Extensive Transfer of Genes for Edible Seaweed Digestion from Marine to Human Gut Bacteria

Humans harbor numerous species of colonic bacteria that digest the fiber polysaccharides in commonly consumed terrestrial plants. More recently in history, regional populations have consumed edible macroalgae seaweeds containing unique polysaccharides. It remains unclear how extensively gut bacteria have adapted to digest these nutrients and use these abilities to colonize microbiomes around the world, especially outside Asia. Here, we show that the ability of gut bacteria to digest seaweed polysaccharides is more pervasive than previously appreciated. We show that enrichment-cultured Bacteroides harbor previously known genes for seaweed degradation, which have mobilized into many members of this genus. We identify previously unknown examples of marine bacteria-derived genes, and their mobile DNA elements, that are involved in degrading seaweed polysaccharides, including genes in gut-resident Firmicutes. Our results are important for understanding the metabolic plasticity of the human gut microbiome and the global exchange of genes in the context of dietary selective pressures.