Selenium bioaccumulation in aquatic ecosystems: 1. Effects of sulfate on the uptake and toxicity of selenate in Daphnia magna

Abstract

Selenium contamination and toxicity have been reported for aquatic ecosystems across the United States. Because waterborne selenium concentrations in natural systems are seldom high enough to be directly toxic, it has been concluded that bioaccumulation of selenium is causing observed toxicity problems. As a result, information regarding the bioaccumulation processes of selenium in aquatic organisms is necessary for appropriate management and remediation of selenium in aquatic ecosystems. Studies of selenium bioaccumulation in bacteria, fungi, and algae have indicated that selenate and sulfate compete for uptake via a common mechanism. However, such competitive interactions have not been examined in higher, multi-cellular organisms. To address this, we conducted experiments to determine how changes in ambient sulfate concentrations affect the uptake of selenate in the freshwater crustacean Daphnia magna. D. magna were exposed to waterborne selenate concentrations ranging from control to 500 μg/L Se with varying concentrations of sulfate ranging from 0 mg/L to concentrations equivalent to EPA “very hard” water. At the higher concentrations of selenate, selenium uptake increased significantly as sulfate concentrations decreased. No toxicity was observed except at the 500 μg/L Se and “no sulfate” treatment, where virtually all of the daphnids died within 72 h.