Modulation of gut microbiota and microbial metabolites during in vitro colonic fermentation of Ulva lactuca: Release and bioconversion of phlorotannins.

Ulva lactuca is a marine macroalga rich in bioactive compounds such as phlorotannins (PhT), whose transformation during colonic fermentation may modulate the gut microbiota and lead to the production of functional metabolites. This study evaluated the release and bioconversion of PhT, as well as the microbial, functional, and metabolic shifts induced by the in vitro colonic fermentation of Ulva lactuca. LC-ESI-QToF identified a total of 24 PhT and microbial derivatives, and volatile compounds were characterized by GC-MS. The fermentation of Ulva lactuca induced a robust proteolytic profile, characterized by the continuous absence of detectable SCFAs (in contrast to raffinose) and the generation of key nitrogenous and aromatic volatile compounds.The microbiota shifted toward genera such as Enterobacteriaceae and Escherichia/Shigella, while functional prediction revealed an enrichment in amino acid degradation and cofactor biosynthesis pathways. Microbial metabolite correlation confirmed associations between proteolytic genera and specific volatile compounds. In contrast to the saccharolytic control (raffinose), Ulva lactuca induced a differential fermentation linked to its protein-rich composition and marine phenolic compounds. These findings provide evidence on the fermentative behavior of marine matrices and their relevance as potential functional sources for gut health.