Exploring microbial diversity in relation to hydrological signatures and anthropic pressures in the Beauce Aquifer, France

Groundwater is one of the largest reservoirs of liquid fresh water on earth. Aquifers are complex environments where water quality can be affected by geological origins, geochemistry and biological activity. These environments are under pressure due to climate change and pollution. Strongly involved in the biogeochemistry of groundwater ecosystems, microorganisms can also contribute to reducing pollution and potentially be used as indicators of water quality. However, the diversity and role of microorganisms in groundwater remains largely unknown. The present work aimed to characterize bacterial diversity in 60 wells over 8000 km² in the Beauce Aquifer, France, determine the baseline diversity expected in relation to the hydro geochemical signatures and identify variations that could be due to anthropogenic pressures. We analysed amplicon sequence variants (ASVs) of the 16S rRNA gene as well as the abundance of functional genes associated to arsenic and nitrate (aioA, arsB, narG), organic carbon contents and the geochemical composition. Bacterial diversity and bacterial co-occurrences were found to change according to the hydrogeochemical signatures identified in the aquifer. A lower diversity was measured in confined aquifers, where low oxygen and reducing conditions occurred with high levels of dissolved iron and manganese. This could indicate a higher vulnerability in the face of pressure. Finally, a baseline bacterial diversity composition was found to be linked to each hydrogeochemical signature and several wells that differed from the expected variations of this diversity were identified.