Acute toxicity of binary and ternary mixtures of Cd, Cu, and Zn to Daphnia magna

Abstract

Standard static-exposure acute lethality tests were conducted with Daphnia magna neonates exposed to binary or ternary mixtures of Cd, Cu, and Zn in moderately hard reconstituted water that contained 3 mg dissolved organic carbon/L added as Suwannee River fulvic acid. These experiments were conducted to test for additive toxicity (i.e., the response to the mixture can be predicted by combining the responses obtained in single-metal toxicity tests) or nonadditive toxicity (i.e., the response is less than or greater than additive). Based on total metal concentrations (>90% dissolved) the toxicity of the tested metal mixtures could be categorized into all 3 possible additivity categories: less-than-additive toxicity (e.g., Cd-Zn and Cd-Cu-Zn mixtures and Cd-Cu mixtures when Cu was titrated into Cd-containing waters), additive toxicity (e.g., some Cu-Zn mixtures), or more-than-additive toxicity (some Cu-Zn mixtures and Cd-Cu mixtures when Cd was titrated into Cu-containing waters). Exposing the organisms to a range of sublethal to supralethal concentrations of the titrated metal was especially helpful in identifying nonadditive interactions. Geochemical processes (e.g., metal-metal competition for binding to dissolved organic matter and/or the biotic ligand, and possibly supersaturation of exposure waters with the metals in some high-concentration exposures) can explain much of the observed metal-metal interactions. Therefore, bioavailability models that incorporate those geochemical (and possibly some physiological) processes might be able to predict metal mixture toxicity accurately.

Explanation of non-additive effects in mixtures of similar mode of action chemicals

“…In the right panel in Fig. 8 with [C A ] fixed at EC40, the introduction of C B leaded to the reduction of death rate and this is a similar pattern to the result of Cd-Zn mixture in Meyer et al (2015). The result also suggests that the same or similar reaction system of our Model III is involved in the combined effect of Cd-Zn mixture, although more detailed studies are definitely necessary.…”

“…Although the C A is a reasonable model for the combined effects, it is important to investigate the limit of the model. Meyer et al (2015) showed that an introduction of low concentration leaded to the sharp reduction of death rate in Cd-Zn mixture. In the right panel in Fig.…”

“…They found that the combined effects of cadmium and zinc were antagonistic when the cadmium was responsible for the toxicity, and were synergistic when zinc was responsible. Meyer et al (2015) conducted binary and ternary mixture of cadmium, copper and zinc with various concentrations. They found that combined effects of Cu-Zn mixture were additive or slightly synergistic, those of Cd-Zn were additive or slightly antagonistic, and those of Cd-Cu were additive, slightly antagonistic and synergistic depending on the concentrations in toxicity tests.…”

Age‐related differences in sensitivity to metals can matter for Daphnia magna neonates

“…Because organisms of different developmental stages may differ in their sensitivity to a chemical, many regulatory agencies (USEPA 2002; OECD 2004) specify that acute toxicity tests should be started with D. magna younger than 1 day old. However, we have observed relatively high variability in the toxicity of Cd and Zn to D. magna neonates (Meyer et al 2015; Traudt et al 2015) and have demonstrated that this variablity is at least partly caused by major changes in sensitivity to the metals even within their first day postbirth (Traudt et al 2016). This methodologically induced variability can have important consequences, especially for predictions of metal-mixture toxicity when using results from individual-metal tests.…”

“…We previously exposed D. magna to binary and ternary mixtures of Cd, Cu, Ni, and Zn in 48-h, static, nonrenewal lethality tests following USEPA-recommended procedures (Meyer et al 2015; Traudt et al 2015). We observed high variability in mortality in the binary and ternary mixtures that contained either Cd or Zn but relatively lower variability in mixtures containing Cu or Ni.…”