Bacterial vitamin sharing emerges from a balance between release and uptake

Many microbes use vitamins they cannot synthesize. As a result, vitamin availability impacts microbial community composition and productivity. Yet how vitamins become available to users remains poorly understood. To understand the basis of vitamin sharing, we quantified vitamin B12 synthesis and uptake in over 200 soil, freshwater, and marine bacterial isolates. We reveal the existence of four strategies demarcated by all combinations of high or limited B12 synthesis and uptake. B12 synthesis, but not uptake, was predictable from genomes without pathway knowledge. Among synthesizers, the fraction of vitamin that was extracellular varied over 100-fold. Extracellular B12 was quantitatively predicted from a dynamic model capturing only release via death and reuptake by living cells. A subset of synthesizers rapidly reclaim released B12, resulting in limited sharing, while retainers exhibit limited release or uptake, and providers release without reuptake. Thus, extracellular B12 availability is governed not by synthesis alone, but by the balance between release and uptake.