Metal accumulation and biomarker responses in Daphnia magna following cadmium and zinc exposure.

Metal accumulation (quantified as body burden) and the responses of two biomarkers (metallothionein [MT] induction and superoxide dismutase [SOD] activity) in a freshwater cladoceran Daphnia magna were quantified after being exposed to different concentrations of Cd and Zn for 3 d. The Cd and Zn body burdens increased with increasing exposure concentrations. The responses of biomarkers were metal-specific and concentration-dependent. As a detoxification mechanism, MT was induced by exposure to Cd or Zn or Cd + Zn and was dependent on the exposure concentrations. Low concentrations of Cd or Zn exposure increased the SOD activity, which was, however, inhibited at higher metal exposure concentrations. Metallothionein concentrations in daphnids were significantly related to the metal body burden. The SOD activity was inversely related to Cd body burden, while it was independent of the Zn body burden. Survivorship of D. magna was high at lower Cd/MT ratios in the animals but then decreased linearly when the ratio exceeded six. In contrast, the relationship between survivorship and Zn/MT ratio was different due to the essentiality of Zn. Overall, MT induction was a useful indicator of metal pollution in cladocerans.