Interactive toxicity of two commonly co-occurring metals, zinc and cadmium, to earthworms in a natural soil

Mixture toxicity and bioavailability are important topics in ecotoxicological research. Here, we assess the role of bioavailability in determining the combined effects of two metals (Cd, Zn) on Eisenia andrei reproduction. When assessed based on total soil metal concentrations, a significant concentration ratio effect was seen. Mixture modeling using the "MixTox" model approach indicated this pattern was characterized by synergism when Zn was the metal at the highest concentration, changing to antagonism when Cd concentration was highest. Using 0.01 M CaCl2 extractable metal concentrations as the exposure metric, effects were also significantly different from additivity, predominantly being synergistic. This indicates that accounting for putative environmental availability did not explain the interaction. Metal analysis for this fraction indicated no effect of Cd on extractable Zn concentrations, but that Zn increased Cd extractability, potentially explaining the synergy. This bioavailability effect could be explained by replacement of Cd on soil binding sites by Zn, possibly enhanced by the formation of soluble Cd-Cl complexes resulting from increased Cl-counter ion presence with greater ZnCl2 addition. Modeling mixture effects based on earthworm tissue metal concentrations indicated no significant deviations from additivity. The tissue measurements indicated that internal Zn was not affected by soil total or extractable Cd levels. However, tissue Cd was strongly reduced by Zn. Such inhibition of Cd uptake could result from Zn competition with Cd at uptake sites and/or the formation of poorly bioavailable Cd-Cl species. Taken together, these mechanisms explain the concentration ratio dependent toxicity of Cd and Zn, why this is greatest when Cd dominates the mixture, and how, when effects are modeled based on tissue concentrations, effects accord with additivity.