The importance of water hardness in determining the reactions of the fish ((Oreochromis niloticus) antioxidant system to the effect of Cr+6, Ni+2, Zn+2 ve Hg+2

In this study, the responses of the antioxidant system of freshwater fish Oreochromis niloticus were investigated following exposure to chromium, nickel, zinc and mercury in differing hardness (30, 60 and 120 mg Ca2+/L) in acute (3 days) and chronic (30 days) durations. For this purpose, the responses of enzymatic and non-enzymatic biomarkers belonging to the antioxidant system were measured in the livers of fish exposed to sublethal concentrations of metals. Antioxidant system parameters showed significant changes (P<0.05) in both acute and chronic experiments in differing hardness. The enzymes that were most affected in both exposure durations were glutathione-S-transferase (GST) and glutathione peroxidase (GPx), while the least affected enzymes were catalase (CAT) and superoxide dismutase (SOD). Furthermore, the responses of the enzymes also varied among different exposure types. There were significant increases in CAT and SOD activities, while there were significant decreases in GPx and GST activities. The levels of the non-enzymatic biomarker, glutathione (GSH), decreased in acute exposures, while they increased in chronic exposures. The results showed that the responses of the antioxidant systems of the fish exposed to metals in differing water hardness were also different. It was evident that the least effect was observed in medium-hardness in acute exposures, while the opposite was observed in chronic exposures, as the most effects were seen in medium-hardness.