Metabolism of a hybrid algal galactan by members of the human gut microbiome

Native porphyran is a hybrid of porphryan and agarose. As a common element of edible seaweed, this algal galactan is a frequent component of the human diet. Bacterial members of the human gut microbiota have acquired polysaccharide utilization loci (PULs) that enable the metabolism of porphyran or agarose. However, the molecular mechanisms that underlie the deconstruction and use of native porphyran remains incompletely defined. Here, we have studied two human gut bacteria, porphyranolytic Bacteroides plebeius and agarolytic Bacteroides uniformis, that target native porphyran. This reveals an exo-based cycle of porphyran depolymerization that incorporates a keystone sulfatase. In both PULs this cycle also works together with a PUL-encoded agarose depolymerizing machinery to synergistically reduce native porphyran to monosaccharides. This provides a framework for understanding the deconstruction of a hybrid algal galactan, and insight into the competitive and/or syntrophic relationship of gut microbiota members that target rare nutrients.