Abstract
Deep aquifers (up to 2km deep) contain massive volumes of water harboring large and diverse microbial communities at high pressure. Aquifers are home to microbial ecosystems that participate in physicochemical balances. These microorganisms can positively or negatively interfere with subsurface (i) energy storage (CH4 and H2), (ii) CO2 sequestration; and (iii) resource (water, rare metals) exploitation. The aquifer studied here (720m deep, 37°C, 88bar) is naturally oligotrophic, with a total organic carbon content of <1mg.L−1 and a phosphate content of 0.02mg.L−1. The influence of natural gas storage locally generates different pressures and formation water displacements, but it also releases organic molecules such as monoaromatic hydrocarbons at the gas/water interface. The hydrocarbon biodegradation ability of the indigenous microbial community was evaluated in this work. The in situ microbial community was dominated by sulfate-reducing (e.g., Sva0485 lineage, Thermodesulfovibriona, Desulfotomaculum, Desulfomonile, and Desulfovibrio), fermentative (e.g., Peptococcaceae SCADC1_2_3, Anaerolineae lineage and Pelotomaculum), and homoacetogenic bacteria (“Candidatus Acetothermia”) with a few archaeal representatives (e.g., Methanomassiliicoccaceae, Methanobacteriaceae, and members of the Bathyarcheia class), suggesting a role of H2 in microenvironment functioning. Monoaromatic hydrocarbon biodegradation is carried out by sulfate reducers and favored by concentrated biomass and slightly acidic conditions, which suggests that biodegradation should preferably occur in biofilms present on the surfaces of aquifer rock, rather than by planktonic bacteria. A simplified bacterial community, which was able to degrade monoaromatic hydrocarbons at atmospheric pressure over several months, was selected for incubation experiments at in situ pressure (i.e., 90bar). These showed that the abundance of various bacterial genera was altered, while taxonomic diversity was mostly unchanged. The candidate phylum Acetothermia was characteristic of the community incubated at 90bar. This work suggests that even if pressures on the order of 90bar do not seem to select for obligate piezophilic organisms, modifications of the thermodynamic equilibria could favor different microbial assemblages from those observed at atmospheric pressure.
Highlights
Among the different deep continental environments containing high total biomass levels, estimated at 2 to 6·1029 cells (Magnobosco et al, 2018), aquifers located in the uppermost two kilometers of the Earth’s crust are characterized by a diverse microbial biosphere (Wu et al, 2016; Hershey et al, 2018; Kadnikov et al, 2020)
A study carried out on anaerobic digesters, which contain a complex community, identified these microorganisms (Hao et al, 2018). We show that these microorganisms can be maintained and active in a simplified community cultivated at oligotrophic conditions and high pressure
The results showed that even at sites located away from stored gas, the microbial communities in deep aquifers retained the ability to degrade the 6 monoaromatic compounds at sulfate and/or fermentation conditions
Summary
Among the different deep continental environments containing high total biomass levels, estimated at 2 to 6·1029 cells (Magnobosco et al, 2018), aquifers located in the uppermost two kilometers of the Earth’s crust are characterized by a diverse microbial biosphere (Wu et al, 2016; Hershey et al, 2018; Kadnikov et al, 2020). Deep continental aquifers used as geological gas storage facilities are relevant ecosystems for studying the deep continental biosphere and the effect of human activity on it These storage aquifers can be found all around the world, but are primarily located in North America, some occur in Europe and the Commonwealth of Independent States (Russia, Armenia, Azerbaijan, and Belarus; Cornot-Gandolphe, 2017). An understanding of these ecosystems makes it possible to predict the effects of future developments on these deep aquifers depending on their use, which include (i) the sequestration of CO2 or carbon molecules in the broad sense to address climate change; (ii) the temporary energy storage of CH4, H2 or air from renewable energy sources; (iii) rare metal mining; and (iv) exploitation of their water resources (De Silva et al, 2015; Flexer et al, 2018; Ranchou-Peyruse et al, 2019; Lemieux et al, 2020). Because of the high pressures deep underground, the volume of gas stored is less than that at atmospheric pressure, allowing the storage of large amounts of gas
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