Résultat de la recherche
Untapping microbial diversity in geothermal power plants
The Earth hosts a wide variety of environments that are considered extreme for life, from glaciers to volcanoes or from the deep-sea to the deep terrestrial subsurface. However, even if environmental conditions are extreme in these environments, most of them harbor a great diversity of microbial life. In this thesis, we focused on one particular extreme environment: deep geothermal fluids that are used for electricity production. These fluids present several extreme conditions, such as high temperatures, high salinities, radioactivity or the presence of heavy metals. They are nevertheless known to host bacterial and archaeal life, which can impact the efficacy of geothermal power plants by causing microbially-induced corrosion or microbially-induced mineral precipitation. However, the general access to these deep geothermal fluids (generally a few kilometers below the Earth’s surface) has been limited until recently due to the impossibility to reach them. Therefore, general patterns about the presence of microbial life in deep geothermal fluids are still widely unknown. Moreover, the presence of Fungi in these specific systems was, to the best of our knowledge, never assessed before. Given the impacts that microbial life can have on the geothermal power plants themselves as well as the potential of discovering new microbial species from these underexplored environments, assessing the presence of microorganisms in these systems is of great interest. The aim of this thesis was therefore to assess the bacterial and fungal diversity present in different geothermal power plants producing electricity in Europe. First, as the microbial diversity in extreme environments can be challenging to assess due to the low amount of biomass present and to the existence of multiple resistance strategies of the microorganisms in such an extreme environment, culture-independent methods were tested and validated for deep geothermal fluids. A method previously developed to access the DNA from bacterial and archaeal cells resistant to lysis was validated on fungal structures. Then, this method as well as bioinformatic pipelines to analyze bacterial and fungal sequences from deep geothermal fluids were validated, showing the importance to use DNA extractions from the lysis-resistant and non-resistant community fractions to get a broad overview of the microbial diversity within the samples. Once all the methods were validated, the diversity of Bacteria and Fungi in fluids from six different geothermal power plants was analyzed. This revealed an astonishing diversity for the bacterial and fungal communities, and even the most extreme sites in term of temperature (i.e., the Icelandic geothermal power plants) were shown to host life. Afterwards, in order to complement this overview of the diversity of Bacteria and Fungi detected through molecular methods, isolation of microorganisms from the same geothermal fluids was done. This allowed us to isolate two anaerobic bacterial strains, one aerobic thermophilic strain of Thermaerobacter composti, one new bacterial strain belonging to the Niallia genus, as well as 11 fungal strains that included a strain of Penicillium citrinum and yeasts belonging to the Meyerozyma genus. The strain of P. citrinum, which is a species known for its biosorption potential of heavy metals, was further used in a biosorption experiment. The potential of the dead biomass of this fungus to biosorb lead was assessed under conditions mimicking those found in a geothermal fluid and during operations for electricity production, with the idea to decrease the lead concentration in the fluids, thus lessening the negative impact of lead precipitation within power plant systems. However, this showed that while elevated temperatures or the presence of organic acids within the fluids did not impact biosorption, the high salinity of the geothermal fluids drastically hindered the biosorption capacity of the biomass, making lead removal under these conditions negligeable. Overall, this thesis highlighted the bacterial and fungal life present in deep geothermal fluids of different geothermal power plants across Europe by the combination of culture-independent and culture-dependent methods. This provides a better understanding of the system, thus allowing for a more informed management of microbial life in geothermal power plants. Furthermore, the strains isolated within this thesis, by being either new species or new extremophilic strains of known species, hold the potential to be useful for different biotechnology purposes as well as helping us understand how life adapts to varied extreme conditions.