Vitamin B is not shared by all marine prototrophic bacteria with their environment.

Vitamin B (cobalamin, herein B) is an essential cofactor involved in amino acid synthesis and carbon resupply to the TCA cycle for most prokaryotes, eukaryotic microorganisms, and animals. Despite being required by most, B is produced by only a minor fraction of prokaryotes and therefore leads to complex interaction between prototrophs and auxotrophs. However, it is unknown how B is provided by prototrophs to auxotrophs. In this study, 33 B prototrophic alphaproteobacterial strains were grown in co-culture with Thalassiosira pseudonana, a B auxotrophic diatom, to determine the bacterial ability to support the growth of the diatom by sharing B. Among these strains, 18 were identified to share B with the diatom, while nine were identified to retain B and not support growth of the diatom. The other bacteria either shared B with the diatom only with the addition of substrate or inhibited the growth of the diatom. Extracellular B measurements of B-provider and B-retainer strains confirmed that the cofactor could only be detected in the environment of the tested B-provider strains. Intracellular B was measured by LC-MS and showed that the concentrations of the different B-provider as well as B-retainer strains differed substantially. Although B is essential for the vast majority of microorganisms, mechanisms that export this essential cofactor are still unknown. Our results suggest that a large proportion of bacteria that can synthesise B de novo cannot share the cofactor with their environment.