Does upregulation of metallothionein production explain less-than-additive toxicity of cadmium-nickel mixtures to adult Daphnia magna?

Abstract

Trace metals occur in the environment and can be essential nutrients for organisms, but even beneficial metals can be toxic at high concentrations. Understanding processes underlying metal-mixture toxicity to aquatic species is challenging. The toxicity of mixtures of cadmium (Cd) and nickel (Ni) to adult Daphnia magna is less than additive. Geochemical and physiological explanations have been posited to explain the effect, but not thus far tested. In the geochemical explanation, less-toxic Ni competes with Cd for binding sites on the organism, which prevents the more-toxic Cd from binding and thereby decreases mortality. In contrast, a commonly proposed physiological explanation is that the presence of a second, less toxic metal (Ni in this case) stimulates additional upregulation of metallothionein (MT), a metal-binding protein, which will then sequester more Cd and provide more protection against Cd toxicity. In this study, heat-stable proteins (HSPs), measured by the silver (Ag)-saturation method, were used as an indicator of MT production in adult Daphnia magna. Organisms exposed to Cd-Ni mixtures did not produce significantly more HSP than those exposed only to Cd. This result suggests that additional HSP production is not the mechanism by which the mixture mortality is decreased. Further testing is needed to determine if there is a different physiological explanation or if geochemical competition between Cd and Ni for binding to sites of toxic action on the organism is responsible.