Roles and impacts of iron-oxidizing bacteria in the dynamics of the deep critical zone (M/F)

As part of the ANR IRONSTONE project, the recruited person will be responsible for experimentally characterizing the metabolic activity of iron-Oxidizing Bacteria (FeOB) and the associated biogeochemical fluxes (Fe, C, N, S) in a fractured basement aquifer. He or she will implement, alone or collaboratively, microbiology approaches (culture of FeOB isolates, enrichments of microbial consortia sampled from fractured aquifers), isotopic labeling, environmental genomics approaches (metagenomics and metatranscriptomics), geochemical and mineralogical monitoring involving high-resolution microimaging approaches as well as thermodynamic modeling. The results obtained will provide a better understanding of the relationships between hydrological circulations, deep microbial dynamics and the biogeochemical functioning of the critical zone.

- Characterize the rates of transformation of Fe, C, N (and associated elements) by populations of iron oxidizing bacteria in relation to changes in environmental conditions.
- Determine the microbial processes and the rates of biomass production and Fe transformation during the transition to anoxic conditions following the proliferation of iron-oxidizing bacteria.
- Identify new microbial markers (key genes of metabolic pathways, genes involved in adaptation to microaerobic conditions) indicators of changes in environmental conditions.

This position is offered as part of the IRONSTONE interdisciplinary research program funded by the French National Research Agency over the period 2021-2025. Recent works on the subsurface have revealed the importance and potentially global scope of underground microbial biomass. However, the biogeochemical processes regulated by the activity of microorganisms in these environments and the impact of these processes on the quality of water resources, the alteration of rocks or the exchange of greenhouse gases between the subsurface and the atmosphere remain little constrained to this day. Based on this observation, the IRONSTONE program explores, from the microscopic scale to that of the critical zone, the way in which spatial heterogeneities and the temporal variation of environmental conditions in fractured aquifers affect the microbial dynamics, the kinetics of reactions catalyzed by microorganisms, and associated elementary fluxes. The project focuses on iron oxidizing bacteria (FeOB) as its main study model. These FeOB use the oxidation of ferrous dissolved Fe(II) to obtain the energy necessary for their metabolism and growth. They are autotrophic and can excrete organic carbon (in the form of very large mineral-organic filaments) in significant quantities when they are in iron-rich microaerobic environments. Thus, the development of FeOB in the subsurface is very dependent on redox gradients and chemical imbalances associated with groundwater circulation. More precisely, the spatial and temporal distribution of FeOB is controlled by the possibility of mixing between oxygen-rich groundwater and deep anoxic fluids rich in ferrous iron. These mixing zones constitute intermittent hotspots of microbiological activity whose impact on biogeochemical fluxes remains to be determined. Likewise, the degradation pathways and the fate of organic matter produced by FeOB in hotspots remain major unknowns.
The work will be carried out within the ECOFUN theme of UMR 6553 Ecobio (University of Rennes) and in the Geomicrobiology team of the Paris Institute of Globe Physics (UMR 7154 – Université Paris Cité). The postdoctoral researcher will work under the scientific supervision of Bénédicte Ménez (50%, IPGP) and Alexis Dufresne (50%, Ecobio). He/she will thus benefit from a strong national dynamic and privileged collaborations with researchers and students from the Géosciences Rennes laboratories and the Rennes Institute of Physics, partners of the IRONSTONE project. He/she will benefit from an interdisciplinary working environment (microbial ecology, geochemistry, mineralogy, geomicrobiology, hydrology), regular workshops organized within the framework of IRONSTONE as well as the support of cutting-edge technical platforms in partner laboratories in Rennes and Paris.