Bacterial community structure and biogeochemical activity in an aquifer contaminated with pesticides

International audienceOur objective was to assess the effect of cocktails of pesticides on groundwater microbial abundance, community structure and their nitrate reducing activity. We used two complimentary approaches: a 2-year monitoring at a catchment level (Ariege alluvial plain, France) and microcosm with groundwater collected in 2 parts of the aquifer and then spiked with selected herbicides having a high occurrence in this aquifer, atrazine (ATZ), desethylated atrazine (DEA) and ATZ+DEA. Abundance of the universal marker (16S rRNA) and of nitrate-reducing bacteria (narG and napA) was assessed by quantitative PCR (qPCR). Diversity was assessed using fingerprinting technic, CE-SSCP (Capillary Electrophoresis-Single Strand Conformational Polymorphism). Pesticides in water were analyzed by LC-MS/MS following an on line-solid phase extraction, and samples for anions and cations were analyzed by ion chromatography. Preliminary results of the microcosm experiment show that biodiversity was higher in water historically contaminated than in pristine-like water and remained higher under laboratory incubations with ATZ, DEA or ATZ+DEA concentrations. Pesticide concentrations in the water historically contaminated often exceeded the legal EU threshold for groundwater and drinking waters (2-year mean: 0.09 ± 0.01 µg ATZ /L, 0.43 ± 0.06 µg DEA /L (n = 23)). On the other side, during microcosm incubation, biodiversity decreased when spiked-ATZ, -DEA or -ATZ+DEA increased from 1 to 10 µg/L. The undergoing analyses of the two-year monitoring at the catchment level will enable to refine boudaries within this complex relationship between biodiversity and pesticide contamination. ATZ, DEA or ATZ+DEA exhibited similar effect on the microbial community. The nitrate reducing bacterial community strongly decreased during the microcosm experiment. In situ samples are expected to clarify the effect of pesticide contamination on bacterial nitrate reducing activity and the possible risk of nitrate acumulation and/or the possible risk of inhibition effect of nitrate on pesticide biodegradation. Biomass was similar in all conditions, suggesting that this is not a sensitive endpoint to assess water quality. This study has the potential to provide sound-based arguments to be considered when improving the current strategy to manage water quality as well as when proposing end point to monitor the microbial community in the biodiversity objective under the European water directive framework