Production of the cobalamin lower ligand, 5,6-dimethylbenzimidazole, by a diatom under cobalamin-deplete conditions

Cobalamin, or vitamin B 12 , is a micronutrient required by half of all surveyed phytoplankton but produced only by select bacteria and archaea. Cobalamin influences community composition and primary productivity in various regions of the ocean and has been shown to be a critical currency in microbial interactions. Many prokaryotes and some phytoplankton can salvage and remodel cobalamin-related compounds, e.g. cobinamides and/or pseudocobalamin, to generate cobalamin using the lower ligand 5,6-dimethylbenzimidazole (DMB). The sources of DMB remain poorly characterized but are currently attributed to prokaryotes alone. Here, we grew axenic cultures of a recently isolated Thalassiosiraceae diatom over a range of cobalamin concentrations (0, 0.75, 10, 100 pM). Then, using mass spectrometry, we quantified cobalamin and DMB in both particulate (cellular) and dissolved (media) phases. We found that this diatom is a facultative cobalamin consumer: it takes up cobalamin when available but can survive in its absence. Strikingly, under low cobalamin availability, we demonstrate that the diatom produces DMB. This highlights eukaryotic production as a source of the molecule and suggests that cobalamin remodeling by phytoplankton may be less dependent on prokaryotes than previously imagined. We detected DMB in the media during stationary phase, highlighting eukaryotic production of the molecule as a public good. We provide evidence that DMB is produced by this diatom as a mechanism to cope with low cobalamin availability, suggesting that even amongst organisms that don’t absolutely require it, cobalamin use has important consequences for fitness. Importance Section Cobalamin (vitamin B 12 ) is a scarce micronutrient in the ocean that impacts the composition and activity of marine microbial communities. Microbes employ a range of strategies to cope with low cobalamin availability, including remodeling degraded cobalamin compounds into cobalamin if DMB, a critical component of cobalamin, is available. Here we show that a recently isolated Thalassiosiraceae diatom doesn’t need cobalamin to survive, but will take it up and use it when available. Additionally, the diatom produces DMB under low cobalamin conditions. DMB production was previously thought to be restricted to bacteria and archaea, so these results suggest cobalamin remodeling by phytoplankton is not as dependent on interactions with prokaryotes as previously imagined. This work also demonstrates that even organisms that don’t absolutely require cobalamin for survival have adopted metabolic strategies to cope with its absence, emphasizing the importance of this vitamin for phytoplankton metabolism.