Control of encounter between pesticide and bacteria on pesticide degradation in soil at mm-to-cm scales

International audienceSome organic pollutants found in soils, like the herbicide 2,4-D, are very mobile and can easily infiltrate, reach theunderground water and spread more broadly in the environment. The degradation of these pollutants during theirlixiviation through soil is a critical point to understand. This degradation starts with the encounter between thepollutant and the degrading agent, that are mainly bacterial endoenzymes in the case of 2,4-D. Encounter, and thusspatial distributions of bacteria and pollutants, appear to be particularly relevant in soils, where these distributionscan be highly heterogeneous and sparse. This leads us to wonder how the macroscopic spatiotemporal distributionsof bacteria and their substrate impact their encounter and thus pollutant degradation.It is often assumed that the dispersion of bacteria and their substrate in soil eventually promotes their encounter.But the validation and the fine understanding of this assumption, especially of how this encounter isshaped by the interaction between transport processes and bacterial metabolism, is still unclear. We use thisassumption as our working hypothesis.We develop several reactive transport models at mm-to-cm scale (Babey et al., 2017), in a considered homogeneousmedium, aimed at investigating the impacts of initial localizations (mainly co-localization) andconcentrations of 2,4-D and its bacterial degraders on 2,4-D biodegradation, under several transport processes(diffusion and advection) and biochemical processes (sorption and microbial metabolism). These models are builtand calibrated on cm-scale experiments performed on the degradation of 2,4-D spots in natural repacked soil coreswithout dispersion of bacteria (Pinheiro et al., 2015), and compared to homologous experiment with dispersion ofbacteria (Pinheiro et al., 2018).Contrary to our hypothesis, we show through modeling that dispersion of bacteria and substrate decreasestheir encounter in many cases. This is causedby the immediate, continuous, and strongly adverse effect of substrate dilution. We show that dispersion promotesencounter in some very specific cases, when an inversion of substrate gradient occurs, that is when the effectof dilution is at least counterbalanced by a decrease of spatial competition of bacteria for substrate. Whetherbacteria dispersion promotes or not encounter strongly depends on the interplay between transport processes andmetabolism.We also show that the positive effect of dispersion on encounter is theoretically limited, and that in someexperiments it is not high enough to explain the observed increase of pollutant biodegradation caused by bacteriadispersion. This points out that dispersion can act also through processes other than encounter between bacteriaand their substrate. One of these processes could be spatial inhibition between bacteria.ReferencesBabey T, Vieublé-Gonod L, Rapaport A, Pinheiro M, Garnier P, de Dreuzy J-R. Spatiotemporal simulations of2,4-D pesticide degradation by microorganisms in 3D soil-core experiments. Ecol Model. 2017 Jan;344:48–61.Pinheiro M, Garnier P, Beguet J, Martin Laurent F, Vieublé Gonod L. The millimetre-scale distribution of 2,4-Dand its degraders drives the fate of 2,4-D at the soil core scale. Soil Biol Biochem. 2015 Sep;88:90–10