Ligand cross-feeding resolves bacterial vitamin B₁₂ auxotrophies

Cobalamin (vitamin B₁₂, herein referred to as B₁₂) is an essential cofactor for most marine prokaryotes and eukaryotes^1,2. Synthesized by a limited number of prokaryotes, its scarcity affects microbial interactions and community dynamics^2-4. 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.