Effect of sublethal concentration of mercury and zinc on the energetics of a freshwater fish Cyprinus carpio (Linnaeus).

Exposure of a freshwater fish Cyprinus carpio to the sublethal concentration of mercury (0.1 mg L-1) and zinc (6.0 mg L-1) resulted in distinct changes in the energy metabolism of gill, liver and muscle at 1, 15 and 30 days. The changes were: (a) The rate of oxygen consumption and SDH activity decreased in the organs of mercury-exposed fish at all the three exposure periods in the order 1 > 15 < 30 days. Whereas, an increase was observed in these parameters in the organs of zinc-exposed fish in the order 1 > 15 > 30 days. (b) The activity of LDH and the levels of pyruvate and lactate increased in all the three organs of the fish at the three exposure periods studied in both the metal media. But, this increase was also in the order 1 > 15 < 30 days and 1 > 15 > 30 days in the organs of the fish exposed to mercury and zinc, respectively. (c) The results indicated greater reliance of mercury exposed fish on the energetically less efficient anaerobic glycolysis as the oxidative metabolism suppressed, and the dependency of zinc-exposed fish on both the oxidative and anaerobic glycolytic pathways in order to meet the energy requirements. On prolonged exposure zinc-exposed fish could adapt to sublethal toxic stress, such type of adaptive-response was not observed in mercury-exposed fish.