Influence of copper and cadmium toxicity on antioxidant enzyme activity in freshwater ciliates

Abstract

Though metals are biologically important, increase in the threshold concentration of heavy metal in the environment may interfere with the metabolic activity of organisms. Heavy metal stress induces production of reactive oxygen species (ROS) <em>viz</em>. hydroxyl radical, superoxide radical or hydrogen peroxide (H₂O₂). Various antioxidant enzymes such as superoxide dismutase, ascorbate peroxidase, catalase, glutathione peroxidase and peroxiredoxin of SOD-ascorbate-glutathione cycle get activated during such stress. These enzymes are involved in ROS detoxification , and thereby protect the cells from oxidative stress. In this study, we explored activity of the antioxidant enzyme, superoxide dismutase (SOD) in the presence of heavy metals i.e., Cu and Cd in three freshwater ciliates,<em> Tetmemena</em> sp., <em>Notohymena</em> sp. and <em>Euplotes</em> sp. Heavy metals negatively affected the population growth of chosen ciliates which was measured as 24 h-LC₅₀. The 24 h-LC₅₀ doses of Cu and Cd, respectively were 0.14 and 2.44 mg/L for <em>Tetmemena</em> sp.; 0.74 and 5.06 mg/L for <em>Notohymena</em> sp.; and 0.17 and 2.24 mg/L for <em>Euplotes</em> sp. The levels of SOD were found to be significantly higher in the presence of Cu and Cd. Interestingly, exposure with Cu increased the SOD level at LC₃₀ and LC₅₀ doses, however, at higher concentration of Cu (i.e., LC₇₀ dose), SOD level decreased. The results suggest that SOD may be used as an effective enzymatic biomarker to evaluate the toxic effects of heavy metals in the ciliate species.