(A) Structures of the 5 different seaweed-derived polysaccharides used in this study. Note that carrageenan has multiple isomers and only l-carrageenan, which supports growth of gut Bacteroides strains identified here is shown for simplicity. (B) A phylogeny of human gut Bacteroidetes based on core gene alignment (Larsbrink et al., 2014) and illustration of seaweed polysaccharide-degrading abilities present in members of each species. Degradative abilities that were observed in isolates cultured without targeted enrichment are represented as circles and the numbers indicate the number of positive strains over the total number tested. Isolates recovered by enrichment on agarose, porphyran or carrageenan are shown in diamonds and the number indicates the total number of isolates recovered for that species from different donors. B.= Bacteroides, Pa.= Parabacteroides, D.= Dysgonomonas, O.= Odoribacter, Bar.= Barnesiella.