Vie de subsurface dans les aquifères profonds et impact de l’activité microbienne sur les processus de karstification

Karst systems represent an important carbon reservoir sequestered in the form of limestones as well as an important freshwater reservoir. They are also peculiar environments of the subsurface in which specific microbial ecosystems can thrive notably thanks to mineral weathering. Although karsts systems have been studied for many years, a new paradigm has recently emerged that suggests some of them could be formed by ghost rock processes. Contrarily to the classical total karstification, ghost-rock karstification leaves in place a highly-porous alterite (i.e. the ghost-rock) that can constitute a microbial habitat. Pioneering studies have indeed shown the presence of microorganisms in ghost-rocks, and it has therefore been considered but not demonstrated that microorganisms could be involved in ghost-rock processes leading to the formation of caves. In order to test this hypothesis, I am performing a geomicrobiological study of two caves systems presenting a similar mineralogy in France (Larzac region) and in South Africa (Sterkfontein caves).

The implementation of this project will rely on the joint analysis of the microbial diversity and of the physico-geo-chemistry of the fluids and rocks of the environments, and will follow 3 main research axes:

1- Microbial ecology of the system (metabarcoding, metagenomic, single cell genomic, transcriptomics, bioinformatics, and geochemistry of the associated fluids and rocks)

2- Bio-organo-mineral interfaces at the micrometer level (Laser Microdissection, FTIR and Raman spectroscopy, CLSM and FISH, SEM/TEM, S-DUV fluorescence and transmittance)

3- Long-term in situ mineral bioalteration experiments will be set up at depth in the Larzac cave system