Interactions of cadmium and zinc impact their toxicity to the earthworm Aporrectodea caliginosa

Individual and binary mixture effects of Cd and Zn on mortality of the earthworm Aporrectodea caliginosa were investigated in one soil type. Mutual, systemic interactions of Cd and Zn were assessed over a broad range of concentration combinations. Exposures were expressed as measured total concentrations in soil, porewater concentrations, and CaCl(2)-extractable concentrations. The Cd-Zn and Zn-Cd interactions in soil were estimated by comparing partition coefficients (K(d)) of one metal in the presence and absence of the second metal. Simple patterns of mixture toxicity (additive, antagonistic, or synergistic) were evaluated by the toxic unit approach. The more complex patterns (dose ratio dependent or dose level dependent) were quantified by the MIXTOX model. The partitioning of Cd and Zn between soil and pore water was affected neither by their concentration nor presence of the other metal, and metal partitioning remained linear at all exposure levels tested. The effect of the Cd and Zn mixtures on mortality of A. caliginosa was mainly antagonistic, and the magnitude of antagonism (1.1- 2.7 toxic units) was dependent upon both the relative concentrations of Cd and Zn and the concentration magnitudes. Interactions of both metals occurred at the organism level and were manifested in terms of effects (28-d median lethal concentrations). The present study highlights the importance of identifying the relative influence of various interactions from external exposure to internal assimilation in evaluating mixture toxicity.