Studying selenium and sulfur volatilisation by marine algae Emiliania huxleyi and Thalassiosira oceanica in culture

Abstract

Environmental contextVolatile selenium compounds from the oceans may ultimately be an important selenium source for agricultural soils. It has been hypothesised that marine algae are responsible for volatile selenium emissions, but in laboratory experiments, we observed minimal volatile selenium production by two marine algae known to produce large amounts of volatile sulfur. Instead, we found hints that bacterial processes may be important in the production of volatile selenium in the oceans. AbstractVolatile methylated selenium compounds, especially dimethylselenide, are thought to comprise the majority of marine selenium emissions. Despite their potential importance for the global redistribution of this trace element, which is essential for human health, little is known about the algal production of volatile organic selenium compounds. Previous studies have found correlations between dissolved dimethylselenide concentrations, dimethylsulfide concentrations (the sulfur analogue of dimethylselenide) and proxies for algal activity, most notably during a bloom of the coccolithophorid Emiliania huxleyi. In culturing studies, we investigated the ability of three globally important marine algal species, E. huxleyi, Phaeocystis globosa and the diatom Thalassiosira oceanica, to produce dimethylselenide. Despite substantial uptake of selenium and the production of volatile sulfur, E. huxleyi and T. oceanica produced negligible volatile selenium (<2nM). P. globosa produced low amounts of volatile selenium (~8nM), but grew poorly in our laboratory. However, cultures of marine bacteria and mixed bacterial–algal cultures showed that substantial amounts of volatile selenium can be produced in the presence of marine bacteria. In addition, a culture of marine bacteria alone produced ~50nM volatile selenium, far more than axenic cultures of E. huxleyi when exposed to equivalent selenite concentrations. Our results hint that marine algae may be of minor importance in the direct production of volatile selenium in the oceans, and suggest that the production of these compounds in the marine biosphere may instead be controlled by bacterial activity.