Rapid microalgal metabolism of selenate to volatile dimethylselenide

An axenically cultured isolate of single-celled freshwater microalgae (Chlorella sp.) metabolized toxic selenate to volatile dimethylselenide at exceptionally high rates when transferred from mineral-nutrient solution to water for 24 h. The Se-volatilization rates were orders of magnitude higher than those similarly measured for wetland macroalgae and higher plants. Ninety percent of 20 micro m selenate supplied to the microalgae incubated without nutrients was removed through accumulation and volatilization. Additions of 1 mm sulphate but not nitrate, inhibited Se accumulation and volatilization so that only 1.8% of the supplied selenate was removed. The microalgae cultured in nutrient solution without sulphate showed increased 35S-sulphate-transporter activity. Selenium K-edge X-ray absorption spectroscopy of selenate-treated microalgae cultured with or without mineral nutrients, showed that 87% of the selenate accumulated during 24 h was reductively metabolized to intermediate organic compounds such as selenomethionine and selenocystine. This is in complete contrast to higher plants that show very limited reduction of selenate. It appears that high rates of Se accumulation and volatilization by the sulphate-deprived microalgae resulted from reduced competition with chemically analogous sulphate ions for selenate uptake via up-regulated sulphate/selenate transporters and rapid reductive metabolism of selenate. Hyper-volatilization of selenate by microalgal cells may provide a novel detoxification response.