Vitamin auxotrophies shape microbial community assembly on model marine particles

Microbial community assembly is governed by the flow of carbon sources and other primary metabolites between species. However, central metabolism represents only a small fraction of the biosynthetic repertoire of microbes: metabolites such as antimicrobial compounds, signaling molecules, and co-factors are underexplored in their potential to shape microbial communities. Here, we focus on B vitamin exchange in marine bacterial communities that degrade polysaccharides, a key component of particulate organic matter. We found that in a screen of 150 natural isolates, almost a third were auxotrophs for one or more B vitamins. By measuring physiological parameters like uptake affinities and comparing those to ambient seawater concentrations, we showed that marine bacteria live at the edge of vitamin limitation in the environment. To understand how auxotrophs survive in the open oceans, we used our experimental data to model vitamin cross-feeding on particles through both secretion and lysis. Our results highlight the importance of vitamin auxotrophies in shaping microbial community assembly and succession, adding another layer of complexity to the trophic structure of particle-associated communities.