Cobalamin and Cobalamin-like Compounds in the Sunlit Ocean: Inter-Phyla Mutualism or Competition?

Eukaryotic phytoplankton dominate much of the world’s sunlit ocean, yet often rely on prokaryotic production of the organic cofactor cobalamin (Vitamin B12). Cyanobacteria, thaumarchaeota and proteobacteria have been implicated as globally important cobalamin suppliers for the marine community, but little is known about the specific forms of the cofactor produced by these prokaryotes. Here we determine cobalamin and cobalamin-like compounds in the dissolved and particulate pools of laboratory cultures and the north Pacific. Laboratory cultures suggest that some globally important producers of the cofactor produce and use cobalamin-like compounds that have limited biological availability in eukaryotes. Environmental distributions suggest that the amount and type of dissolved cobalamin in surface oceans is dependent on the local prokaryotic community. These communities undergo dramatic shifts across coastal, high nutrient low chlorophyll, and oligotrophic regimes. Cobalamin speciation mirrors this community shift and suggests both mutualism and competition play important roles in the cycling of cobalamin and cobalamin-like compounds in the upper ocean.