Responses of oxidative stress biomarkers of freshwater fish (Oreochromis niloticus) exposed to Cr6+, Hg2+, Ni2+ and Zn2+ in differing calcium levels

Abstract

Freshwaters from different geographical locations show different hardness, affecting metal uptake and toxicity in fish. The most important ion that determines water hardness is calcium. In this study, acute and chronic effects of metals on the oxidative stress biomarkers in the liver of freshwater fish (Oreochromis niloticus) were investigated in differing Ca2+ (30, 60 and 120 mg Ca2+/L) levels. Fish were exposed to Cr6+, Ni2+ and Zn2+ (30 μM) and Hg2+ (0.3 μM) for 3 days in acute experiments, while they were exposed to Cr6+, Ni2+ and Zn+2 (10 μM) and Hg2+ 0.03 μM) for 30 days in chronic experiments. Data showed that the oxidative stress biomarkers significantly (p < 0.05) altered after metal exposures at all calcium levels, though there was no significant change (p > 0.05) among calcium controls. In both acute and chronic exposures, catalase CAT) and superoxide dismutase (SOD) activities increased significantly, while glutathione peroxidase (GPX), glutathione reductase (GR) and glutathione S-transferase (GST) activities decreased. There were significant decreases in total glutathione (GSH) levels in acute exposures, though GSH levels increased in chronic exposures. Malondialdehyde (MDA) levels significantly increased in both durations. The highest significant alterations in the biomarkers occurred at the lowest Ca2+ levels. GPX and GST were found to be the most sensitive enzymes in all exposures and the least alterations in biomarker response occurred in fish exposed to Ni2+. This study demonstrated that calcium levels (hardness) were important factors in the evaluation of metal toxicity for freshwater fish.