Use of molluscs as biomarkers of environmental pollution

Agricultural, industrial, mining and other anthropogenic activities result in the production of an increasing diversity and volume of chemicals. These include pyrethroids, organochlorines, carbamates (CMs) organophosphates (OPs), polychlorobiphenyls, dioxins, hydrocarbons and metals (Cd, Cu, Fe, Hg, Pb, Zn etc), many of which find their way into aquatic ecosystems and biota. In these water-bodies, such foreign compounds (xenobiotics) pose a threat to non-target species through a variety of molecular mechanisms, including genotoxicity, neurotoxicity, endocrine disruption and oxidative stress. We are currently pursuing the possibility of using freshwater snails as sentinels of pollution exposure and damage. This approach involves the development of biochemical measurements as biomarkers of exposure to the above types of contaminants. Data obtained so far indicate that exposure to some pesticides and metals alters (increased or decreased activity) particular biotransformation (xenobiotic-metabolising) and other protection enzymes, namely glutathione S-transferase (EC 2.5.1.18), antioxidant enzymes and esterases, to varying extents depending on the pollutant and its dosage. Unfortunately, in most cases examined so far, the alterations observed are too variable and small to be useful as robust biomarkers. However, using a number of different substrates to measure cholinesterase (EC 3.1.1.8) and general esterase activities, exposure to OP and CM pesticides usually leads to an inhibition in enzyme activity. Although the changes can be pesticide- and substrate-specific, the consistency and magnitude are sufficient to use reduced esterase activity in several aquatic snail species, such as Helisoma duryi and Lymnaea natalensis, as a biomarker of water pollution by OPs and CMs. Of particular interest is the fact that the inhibition is generally dose-dependent and detectable within hours of exposure. In addition, from an ecological standpoint, our data indicate that sub-lethal and chronic exposure to pollutants causes alterations in protective enzymes. This in turn may have ramifications for aspects of organism and ecosystem health.