The Marine Biogeochemistry of Selenium: A Re-Evaluation

Vertical and horizontal profiles from the North and South Pacific Oceans demonstrate the existence of three species of dissolved selenium: selenite, selenate, and organic selenide (operationally defined). In surface waters, organic selenide makes up about 80% of the total dissolved selenium, 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, while organic selenide is nondetectable. In suboxic waters of the tropical northeastern Pacific, organic selenide concentrations rise, while selenite values decrease. The downward flux of particulate selenium generally decreases with depth, and fluxing particulate selenium is found to be primarily in the (-2) oxidation state. These data allow a re-evaluation of the internal biogeochemical cycle of selenium. This cycle includes selective uptake, reductive incorporation, particulate transport, a multistep regeneration, and kinetic stabilization of thermodynamically unstable species. The marine biogeochemical cycles of many trace elements include the processes of uptake from dissolved to particulate form, particulate transport, and regeneration back to the dissolved state. Uptake can occur passively by adsorption onto particle surfaces and coprecipitation into solid phases, or actively by selective incorporation into biological tissues and skeletal material. The vertical transport of trace elements from the surface zone to the deep sea via detrital matter is a function of the type of carrier and of sinking rates. The regeneration of a particulate-bound trace element to the dissolved state can occur by simple dissolution of the carrier or by the complex microbial process of oxidative degradation. The regeneration of nitrogen provides an example of this latter process; several intermediates (e.g. ammonia, nitrite) occur in the production of nitrate from organic nitrogen. Oxidation-reduction reactions may also be an important process for trace elements such as selenium. Dissolved selenium may exist as selenate (+ 6), selenite (+4), or several possible forms of selenide (2). Previous studies have dealt with different aspects of the marine biogeochemical cycle of selenium. Sugimura et al. ( 1977) showed L This work supported by the National Science Foundation (OCE 79-23322 and 79-19928). 2 Present address: Department of Oceanography, Old Dominion University, Norfolk, Virginia 23508. both selenite and selenate to be present in oxic seawater, with higher concentrations in deep waters than at the surface. The presence of substantial amounts of selenite throughout the water column was surprising, since thermodynamics would predict that only selenate should be present in oxic seawater. Measures and Burton (1980a) and Measures et al. (1980) presented the first oceanographically consistent vertical profiles of selenate and selenite in the Atlantic and Pacific Oceans. They concluded that selenium displays nutrient-type behavior. Multiple correlations with silicate and phosphate were made, and it was suggested that selenium has both deep and shallow regeneration cycles, depending on the particulate phase (i.e. selenite is found with silica phases, while selenate is found in soft tissues as is phosphate). It was pointed out, however, that these conclusions assumed preservation of the original oxidation states. If, on the other hand, selenate was reduced during incorporation, oxidative regeneration could then produce the thermodynamically unstable selenite. The work by Measures et al. (1980) during the GEOSECS-I reoccupation emphasized a model involving particulate transport of selenite and selenate to deep water. Cutter (1982) found that in Saanich Inlet selenite and selenate concentrations decrease in the suboxic zone and are at the detection limits in anoxic water. The major