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. •Cu(II) is more toxic than Cd(II) to zebrafish at the level of mortality.•Cu(II) and Cd(II) exhibit a significant synergistic effect in mixture exposures.•Toxicity of metal ions increases with decreasing water hardness.•Whole body metal burden and metal accumulation rate do not explain metal toxicity.•Whole body Na+ level of dead fish is always lower than that of the survivors.