SELENIUM BIOACCUMULATION IN AQUATIC ECOSYSTEMS .1. EFFECTS OF SULFATEON THE UPTAKE AND TOXICITY OF SELENATE IN DAPHNIA-MAGNA

Selenium contamination and toxicity have been reported for aquatic eco systems across the United States. Because waterborne selenium concentr ations in natural systems are seldom high enough to be directly toxic, it has been concluded that bioaccumulation of selenium is causing obs erved toxicity problems. As a result, information regarding the bioacc umulation processes of selenium in aquatic organisms is necessary for appropriate management and remediation of selenium in aquatic ecosyste ms. Studies of selenium bioaccumulation in bacteria, fungi, and algae have indicated that selenate and sulfate compete for uptake via a comm on mechanism. However, such competitive interactions have not been exa mined in higher, multi-cellular organisms. To address this, we conduct ed experiments to determine how changes in ambient sulfate concentrati ons affect the uptake of selenate in the freshwater crustacean Daphnia magna. D. magna were exposed to waterborne selenate concentrations ra nging from control to 500 mu g/L Se with varying concentrations of sul fate ranging from 0 mg/L to concentrations equivalent to EPA ''very ha rd'' water. At the higher concentrations of selenate, selenium uptake increased significantly as sulfate concentrations decreased. No toxici ty was observed except at the 500 mu g/L Se and ''no sulfate'' treatme nt, where virtually all of the daphnids died within 72 h.