Bacterial community structure and biogeochemical activity in an aquifer contaminated with pesticides
Résumé
Our objective was to assess the effect of cocktails of pesticides on groundwater community structure, microbial abundance and denitrifier abundance. We used two complimentary approaches: a 2-year in situ monitoring at a rural alluvial plain (France) (n = 37) and microcosms with groundwater with contrasted contamination history spiked with selected herbicides having a high occurrence in this aquifer, atrazine (ATZ), desethylated atrazine (DEA) and ATZ+DEA (n = 50). Abundance of the universal marker (16S rRNA) and of nitrate-reducing bacteria (narG and napA) was assessed by qPCR. Biodiversity was assessed using a fingerprinting technic (CE-SSCP). Pesticides in water were analyzed by LC-MS/MS. In microcosms, biodiversity was higher in historically contaminated water than in pristine-like one. The community structure was affected by the concentration of the incubation with community different from the control at 1 ug/L in the pristine-like water and 10 ug/L in the historically contaminated one suggesting a community tolerance to triazine induced by its chronic exposure. The time duration of the incubation also affected the community structure, however, the triazine type had similar effect on the community structure. Biomass was higher at 10 ug/L than at 1 ug/L or in control, especially at 30-d incubation in both water types. This was surprising since triazine degradation was not observed during the 4-week incubation. During the two-year in situ monitoring, triazine affected the community structure, while denitrifier abundance slightly but significantly increased when chloroacetanilide concentrations increased. A significant relationship was not observed between nitrate concentration and microbial biomass or denitrifier abundance. Microbial end points based on molecular indicators should be proposed to complete the biodiversity objective under the European water directive framework with the microbial compartment.