Comparative toxicity of selenate, selenite, seleno-<scp>DL</scp>-methionine and seleno-<scp>DL</scp>-cystine to <i>Daphnia magna</i>

Abstract

Abstract Elevated concentrations of the trace element selenium (Se) have resulted in the degradation of several aquatic ecosystems. This study evaluated the comparative toxicity of several aqueous chemical species of selenium to an aquatic cladoceran, Daphnia magna. Responses to mixtures of these selenium forms, varying the sulfate concentration, were also examined. Initial experiments compared the toxicity of aqueous forms of selenate, selenite, seleno-DL-methionine, and seleno-DL-cystine to neonate Daphnia magna, resulting in 48-h LC50 values of 2.84, 0.55, 0.31, and 2.01 mg Se per liter, respectively. Immobilization was an acute sublethal response observed during exposure to the organic selenium forms only. The 48-h IC50 values were 0.045 and 0.52 mg Se per liter for seleno-DL-methionine and seleno-DL-cystine, respectively. Evaluation of the invertebrate response to various combinations of selenate, selenite, and seleno-DL-methionine demonstrated that the toxicities of these forms of selenium are additive. Increasing the concentration of sulfate decreased, varied, and left unaffected the toxicities of selenate, selenite, and seleno-DL-methionine, respectively. These results indicate that both the chemical form of selenium and the sulfate concentration can influence the toxicity of selenium.