Acute toxicity of cadmium in daphnia magna under different calcium and pH conditions: Importance of influx rate

Water chemistry is generally thought to influence metal toxicity via affecting metal bioavailability and bioaccumulation, but its effects on tissue residue-based toxicity are poorly known. We conducted toxicity tests in parallel with uptake kinetics experiments of cadmium (Cd) in waters of different calcium (Ca) concentrations and pH levels using acclimated Daphnia magna as a model organism. Both the acute toxicity and uptake of Cd were reduced by higher Ca concentration and lower pH. Strikingly constant median effective influx rates (EJ₅₀, 1.3-1.6 μg g^-1 h^-1) of Cd were observed when the concentration of Ca varied from 0.5 to 200 mg L^-1, indicating that acclimation to different Ca levels did not affect the tissue residue-based toxicity. The EJ₅₀ values increased consistently with decreasing pH level, showing that acclimation to acidic water decreased the tissue residue-based toxicity. With the use of calcium uptake inhibitors, we demonstrated that both Ca channel and Ca²⁺/Na⁺ exchanger were involved in Cd uptake in daphnids, but there were also other possible pathways with higher affinity. The relative importance of different pathways was clearly dependent on the ambient Ca availability. Our findings are helpful for the development of a more accurate biotic ligand model in predicting the acute toxicity of Cd to daphnids.