Description du sujet de thèse

Metallic/metalloid trace elements (TE) are naturally present in varying amounts in all environmental compartments. Depending on their concentration and their chemical speciation, these TEs can be toxic and potentially threaten the natural balance of ecosystems. To limit their dispersion in the environment, their immobilization by trapping in the structure of biominerals such as iron sulfides, and in particular pyrite, appears to be an advantageous and eco-compatible solution. As part of the ANR PYRISOFE project, which is focused on recording the isotopic signal of iron during the formation of pyrite in the presence of trace elements of interest, the PhD project aims to study the mechanisms which control the biogenic formation of pyrite and their impact on Fe isotopes fractionation. Indeed, despite its presence in various sedimentary contexts, the processes that govern the biomineralization of pyrite at neutral pH under environmental conditions, in particular at the molecular scale, are still poorly described in the literature, even though these mechanisms could have a strong influence on the diversity of iron isotope signatures measured in sedimentary archives and surficial environments. The PhD project will initially focus on (1) the characterization and concomitant use of microbial activities (ferri-reduction, sulfide production and sulfo-oxidation) leading to the biomineralization of pyrites in the presence of toxic metals/metalloids (e.g. Ni, Cr, Co, As, Sb) using specific experimental devices developed in the laboratory and whose prior study has shown the potential, and (2) on the measurement of iron isotopes of the different mineral components of the system in collaboration with researchers from GET Toulouse (Franck Poitrasson, Romain Guilbaud, Marc Blanchard). In a second step, the experimental work will focus on the spatio-temporal study in 3 dimensions of the relationships between bacteria, iron sulfides and TE in our experimental system. The speciation of elements and mineral phases will be studied using X-ray diffraction, electron microscopy associated with microanalysis (SEM, Cryo-FIB-SEM and TEM-EDX), as well as X-ray absorption spectroscopy and possibly synchrotron-based X-Ray tomography. This work will provide leads for explaining the methods of acquisition of the isotopic signal of iron and its significance in environmental conditions propitious to the formation of iron sulfides, and will also make it possible to consider eco-compatible bioremediation strategies to limit the dispersion of TE in contaminated environments.

Contexte de travail

The IMPMC laboratory (http://impmc.sorbonne-universite.fr/fr/equipes.html), Institute of Mineralogy, Physics of Materials and Cosmochemistry) is a joint research unit between the CNRS, Sorbonne University, the Muséum of Natural History (MNHN), and the Institute for Research and Development (IRD). It is located on the Pierre and Marie Curie campus. The laboratory accounts for approximately 175 people, including 108 permanent staff, divided into research teams that conduct multidisciplinary research in the fields of physics, biology, geosciences and chemistry. At IMPMC, the doctoral student will join the environmental mineralogy team (http://impmc.sorbonne universite.fr/fr/equipes/mineralogie_environnement.html). He/She will work under the supervision of Georges Ona-Nguema (MCF SU HDR) and Pierre Le Pape (CR CNRS), and will also have strong interaction with the other members of the team involved in the ANR PYRISOFE. He/She will have access to the laboratory analysis instruments. The part concerning iron isotopes measurements will be carried out in collaboration with the members of the ANR PYRISOFE at the GET laboratory (https://www.get.omp.eu/) Geosciences Environnement Toulouse. The GET is a joint research unit between CNRS, IRD, Toulouse 3 University and CNES. The unit is a multi- and interdisciplinary research laboratory in Earth and Environmental Sciences attached to the Midi-Pyrenees Observatory (OSU OMP), bringing together around 226 people, including 156 permanent staff. At GET, the recruited person will work under the co-direction of Franck Poitrasson (DR CNRS) and in collaboration with Romain Guilbaud (CR CNRS) and Marc Blanchard (DR CNRS).

Contraintes et risques

Trips has to be planned to Toulouse for collaboration with local collegues, including for isotopic measurement campaigns.

Informations complémentaires

The candidate must hold a Master 2 degree in geochemistry/mineralogy or environmental chemistry with knowledge in geomicrobiology. A dual training in geochemistry/microbiology would be highly appreciated. The study proposed in this PhD project involves concepts of solution geochemistry, mineralogical characterization methods, data analysis, and requires good oral and written communication skills. In particular, we are looking for a person with good autonomy, curious and with a strong motivation to work in a team on an interdisciplinary project. Applications must include a CV, the address of at least one contact person likely to be contacted, a letter of motivation, a summary of the master's 2 thesis as well as the marks of Master 1 and 2.