Evidence for adaptation of riverine sediment microbial communities to diuron mineralization: incidence of runoff and soil erosion

Abstract : PurposeSurface runoff and erosion are major drivers of pesticide transport from soils to rivers draining vineyard watersheds. A recent study showed that applications of diuron on vineyards and diuron dispersal could lead to microbial adaptation to diuron biodegradation from treated soils to the receiving hydrosystem. Given the limited knowledge on microbial adaptation to pesticide degradation in aquatic environments, we conducted a microcosm study designed to assess the impact of runoff and erosion processes on the adaptation of riverine-sediment microbial communities to diuron mineralization.Materials and methodsThe experimental laboratory set-up consisted in aquariums filled with natural riverine sediments and water and supplemented or not in triplicate (a) with diuron to simulate surface water runoff and (b) with diuron-treated vineyard soil to simulate erosion following a strong rainfall event. The resulting effects were estimated by assessing and comparing (a) the fate of diuron and diuron partitioning between sediment and water phases, (b) the evolution of sediment-based bacterial community density and community structure, and (c) the evolution of diuron mineralization potential in sediment samples.Results and discussionDiuron dissipated rapidly in all the microcosms, with half-life values varying between 1 and 3 weeks. The treated soil (and then soil microbiota) or dissolved diuron inputs to microcosms had no significant effect on sediment bacterial density and community structure. After 2 to 4 weeks, both contamination procedures led to a significant increase in sediment diuron mineralization potential.Despite the lack of effects on sediment bacterial density and community structure, both diuron runoff and/or diuron-contaminated soil erosion led to sediment community adaptation to diuron mineralization. This confirmed that chronic exposure to diuron may lead riverine sediment communities to adapt to the degradation of this herbicide. The shorter initial lag-phase in mineralization kinetics observed in microcosms treated with diuron-contaminated soil suggests transfer of degradative potential from soil to sediment.ConclusionsOur observations confirmed evidence of a high diuron biodegradation potential by benthic microbial communities in chronically exposed watersheds and highlighted that inter-linkages between soil and sediments may also modify biological functions that regulate aquatic ecosystems.Microbial communities adapted to pesticide biodegradation are likely to play a key environmental role in reducing pesticide exposure in contaminated ecosystems. To assess this self-purifying function, further research is needed to improve the estimation of real in-field pesticide biodegradation kinetics and processes in both soil and aquatic ecosystems without overlooking the inter-compartments linkages that can occur.