THE MARINE BIOGEOCHEMISTRY OF SELENIUM - A RE-EVALUATION

Vertical and horizontal profiles from the North and South Pacific Ocean demonstrated the existence of 3 spp. of dissolved Se: selenite, selenate and organic selenide (operationally defined). In surface waters, organic selenide makes up about 80% of the total dissolved Se, selenite concentrations are uniformly low, and selenate concentrations rise with increased vertical mixing. The organic selenide maximum (thought to consist of seleno-amino acids in peptides) coincides with the maxima of primary productivity, pigments, bioluminescence and dissolved free amino acids. Deep ocean waters are enriched in selenite and selenate; organic selenide is nondetectable. In suboxic waters of the tropical northeastern Pacific, organic selenide concentrations rise; selenite values decrease. The downward flux of particulate Se generally decreases with depth, and fluxing particulate Se is primarily in the (-2) oxidation state. These data allow a re-evaluation of the internal biogeochemical cycle of Se. This cycle includes selective uptake, reductive incorporation, particulate transport, a multistep regeneration and kinetic stabilization of thermodynamically unstable species.