Dynamics of reactive microbial hotspots in concentration and velocity gradients

International audience; n subsurface environments, bacteria play a major role in controlling the kinetics of a broad range of bio-geochemical reactions. In such environments, nutrients uxes and solute concentrations needed for bacteriametabolism may be highly variable in space and intermittent in time. This can lead to the formation of reactivehotspots where and when conditions are favorable to particular microorganisms, hence inducing biogeochemicalreaction kinetics that dier signicantly from those measured in homogeneous model environments. To investi-gate the impact of chemical gradients on the spatial structure and growth dynamics of subsurface microorganismpopulations, we develop microuidic cells allowing for a precise control of ow and chemical gradient conditions,as well as quantitative monitoring of the bacteria's spatial distribution and early-stage biolm development.Using the non-motile Escherichia coli JW1908-1 strain and Gallionella capsiferriformans ES-2 as modelorganisms, we investigate the behavior and development of bacteria over a range of single and double con-centration gradients in the concentrations of nutrients, electron donors and electron acceptors. We measurebacterial activity and population growth locally in precisely known hydrodynamic and chemical environments.This approach allows time-resolved monitoring of the location and intensity of reactive hotspots in micromodelsas a function of the ow and chemical gradient conditions. We compare reactive microbial hotspot dynamics inour micromodels to classic growth laws and well-known growth parameters for the laboratory model bacteriaEscherichia coli, namely Michaelis-Menten-Monod nutrients uptake and Doop's growth law.The validated growth laws are then integrated into a mixing model quantifying the dynamics of nutrientgradients in shear ows. The main objective is to investigate the inuence of combined chemical and velocitygradients on biogeochemical reactions kinetics and biomass production. We discuss the consequences of theseresults in the context of biomass production in heterogeneous velocity and chemical gradients.