Ascophyllum nodosum is a raw material for the industrial production of alginate, and the fucoidan within it is also a bioactive substance awaiting development and utilization. This study employed a complex enzyme method to extract the polysaccharide from Ascophyllum nodosum (ANP). Homogeneous components ANP-3, ANP-6, and ANP-7 were obtained through DEAE-52 column chromatography and ultrafiltration purification. The molecular weights of these three fucoidans were 124.5 kDa, 63.2 kDa, and 124.5 kDa, respectively, with sulfate group contents of 30.7%, 39.8%, and 42.0%, respectively. Among them, the backbone structure of ANP-3 consisted of →3)-α-Fucp2S4S-(1→, →3,6)-β-Galp4S-(1→, →3,6)-β-Manp4S-(1→, →2)-α-Fucp3S-(1→, and →3,6)-β-Galp4S-(1→ residues, while the branched chain structure consisted of →2)-α-Fucp3S-(1→, →3)-α-Fucp2S4S-(1→, →6)-β-Manp-(1→, →3)-β-Galp-(1→, α-GlcAp-(1→, and α-Fucp-(1→ residues. In contrast, the backbone structures of both ANP-6 and ANP-7 were primarily composed of →3)-α-L-Fucp2S4S-(1→, →2)-α-L-Fucp4S-(1→, →6)-β-D-Galp-(1→, and →3,6)-β-D-Galp4S-(1→ residues, while their branches consisted of →2)-α-L-Fucp4S-(1→ and →3)-β-D-Galp-(1→ residues.

Using an antibiotic-induced chronic colitis mouse model, the study investigated the mitigating effect of ANP on chronic colitis. The results indicated that ANP, in both the prevention and recovery groups, was able to prevent the shortening of the colon and inflammatory cell infiltration, and reduced crypt depth in the colon tissue of mice with chronic colitis, significantly reducing the expression levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6), and increasing the expression level of interleukin-10 (IL-10) . Gut microbiota sequencing results showed that ANP could upregulate the relative abundance of the beneficial bacterium Akkermansia and downregulate the relative abundances of the harmful bacteria Proteus and Enterococcus .

Finally, the colon health-promoting effects of low molecular weight ANP were explored. It was found that the degraded ANP (D-ANP-3.5) prepared by photocatalytic reaction showed better capabilities to prevent colon shortening in mice with colitis, increase the spleen index, increase the number of goblet cells, and alleviate pathological damage in mice with colitis than ANP. Furthermore, gut microbiota sequencing and fecal metabolomics analysis demonstrated that compared to ANP, D-ANP-3.5 could more effectively enrich the relative abundance of the beneficial bacterium Akkermansia, upregulate the contents of metabolites such as betaine, acetic acid, propionic acid, and butyric acid in mice.

Thus, this study shed mechanistic insights into the anti-colitis effects of the fucoidan from Ascophyllum nodosum, and provided references for the development of its functional food and pharmaceutical applications.