Distribution and biogeochemical controls of dissolved cobalamin (vitamin B₁₂) in the western North Pacific

Abstract. Vitamin B₁₂ (cobalamin) is an essential micronutrient for many marine microorganisms, yet its distribution and biogeochemical controls in the open ocean, particularly below the euphotic zone, remain poorly constrained. We investigated the vertical and zonal distributions of four dissolved cobalamin species from the surface to the deep western North Pacific. Total dissolved cobalamin (TD-B₁₂) concentrations ranged from below detection to 1.63 pM and were generally low in surface waters. Elevated concentrations occurred below the subsurface chlorophyll maximum (SCM), extended into the upper North Pacific Intermediate Water (NPIW) at northern stations, and were also observed locally in bottom waters. Hydroxocobalamin (OH-B₁₂) was the dominant dissolved form throughout most of the water column. From the surface to the upper NPIW, TD-B₁₂ increased with apparent oxygen utilization (AOU), consistent with a contribution of organic matter remineralization to cobalamin accumulation. TD-B₁₂ was also positively associated with the relative abundance of Nitrosopumiliaceae inferred from 16S rRNA gene amplicon sequencing, indicating an association between cobalamin accumulation and microbial community composition. In contrast, the relationship between TD-B₁₂ and AOU disappeared below the lower NPIW, suggesting that remineralization alone cannot explain cobalamin distributions at greater depths. Elevated cobalamin within the upper NPIW suggests a role for intermediate-water transport, whereas localized bottom-water enrichment may reflect benthic sources and deep-water circulation. Overall, dissolved cobalamin distributions reflect a depth-dependent balance among microbial cycling, degradation, and physical transport.