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PhD thesis in Environmental Science : Mercury methylation by microorganisms

This offer is available in the following languages:
Français - Anglais

Date Limite Candidature : lundi 4 juillet 2022

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General information

Reference : UMR5254-SOPPUY-045
Workplace : PAU
Date of publication : Monday, June 13, 2022
Scientific Responsible name : Marisol GONI URRIZA
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 3 October 2022
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

I- Scientific context
Mercury (Hg) is a global pollutant on Earth, and is able to convert into highly toxic methylmercury (MeHg), which is bioaccumulated and bioamplified in the aquatic food web. Microorganisms, and particularly sulfate reducing bacteria (SRBs), play a central role in MeHg conversion, but the cellular and environmental mechanisms favoring MeHg production are not still clearly understood. Although hgcA and hgcB genes have been identified as necessary for Hg methylation in some bacteria (Parks et al. 2013), the methylation process cannot be fully described, and we focus on the characterization of Hg at the bacterial cell level to progress in the understanding of Hg methylation, from its uptake to the export of MeHg. We have developed our own Hg methylating sulfate-reducing bacteria model namely, Pseudodesulfovibrio hydrargyri strain BerOc1. We apply an interdisciplinary approach combining genetics, analytical chemistry based on mass spectrometry, and chemical imaging and X-ray absorption spectroscopy to clarify the cascade of events leading to Hg methylation.
Very recently, we identified new genes as players in the trafficking of Hg at the cell level of P. hydrargyri (PhD thesis S. Barrouilhet), and some of them have important impact on Hg methylation/demethylation. The aim of our project (ANR project Micromer) is to better understand the involvement of these genes.

II- Objectives
The global objective of this PhD thesis is to clarify the mechanisms of Hg methylation/demethylation by the sulfate reducing strain P. hydrargyri BerOc1. For that, we will focus on the role of several genes suspected to be involved in the processes by comparing the wild strain with knockout mutants, as well as other SRBs that only demethylate MeHg. Our specific objectives are to specify:
1. Hg uptake: Is Hg methylation governed by the first steps of the process (ie, recognition and uptake of
mercury)? What is the external form of the metal and, how the Hg form affects its uptake?
2. Hg export: Does Hg and/or MeHg export also control (at least partially) the Hg methylation/demethylation processes?
3. Role of metabolites: How metabolites (thiols, S-compounds and others) produced by bacteria affect Hg
species trafficking across the cell?

For that, the candidate will implement an approach based on elemental imaging techniques (transmission electron microscopy, synchrotron nano-X ray fluorescence…) and X-ray absorption spectroscopy (Xanes, Exafs) to localize Hg at the cell level and determine its speciation (Isaure et al. 2020). The PhD student will also carry out bacterial culture, and quantification of Hg and MeHg (GC-ICP MS).

III- Work plan
1- Bibliographic study
2- Microbes cultures, growth and quantification of inorganic Hg and MeHg (GC-ICP MS)
3- Set-up of chemical imaging (TEM-EDS, nano XRF)
4- Hg speciation by X-ray absorption spectroscopy (XAS).
The PhD work requires travels (collaboration with BIC in Bordeaux, synchrotron experiments)

IV- Competences required
The candidate should hold a master degree in environmental chemistry, geosciences, physico-chemistry, microbiology or equivalent. He/she should have strong interest in performing laboratory work. He/she will be able to work autonomously and rigorously. He/she will be interested by interdisciplinary work (work with microbiologists, biogeochemists, chemists, and physicists). Skills in writing. French speaking would be a plus.
Contacts : marie-pierre.isaure@univ-pau.fr / marisol.goni@univ-pau.fr

Work Context

The PhD student will work at IPREM institute that is settled in two sites Pau and Anglet. The work will be mainly carried out in Pau site. The PhD work requires travels (collaboration with BIC in Bordeaux, synchrotron experiments).
IPREM institute is a joint research unit (UMR 5254) CNRS and Université de Pau et des Pays de l'Adour. Research activities focus on the development of fundamental knowledge in physical chemistry, analytical chemistry and microbiology, in relation to environment and materials. IPREM is composed of three scientific clusters and the CME cluster (Environmental Chemistry and Microbiology) focuses on contaminants and trace elements in the environment and their interactions with microorganisms. To study chemical species, their transformations and their interactions with living organisms, our laboratory uses various analytical chemistry techniques based on mass spectrometry, as well as imaging and speciation techniques based on synchrotron radiation.

Laboratory Keywords: Environment, Microbiology, Speciation, Analytical chemistry, Mass spectrometry, Imaging.

Constraints and risks

The PhD will work with toxic chemicals. Good laboratory practices must be achieved to limit environmental and health hazards.

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