Abstract:
Microbial communities are shaped by environmental conditions and ecological interactions, but the mechanisms underlying these interactions often remain unclear. In many cases, these can be understood mechanistically as being mediated by external metabolite pools. Vitamin B12 is a particularly consequential example: it is produced by relatively few prokaryotes, used by many bacteria, and required by over half of algal species, linking bacterial provisioning to primary productivity in aquatic systems. Here, I ask how B12 is provisioned by bacterial producers and what determines the size of extracellular B12 pools. To address this, we quantified B12 synthesis, release, and uptake across hundreds of bacterial isolates from terrestrial and marine environments. Four broad strategies emerged: Retainers (synthesis and high uptake), Providers (synthesis and no uptake), Scavengers (no synthesis, high uptake), and Bystanders (no synthesis and no uptake). For individual taxa, extracellular B12 concentrations could be quantitatively predicted from release during cell lysis together with reuptake by the remaining living cells, suggesting that extracellular B12 pools reflect a flux balance between passive release and subsequent uptake. In this view, reuptake by producers is a major control on the size of extracellular metabolite pools and thus on the degree of metabolite sharing.