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This study involves an investigation on the effects of a pesticide, metal and a detergent as individual and mixtures on esterase and antioxidant enzyme activity of the freshwater snail Helisoma duryi. Adult snails were exposed to sublethal concentrations of copper (5 µg/L), industrial detergent, oxyfoam (15 µg/L), carbaryl (25 µg/L) as well as mixtures of these pollutants for 96 hours. Carboxylesterase and cholinesterase activities were measured using 4-nitrophenyl acetate and acetylthiocholine iodide as substrates respectively. The activities of superoxide dismutase, catalase, glutathione peroxidase and glutathione S-transferase were also measured as indices of oxidative stress. Esterase activity was inhibited in snails exposed to carbaryl, copper or detergent. Mixtures of the different chemicals also caused inhibitions of esterase activity when compared to the controls. All the chemicals individually and as mixtures, caused enhanced activities of antioxidant enzymes. When comparing all the antioxidant enzymes analyzed, the highest activity was caused by the triple mixture of the pollutants. The results suggest that aquatic life is at risk to adverse effects of pollutant mixture as reflected by increased alteration of enzyme activity in mixture-exposed snails though the increase was less than sum of effects of individual pollutants. More studies on the effects of a wider range of pollutant mixtures on aquatic organisms are needed however, for the full appreciation of reactive interactions that takes place in complex mixtures which ultimately affect the health of aquatic biota.

Pesticides extensively used in agricultural fields to ensure high quality crop yields indirectly find their way to aquatic bodies where they affect aquatic biota. We investigated the effects of pesticides in different dam waters on esterase enzyme activity of the freshwater snail species Lymnaea natalensis. Groups of adult snails were exposed to 0.006 ppm chlorpyrifos and 0.003 ppm aldicarb in polluted water from Umguza dam and relatively pristine water from Hillside dam for 14 days. Carboxylesterase, acetylcholinesterase and arylesterase activities were measured. Both pesticides caused significant inhibition of esterase activity after the 14 day exposure period, with exposures to Umguza dam water showing higher inhibition as compared to exposures to Hillside dam water. Aldicarb and chlorpyrifos both showed a time-dependent inhibition of enzyme activity, the former causing a higher inhibitory effect as compared to the latter. Acetylcholinesterase was inhibited up to 80% following exposure to aldicarb while exposure to chlorpyrifos in Umguza water caused only 40% inhibition. Carboxylesterases were similarly inhibited with higher inhibition observed in snails exposed to Umguza dam water when compared to snails exposed to Hillside dam water, while arylesterases were inhibited in the range 80-90%, with an exception of chlorpyrifos spiked Hillside dam water which caused 45% inhibition. Contaminated Umguza dam water also appeared to enhance the effects of pesticides when compared to the relatively pristine Hillside dam water. Alteration of esterase activity can be used as an early warning signal indicating exposure to environmental pollutants. The results of this study therefore, highlight the adverse effects of pesticides on non-target aquatic organisms, evidenced by the inhibition of esterase activity.

Aquatic reservoirs remain the ultimate sink of chemical pollutants emanating from anthropogenic activities such as agriculture, mining and industry. Freshwater biota undoubtedly is at risk from the adverse effects of these water pollutants and there is therefore, a need to monitor effects of these chemical pollutants in order to safeguard the health of aquatic biota. We investigated the oxidative stress effects of chlorpyrifos and lead on the freshwater snail Helisoma duryi to assess the potential of using this enzyme system as a biondicator of exposure to environmental pollutants. Groups of snails were exposed to 5 ppb lead acetate and 25 ppb chlorpyrifos for 7 days after which half of the snails were sacrificed and the other half were allowed to recover in clean water and sacrificed after another 7 days. Post mitochondrial fractions were used to measure the activities of the following antioxidant enzymes: superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase and diphosphotriphosphodiaphorase. Both pollutants enhanced the activities of all the antioxidant enzymes suggesting a defensive mechanism by the snail to combat the oxidative stress due to the organophosphate chlopryrifos and metal pollutant lead. There was a significant recovery of the antioxidant defense system of the snails allowed to recover in clean water shown by the reduced alteration of the antioxidant enzyme activities of the snails allowed to recover for 7 days. This suggests the need to minimize exposure of aquatic biota to chemical pollutants and remediate the polluted water reservoirs in order to safeguard the health of aquatic life.