Aqueous and dietary copper uptake and elimination in Daphnia magna determined by the 67CU radiotracer

Abstract

Among the many toxic metals, the biokinetics of copper (Cu) in the freshwater cladoceran Daphnia magna have not been studied due to the lack of an ideal radiotracer. In the present study, a gamma radiotracer, ⁶⁷Cu (half-life = 61.9 h), was used to study the uptake of copper from the dissolved and dietary phase and efflux in D. magna, an important toxicity testing species. The influx rate of Cu from the dissolved phase increased with dissolved Cu concentration, with a calculated uptake rate constant of 0.055 L/g/h. The assimilation efficiency (AE) of Cu decreased significantly (from 92 to 16%) as the available food concentration increased, and the AE differed among the food types. As low as 1% of Cu AE was found in daphnids fed high concentrations (1.54 mg/L) of the green algae Chlorella pyrenoidosa. The AE decreased linearly as the ingestion rate of the daphnids increased. The efflux rate constant was 0.20/d at high food concentrations. Excretion accounted for 82 to 94% of total Cu loss from the animals, although Cu also was transferred maternally from female adults to their offspring. Under conditions of high food concentrations, approximately 6.5% of the mother's Cu was transferred to the offspring over 7 d. It was concluded that Cu accumulation is dominated by uptake from dietary sources, and there is a substantial need to understand the dietary toxicity of Cu to daphnids. The present study has implications for the choice of food particles in conducting the Cu toxicity testing in cladocerans.