Water Hardness Improves the Antioxidant Response of Zinc-Exposed Goldfish (Carassius auratus)

Simple Summary Zinc (Zn), known as a heavy metal and an essential element, is beneficial to fish in small amounts, but exposure to high concentrations of Zn induces physiological changes in fish. These changes may disrupt homeostasis in fish and cause stress. The water hardness of fresh water has the effect of reducing stress caused by Zn. In this study, to investigate the effect of water hardness on zinc-induced oxidative stress, we confirmed the extent of oxidative stress and apoptosis by exposing goldfish to various Zn concentrations and water hardness. Ultimately, our results suggest that a high water hardness level decreases Zn-induced toxicity, which is expected to be able to provide new insight into our understanding of how aquatic environmental factors, such as water hardness, affect the maintaining of the antioxidant system of fish. Zinc (Zn), a heavy metal, is an essential element in fish; however, exposure to high concentrations causes oxidative stress. Water hardness reduces oxidative stress reactions caused by heavy metals. To confirm the effect of water hardness on oxidative stress caused by Zn, goldfish were exposed to various Zn concentrations (1.0, 2.0, and 5.0 mg/L) and water hardness (soft (S), hard (H), and very hard (V)). The activity of superoxide dismutase (SOD) and catalase (CAT) in plasma increased with 1.0, 2.0, and 5.0 mg/L of Zn, and decreased with H and V water hardness. The levels of H2O2 and lipid peroxide (LPO) increased with Zn above 1.0 mg/L and decreased with H and V of water hardness. Caspase-9 mRNA expression in the liver increased after 7 and 14 days of Zn exposure and decreased with H and V water hardness. It was confirmed that DNA damage was less dependent on H and V water hardness. Based on the results of this study, at least 1.0 mg/L Zn causes oxidative stress in goldfish, and a high level of apoptosis occurs when exposed for more than 7 days. It appears that the oxidative stress generated by Zn can be alleviated by water hardness of at least 270 mg/L CaCO3. This study provides information on the relationship between the antioxidant response caused by heavy metals and water hardness in fish.