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.