Shedding light on cobalamin photodegradation in the ocean

Cobalamin, vitamin B12, is an important micronutrient that has been investigated for decades in the marine context because it is required for phytoplankton growth. The biologically active forms (Me‐B12, Ado‐B12) and the synthetic form (CN‐B12) quickly convert to OH‐B12 after light exposure in various aqueous solutions, but puzzlingly have been frequently reported to dominate dissolved cobalamin pools in the sunlit ocean. Here, we document photodegradation timescales for these cobalamin forms in natural seawater using targeted mass spectrometry, providing quantitative evidence that OH‐B12 is expected to be the dominant dissolved form in irradiated seawater. Then, through high‐resolution mass spectrometry, we identify four photodegradation products of OH‐B12 which represent potential building blocks microbes could salvage and remodel to satisfy cellular cobalamin requirements. Taken together, these results clarify the impact of light on marine cobalamin dynamics, laying a foundation for a more quantitative understanding of the role of cobalamin in microbial communities and biogeochemical cycles.