) were studied. The results demonstrated that WL-EPS-1, NaCl-EPS-1 and Glu-EPS-1 were heteropolysaccharides comprising different proportions of monosaccharides and uronic acid, with different average molecular weights of 0.99 × 10³, 1.09 × 10³ and 1.18 × 10³ kDa, respectively. Their intrinsic viscosity were significantly different, at 24.72, 29.98, and 41.06 dL/g, respectively. The functional groups of polysaccharides were not greatly affected, but the chemical composition, triple-helix structure, chain length and surface morphology were significantly influenced by culture conditions. In vitro bioactivity assays showed that the antioxidant activity generally increased in the order of WL-EPS-1 < NaCl-EPS-1 < Glu-EPS-1, while NaCl-EPS-1 had the best radioprotective effect, and Glu-EPS-1 had the best lipid-lowering effect. In addition, the structure-activity relationship of the polysaccharides was analyzed by partial least squares, which revealed that the most important factors affecting the antioxidant, radioprotective and lipid-lowering activities of polysaccharides were viscosity and molecular weight. This study provides a strategy for obtaining high-bioactivity polysaccharides by appropriate regulation of culture conditions, as well as opening new directions for the molecular modification of polysaccharides.

This study investigated the effect of Nostoc flagelliforme powder on the intestinal flora and immune regulation of mice. The chemical composition of wild and liquid cultured N. flagelliforme powder was determined. Mice were gavaged with different doses of N. flagelliforme powder (200 and 400 mg/kg m_b). Afterwards, body mass, immune organ indices, the serum concentrations of interleukin 1β (IL-1β), tumor necrosis factor α (TNF-α), interferon γ (IFN-γ) and the level of intestinal secretory immunoglobulin A (SIgA), cytokine expression in immune organs, and the water content, pH, and short-chain fatty acids in intestinal contents were measured and the effect of Nostoc flagelliforme powder on the intestinal microecology of mice was evaluated. The results showed that the nutritional value of liquid cultured N. flagelliforme powder was higher than that of wild N. flagelliforme powder. Compared to the control group, high-dose liquid cultured N. flagelliforme powder increased immune organ indexes, serum IFN-γ and TNF-α levels and intestinal SIgA levels, up-regulated the mRNA expression of IFN-γ, TNF-α and immunoglobulin A (IgA) in the spleen promoting the synthesis of relevant immune factors, reduced the pH of intestinal fluid, and increased the content of short-chain fatty acids in cecum contents. The intervention of N. flagelliforme powder improved the species richness and diversity of the intestinal microflora in mice, decreased the relative abundance of Bacteroidetes in the colon and cecum, and increased the relative abundance of Firmicutes in the colon, and the relative abundance of Verrucomicrobia and the immune-related probiotics Akkermansia, Parabacteroides, Alistipes and Ruminiclostridium. Conclusion: Liquid cultured N. flagelliforme powder can regulate the intestinal flora and improve immune function in mice.