Control of transport processes on microbial dynamics and pesticide degradation from µm to mm scales

International audienceSoil microorganisms play a major ecosystem function by degrading agricultural pollutants such as pesticides,thus mitigating their dispersion in the environment and damping their consequences on Earth’s biochemicalcycles. Understanding the mechanisms at the microbial scale is key to understand the control of remediationefficiency among soils and to propose improvements of ecologically-based agricultural practices.Soils are among the most heterogeneous microbial habitats on Earth, with separation distances between pesticidesand pesticide degraders large enough to see this habitat as a “desert” for microorganisms. In such context, besidesphysicochemical and catabolic limitations, spatiotemporal access of pesticide degraders to their substrate ispotentially a strongly limiting factor of biodegradation.Unlike 2,4-D, a pesticide that can easily diffuse through soil water films, 2,4-D degraders have been shownby Pinheiro et al. [2015] to be immobile at field capacity at fixed saturation. Infiltration can however movethe degraders and enhance biodegradation, suggesting potential positive feedbacks between dispersion andbiodegradation.The objective of this presentation is to formalize the control of the spatiotemporal organizations on pesticideconcentration and degradation rates. And more generally, is it systematically beneficial for degraders todisperse, given that their substrate diffuses?We develop two reactive transport models, from m to mm scales [Babey et al., 2017], to investigate theinterplay between initial distributions, transport processes and metabolic degradative processes. We considerspace-limited spots of substrate as substrate initial distributions, and homogeneous isotropic diffusion as transportprocess. The temporal biological response relies on classical biological metabolic processes such as substratelimitation, microbial growth, mortality, and microbial lag phase. Previously calibrated values were used fordiffusion and biological parameters [Babey et al., 2017].Synthetic simulations show that the interaction between microbial exposition to pesticide and microbial uptakecapacities leads to contrasted biodegradation efficiencies. We show that degrader dispersion generallyenhances their probability of contact with the substrate from the time substrate gradients are reversed, potentiallyat the expense of their activity in more diluted environments. Counteracting processes may spatially modulate theefficiency of degradation.Strong biodegradation heterogeneities can emerge from no-linear interactions between transport and biologicaleffects. Therefore degrader dispersion would be favorable only in specific situations. Results giveguidelines to design future experiments to validate the control of respective spatiotemporal organizations ofpesticide concentration and degraders