The Comet assay, a sensitive biomarker of water quality improvement following adoption of beneficial agricultural practices?

Numerous actions have been undertaken by farmers to attenuate the impact of agricultural activities on aquatic ecosystems. The identification of biomarkers that respond quickly to water quality improvement could facilitate the assessment of adopted alternative practices and help maintain mobilization among stakeholders. Here, we evaluated the potential of the Comet assay, a biomarker of genotoxic effects, using a freshwater mussel, Elliptio complanata, as a model animal. The frequency of DNA damage was assessed in hemocytes of mussels collected from a pristine habitat and caged for 8 weeks in the Pot au Beurre River (PAB), a tributary of the fluvial Lake St-Pierre (QC, Canada) impacted by agricultural activities. We found that the level of DNA damage naturally induced in mussel hemocytes was low and showed very limited variations over time. Compared to these baseline levels and to laboratory controls, we observed a doubling in DNA alterations in mussels exposed to agricultural runoff in the 3 branch of the PAB River. The genotoxic response was significantly lower in mussels caged in the 1 branch of the PAB river, where longer stretches of shoreline have been restored as buffer strips. Glyphosate, mesotrione, imazethapyr and metolachlor were the main discriminant pesticides between these two branches. Metolachlor was found in sufficient concentrations to induce DNA damage, but it is more likely that the observed genotoxicity was the result of a "cocktail effect" i.e. the cumulative contribution of co-existing genotoxicants including the above-mentioned herbicides and ingredients in their formulation. Our findings suggest that the Comet assay is a sensitive tool for the early detection of changes in water toxicity following the adoption of agricultural beneficial practices.