Efficient cobalamin uptake and cycling contribute to the lack of cobalamins in the surface cobalt‐binding ligand pool in the North Pacific

Abstract Cobalt is a central component of cobalamins, which are nutrients essential for various metabolic processes in marine organisms. Dissolved cobalt in seawater is mostly bound to organic ligands, and the prevailing assumption to date is that these ligands are cobalamin‐related compounds, yet the identity and impact of these ligands on cobalt bioavailability remain unknown. In this study, we examined cobalt ligand distributions and cobalamin cycling in surface waters across a North Pacific meridional transect. While we did not detect cobalamin derivatives in the dissolved cobalt ligand pool, the detection of transcripts associated with cobalamin synthesis and salvage pathways suggests that cobalamins may not be accumulating in seawater as cobalt‐binding ligands and thus represent only a small fraction of the cobalt ligand pool in the North Pacific.