Control of encounter between pesticide and bacteria on pesticide degradation in soil at mm-to-cm scales
Résumé
Some organic pollutants found in soils, like the herbicide 2,4-D, are very mobile and can easily infiltrate, reach the
underground water and spread more broadly in the environment. The degradation of these pollutants during their
lixiviation through soil is a critical point to understand. This degradation starts with the encounter between the
pollutant and the degrading agent, that are mainly bacterial endoenzymes in the case of 2,4-D. Encounter, and thus
spatial distributions of bacteria and pollutants, appear to be particularly relevant in soils, where these distributions
can be highly heterogeneous and sparse. This leads us to wonder how the macroscopic spatiotemporal distributions
of 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 is
shaped by the interaction between transport processes and bacterial metabolism, is still unclear. We use this
assumption as our working hypothesis.
We develop several reactive transport models at mm-to-cm scale (Babey et al., 2017), in a considered homogeneous
medium, aimed at investigating the impacts of initial localizations (mainly co-localization) and
concentrations 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 built
and calibrated on cm-scale experiments performed on the degradation of 2,4-D spots in natural repacked soil cores
without dispersion of bacteria (Pinheiro et al., 2015), and compared to homologous experiment with dispersion of
bacteria (Pinheiro et al., 2018).
Contrary to our hypothesis, we show through modeling that dispersion of bacteria and substrate decreases
their encounter in many cases. This is caused
by the immediate, continuous, and strongly adverse effect of substrate dilution. We show that dispersion promotes
encounter in some very specific cases, when an inversion of substrate gradient occurs, that is when the effect
of dilution is at least counterbalanced by a decrease of spatial competition of bacteria for substrate. Whether
bacteria dispersion promotes or not encounter strongly depends on the interplay between transport processes and
metabolism.
We also show that the positive effect of dispersion on encounter is theoretically limited, and that in some
experiments it is not high enough to explain the observed increase of pollutant biodegradation caused by bacteria
dispersion. This points out that dispersion can act also through processes other than encounter between bacteria
and their substrate. One of these processes could be spatial inhibition between bacteria.
References
Babey T, Vieublé-Gonod L, Rapaport A, Pinheiro M, Garnier P, de Dreuzy J-R. Spatiotemporal simulations of
2,4-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-D
and its degraders drives the fate of 2,4-D at the soil core scale. Soil Biol Biochem. 2015 Sep;88:90–100