The synergistic toxicity of Cd(II) and Cu(II) to zebrafish (Danio rerio): Effect of water hardness.

We have evaluated the interactive toxicity of Cu(II) and Cd(II) in water with different hardness levels using adult zebrafish (Danio rerio). Zebrafish were exposed to Cd(II) (0.2-22 µM) or Cu(II) (0.1-8 µM) in single or binary exposures in very soft, moderately hard or very hard water. The whole body burdens of Cd(II) and Cu(II) reflect the net effect of biouptake and elimination, mortality was the indicator of toxicity, and whole body major ion content was measured to assess ion regulatory functions. Cu(II) was found to be more toxic than Cd(II) for zebrafish, and Cu(II) and Cd(II) exhibited a significant synergistic effect. The toxicity of metal ions increased upon decreasing the ionic strength of the exposure medium, probably due to elevated competition between metal ions with other cations in hard water and increased activity of Ca2+ pathways in soft water treatments. Whole body metal accumulation and the accumulation rate of both Cu and Cd increased as the metal ion concentration in the exposure medium increased. Nevertheless, neither parameter explained the observed synergistic effect on mortality. Finally, we observed a significant loss of whole body Na+ in fish which died during the metal exposure compared to surviving fish, irrespective of exposure conditions. Such an effect was not observed for other major cations (K+, Ca2+ and Mg2+). This observation suggests that, under the applied exposure conditions, survival was correlated to the capacity of the organism to maintain Na+ homeostasis.

Systematic Evaluation of Chronic Metal-Mixture Toxicity to Three Species and Implications for Risk Assessment.

Metal contamination generally occurs as mixtures. However, it is yet unresolved how to address metal mixtures in risk assessment. Therefore, using consistent methodologies, we have set up experiments to identify which mixture model applies best at low-level effects, i.e., the independent action (IA) or concentration addition (CA) reference model. The toxicity of metal mixtures (Ni, Zn, Cu, Cd, and Pb) to Daphnia magna, Ceriodaphnia dubia, and Hordeum vulgare was investigated in different waters or soils, totaling 30 different experiments. Some mixtures of different metals, each individually causing <10% inhibition, yielded much larger inhibition (up to 66%) when dosed in combination. In general, IA was most accurate in predicting mixture toxicity, while CA was the most conservative. At low-effect levels important in risk assessments, CA overestimated mixture toxicity to daphnids and H. vulgare, on average, with a factor 1.4 to 3.6. Observed mixture interactions could be related to bioavailability or by competition interactions, either for binding sites of dissolved organic carbon or for biotic ligand sites. Our study suggests that the current metal-by-metal approach in risk evaluations may not be conservative enough for metal mixtures.