Abstract
Exposure to even low concentrations of heavy metals can be toxic to aquatic organisms, especially during embryonic development. Thus, this study aimed to investigate the toxicity of nickel and cadmium in zebrafish (Danio rerio) embryos exposed to environmentally relevant concentrations of each metal alone or in combination from 4 h through to 72 h postfertilization. Neither metal altered survival, but individual and combined exposures decreased hatching rate. Whereas cadmium did not affect total body length, trunk area, eye diameter, or eye area, nickel alone and in combination with cadmium decreased each morphological parameter. Yolk sac area, an index of metabolic rate, was not affected by nickel, but was larger in embryos exposed to high cadmium concentrations or nickel and cadmium combined at high concentrations. Nickel decreased spontaneous movement, whereas cadmium alone or nickel and cadmium combined had no effect. Neither metal altered elicited movement, but nickel and cadmium combined decreased elicited movement. Myosin protein expression in skeletal muscle was not altered by cadmium exposure. However, exposure to nickel at low concentrations and combined exposure to nickel and cadmium decreased myosin expression. Overall, nickel was more toxic than cadmium. In conclusion, we observed that combined exposures had a greater effect on movement than gross morphology, and no significant additive or synergistic interactions were present. These results imply that nickel and cadmium are toxic to developing embryos, even at very low exposure concentrations, and that these metals act via different mechanisms.
Highlights
Environmental heavy metal contamination is an increasing global concern
Zebrafish embryos (ZFE) typically hatch at 48–96 hpf, with the majority hatching by 72 hpf [35]; we assessed the hatching rate at 48 and 72 hpf (Figure 1)
At 72 hpf (Figure 1D–F), the hatching rate in embryos exposed to the two lowest nickel concentrations and embryos exposed to all cadmium concentrations were comparable to that of age-matched controls (ANOVA; nickel, p < 0.00001; cadmium, p = 0.46)
Summary
Environmental heavy metal contamination is an increasing global concern. Nickel and cadmium content in plants appears to be linear with respect to metal contamination of ground soil, which can contaminate aquatic systems due to runoff [4,5,6]. Subsequent leakage of industrial waste and sewage into groundwater or the application of sewage sludge onto soil and mobilization of metals from sediments during floods further increase the environmental heavy metal contamination of aquatic systems [10]. Heavy metal contamination of fresh and salt waters is of growing concern and even in trace amounts, metals such as mercury, lead, cadmium, and nickel are considered to be a severe threat to the aquatic environment
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