Facilitated depuration of hazardous non-essential elements in pre-exposed fish

In this study, we investigated the facilitated depuration of non-essential elements from the tissues of Clarias gariepinus by interactions with essential elements. Fish were exposed to lead, cadmium and mercury at a range of concentrations previously detected in aquatic environments, for 28 days. Subsets of fish exposed to each of these elements were then exposed to the essential elements, such as calcium, chromium, cobalt, magnesium, potassium or zinc and untreated controls for another 28 days. After exposures, the residual concentrations of lead, cadmium and mercury in the tissues and faeces of the fish were analysed using flame atomic absorption spectrophotometry. The activities of antioxidant enzymes, such as superoxide dismutase, catalase and glutathione S-transferase in the exposed fish, were also assessed using referenced methods. The essential elements, calcium, chromium, cobalt and zinc, were found to facilitate the depuration of lead, cadmium and mercury in the exposed fish. The concentration of lead and mercury in the flesh of exposed fish decreased by 16% and 12%, respectively, after exposures to cobalt and that of cadmium decreased by 16% after exposure to zinc. The activities of the antioxidant enzymes in fish exposed to lead also decreased by at least 30%, respectively, after exposures to the essential elements. This study has shown that interactions among essential elements and non-essential elements can be beneficial, facilitating the elimination of the latter in aquatic organisms. Further studies are needed to determine the underlying mechanisms promoting this process and the possible adoption of the process to rehabilitate aquatic organisms in polluted ecosystems.