Fate of the Herbicide Alachlor Exposed to Different Microbial Consortia in Aquatic Systems

Abstract

This work explored the influence of different microbial consortia on the attenuation of the herbicide alachlor in the aqueous phase and its impact on the bacterial communities. Experiments were performed using microcosms (450 mL) at two initial alachlor concentrations (1 and 20 μg L−1) with or without suspended microbial inoculum monitored for 10 days. The microbial consortia from different habitats included activated sludge (site W) from a wastewater treatment plant and a suspension of natural phototrophic biofilms from two distinct lotic environments (site G, mid-slope of the river Garonne, France and site M, in an agricultural watershed basin, the stream Montousse, France), different colonization substrate types (glass slides versus pebbles) and exhibited different in situ culture periods. Comparison of biomass levels and bacterial community structure measured by PCR-denaturing gradient gel electrophoresis (DGGE) revealed differences according to habitat type (wastewater treatment plant versus lotic environments) and two distinct lotic environments (the river Garonne versus the stream Montousse) among microbial consortia. The disappearance curves of water soluble alachlor in all microcosms were exponential; thus, a first-order model was used to estimate the rate of disappearance and the half-life of alachlor. Alachlor decay coefficients were similar between the 1 and 20 μg L−1 treatments for communities from activated sludge but differed significantly for communities from phototrophic biofilms with the greatest values for 1 μg L−1 treatments. For both treatments (respectively at 1 and 20 μg L−1), the shortest half-life values were observed for experiments with biofilms from site G (3.9 and 8 days) followed by biofilms from site M (16.3 and 30.8 days) and then by activated sludge from site W (21.8 and 21.6 days). Maximum specific alachlor removal rates obtained in 20 μg L−1 treatments were 62.72, 9.21 and 5.46 μg alachlor removed g−1 AFDM day−1, respectively, for biomasses from sites W, G and M. The structural (PCR-DGGE) and functional diversity (carbon source utilization using commercial Eco-plates) responses of bacterial communities during the microcosm incubation were different depending upon the nominal alachlor concentrations and site of origin. This study confirmed the influence of habitat type and thus environmental conditions on the structure and metabolic activity (alachlor degradation) of microbial communities, the importance of considering the influence of operating conditions (incubation period) when investigating microcosm study and the nature of chemical studied.