Availability of vitamin B(12) and its lower ligand intermediate α-ribazole impact prokaryotic and protist communities in oceanic systems

Genome analyses predict that the cofactor cobalamin (vitamin B(12), called B(12) herein) is produced by only one-third of all prokaryotes but almost all encode at least one B(12)-dependent enzyme, in most cases methionine synthase. This implies that the majority of prokaryotes relies on exogenous B(12) supply and interacts with producers. B(12) consists of a corrin ring centred around a cobalt ion and the lower ligand 5’6-dimethylbenzimidazole (DMB). It has never been tested whether availability of this pivotal cofactor, DMB or its intermediate α-ribazole affect growth and composition of prokaryotic microbial communities. Here we show that in the subtropical, equatorial and polar frontal Pacific Ocean supply of B(12) and α-ribazole enhances heterotrophic prokaryotic production and alters the composition of prokaryotic and heterotrophic protist communities. In the polar frontal Pacific, the SAR11 clade and Oceanospirillales increased their relative abundances upon B(12) supply. In the subtropical Pacific, Oceanospirillales increased their relative abundance upon B(12) supply as well but also downregulated the transcription of the btuB gene, encoding the outer membrane permease for B(12). Surprisingly, Prochlorococcus, known to produce pseudo-B(12) and not B(12), exhibited significant upregulation of genes encoding key proteins of photosystem I + II, carbon fixation and nitrate reduction upon B(12) supply in the subtropical Pacific. These findings show that availability of B(12) and α-ribazole affect growth and composition of prokaryotic and protist communities in oceanic systems thus revealing far-reaching consequences of methionine biosynthesis and other B(12)-dependent enzymatic reactions on a community level.