Joint effects of binary mixtures of cadmium, zinc and mercury on selected test animals and plants

Cadmium, zinc and mercuray have a central position in ecotoxicological studies. The mode of action of heavy metal ions is described to be dissimilar, at least at the subcellular level. Contrary to many other chemicals, the joint effects of metals in simple mixtures have strong deviations from both nonadditive and response additive effects (synergism, antagonism). In the literature, no general trend for metal joint effects can be found. There is a belief that nonaditive, synergistic or antagonistic effects depend on the metals, organisms and toxicity endpoints. Recent studies have revealed, that the type of mixture effect strongly depends on the metal concentration in the mixture. One can speculate that the type of joint effect could be predicted from the metal concentration in the mixture. The aim of our study is to find out whether a shift of the type of a metal joint effect above a certain threshold exposure level is a general trend or an exception. Recognition of the trend in metal joint effects would make a risk assessment much easier because it would allow some generalisation of toxicity data. In our work, toxicity of Cd, Zn and Hg in binary mixtures will be studied at biochemical and physiological level of two plants and two animal species, one of each is terrestrial and the other one aquatic. The two aquatic test organisms are Daphnia magna and Lemna minor, the two terrestrial test organisms are Porcellio scaber and Thlaspi praecox. Data on NOEC and LOEC for target and non-target endpoints will show the sensitivity of the endpoint. While data on ECs of single metals and that of mixtures are crucial for interpretation of the metal joint effects and subsequently for risk assessment. Laboratory data will be supplemented with field analyses of Zn, Cd and Hg. On the basis of metal concentrations in polluted environment and laboratory toxicity data, possible effects of Zn, Cd and Hg will be assessed. The need to considere metal joint effects in risk assessment will be discussed.