Vitamin B(12) is not shared by all marine prototrophic bacteria with their environment

Vitamin B(12) (cobalamin, herein B(12)) 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(12) 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(12) is provided by prototrophs to auxotrophs. In this study, 33 B(12) prototrophic alphaproteobacterial strains were grown in co-culture with Thalassiosira pseudonana, a B(12) auxotrophic diatom, to determine the bacterial ability to support the growth of the diatom by sharing B(12). Among these strains, 18 were identified to share B(12) with the diatom, while nine were identified to retain B(12) and not support growth of the diatom. The other bacteria either shared B(12) with the diatom only with the addition of substrate or inhibited the growth of the diatom. Extracellular B(12) measurements of B(12)-provider and B(12)-retainer strains confirmed that the cofactor could only be detected in the environment of the tested B(12)-provider strains. Intracellular B(12) was measured by LC-MS and showed that the concentrations of the different B(12)-provider as well as B(12)-retainer strains differed substantially. Although B(12) 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(12) de novo cannot share the cofactor with their environment.