Seaweed Fermentation Modulates Gut Microbiome Composition Without Altering Metabolite Output In Vitro

Purpose Seaweed consumption gains popularity due to its high micronutrient, protein and fiber content. While seaweed extracts show promising effects on the gut, it remains unclear if processing of raw seaweed changes the nutrient accessibility for the gut microbiota. This study aimed to investigate how different processing methods affect the prebiotic potential of a common brown seaweed, Saccharina latissima . Methods The seaweed was processed (blanched, oven-dried or fermented by Lactiplantibacillus plantarum ) and underwent simulated upper-gastrointestinal digestion with INFOGEST 2.0 prior to in vitro fermentation, using four adult human fecal donors. Samples were analyzed for their microbiome composition and produced metabolites. Results Seaweed-supplemented media changed the fecal microbial composition compared to low-fiber and high-fiber controls. Specifically, seaweed increased the relative abundances of Bacteroidales, Escherichia/Shigella and unclassified Lachnospiraceae, while Bifidobacteria decreased. Regardless of processing, seaweed-supplemented cultures showed similar alpha and beta diversity indicating that processing did not modulate seaweed’s impact on the microbial composition. The short-chain fatty acid and amino acid concentrations were similar in the seaweed-supplemented media and the low-fiber medium after fermentation, while formic acid and lactic acid were significantly higher in the high-fiber medium, compared to other media, and negatively correlated with the final pH after fermentation. Conclusion Seaweed altered the microbiome composition, however with minor effects of seaweed processing. These compositional shifts did not translate into metabolic changes, suggesting that seaweed components were largely inaccessible to microbes under the tested conditions. Further studies are recommended to investigate alternative processing and co-formulations to enhance seaweed fermentability.