The Global Ocean Selenium Cycle Shaped by Phytoplankton Uptake and Community Structure

Selenium (Se) is an essential micronutrient for humans, with seafood as a major dietary source, yet its cycling in the ocean remains poorly understood. Here, we present the first global 3-D ocean Se model, constrained by field observations and laboratory experiments, to identify the processes that control marine Se speciation and distribution. Biological uptake by phytoplankton emerges as the dominant driver of horizontal gradients and vertical nutrient-like profiles of inorganic Se (selenite, Se(IV) + selenate, Se(VI)). Surprisingly, oligotrophic regions dominated by cyanobacteria (Prochlorococcus and Synechococcus)─thought not to require Se─show the strongest inorganic Se depletion, suggesting previously unrecognized Se utilization by cyanobacteria. Co-occurring bacteria may also contribute to this depletion but are not explicitly represented in the model. Our model reveals a short residence time of 0.57 years for the biologically preferred Se(IV) in the euphotic zone, indicating the potential for Se(IV) limitation. When Se(IV) becomes scarce, we find a shift toward Se(VI) uptake, supported by a net conversion from Se(VI) to Se(IV) through rapid cycling of Se(VI) assimilation, organic Se production, and Se(IV) regeneration. These results demonstrate that phytoplankton community composition exerts strong control over ocean Se cycling. Because community structure responds to temperature, stratification, and oxidative stress, our findings suggest that a changing ocean will reshape Se availability, microbial resilience, and the flow of this essential micronutrient through food webs.