Widespread B-vitamin auxotrophy in marine particle-associated bacteria

Microbial community assembly is governed by complex interaction networks based on the secretion and exchange of metabolites. While the importance of trophic interactions (e.g. cross-feeding of metabolic byproducts) in structuring microbial communities is well-established, the roles of myriad natural products such as vitamins, siderophores, and antibiotics remain unclear. Here, we focus on the role of B vitamins in coastal marine bacterial communities that degrade particulate organic matter. We find that natural seawater particle-associated communities are vitamin limited and almost a third of bacterial isolates from these communities are B vitamin auxotrophs. Auxotroph growth rates are limited under even maximal environmental vitamin concentrations, indicating that auxotrophs likely survive through cross-feeding with community members. We find that polysaccharide-degrading bacteria tend to be vitamin prototrophs, suggesting that the initial arrival of degraders to a particle may promote a succession to auxotrophic taxa partially through vitamin cross-feeding. However, auxotrophs with complementary vitamin requirements were generally not able to grow in co-culture, and auxotroph growth was only partially rescued by prototrophs. We conclude that while vitamin auxotrophies are important metabolic dependencies shaping community structure, vitamin cross-feeding may primarily take place through cell lysis. ### Competing Interest Statement The authors have declared no competing interest.