Ligand cross-feeding resolves bacterial vitamin B 12 auxotrophies.

Cobalamin (vitamin B12 , herein referred to as B12 ) is an essential cofactor for most marine prokaryotes and eukaryotes1,2 . Synthesized by a limited number of prokaryotes, its scarcity affects microbial interactions and community dynamics2-4 . Here we show that two bacterial B12 auxotrophs can salvage different B12 building blocks and cooperate to synthesize B12 . A Colwellia sp. synthesizes and releases the activated lower ligand α-ribazole, which is used by another B12 auxotroph, a Roseovarius sp., to produce the corrin ring and synthesize B12 . Release of B12 by Roseovarius sp. happens only in co-culture with Colwellia sp. and only coincidently with the induction of a prophage encoded in Roseovarius sp. Subsequent growth of Colwellia sp. in these conditions may be due to the provision of B12 by lysed cells of Roseovarius sp. Further evidence is required to support a causative role for prophage induction in the release of B12 . These complex microbial interactions of ligand cross-feeding and joint B12 biosynthesis seem to be widespread in marine pelagic ecosystems. In the western and northern tropical Atlantic Ocean, bacteria predicted to be capable of salvaging cobinamide and synthesizing only the activated lower ligand outnumber B12 producers. These findings add new players to our understanding of B12 supply to auxotrophic microorganisms in the ocean and possibly in other ecosystems.