Guillaume Holub ^1,2* Claire Sergeant ^1,2 Céline Bailly ^3,4 Aude Beauger ^5,6 Vincent Breton ^5,7 Patrick Chardon ^5,7 Gilles Montavon ^3,4 Marie-Hélène Vesvres ^1,2 Clarisse Mallet ^5,8

• ¹ Université de Bordeaux, Bordeaux, France
• ² UMR5797 Laboratoire Physique des deux Infinis Bordeaux (LP2iB), Gradignan, France
• ³ IMT Atlantique Bretagne-Pays de la Loire, Nantes, Pays de la Loire, France
• ⁴ UMR6457 Laboratoire de Physique Subatomique et des Technologies Associées (SUBATECH), Nantes, Pays de la Loire, France
• ⁵ Université Clermont Auvergne, Clermont-Ferrand, Auvergne, France
• ⁶ UMR6042 Laboratoire de Géographie Physique et Environnementale (GEOLAB), Clermont-Ferrand, Auvergne, France
• ⁷ UMR6533 Laboratoire de Physique de Clermont (LPC), Aubiere, Auvergne, France
• ⁸ UMR6023 Laboratoire Microorganismes Génome Et Environnement (LMGE), Aubière, Auvergne, France

Some natural environments on Earth are characterised by high levels of radiation, including naturally radioelement enriched mineral springs in the French Massif Central. Therefore naturally radioactive mineral springs are interesting ecosystems for understanding how bacterial populations in these springs have adapted to high levels of natural and chronic radioactivity over the very long term. The aim of this study was to analyse the bacterial communities of sediments from five naturally radioactive mineral springs in the French Massif Central, sampled in autumn 2019 and spring 2020, and to observe whether radionuclides, compared to other physicochemical parameters, are drivers of the bacterial community structuring in these extreme environments. Physicochemical measurements showed that two springs, Dourioux and Montagne had high radioelement concentrations/activities (uranium, thorium and radon). Analysis of the structure of the bacterial communities, by next generation sequencing based on 16S rRNA gene sequencing, showed that the presence of radionuclides in Dourioux and Montagne, did not lead to a reduction in bacterial diversity and richness compared to the other springs. However, Dourioux and Montagne were characterised by specific bacterial populations, whose presence correlates with the radioelement concentrations/activities measured in these springs. This suggests that radioelements could partly explain the structuring of bacterial communities in these springs. In addition, several of these operational taxonomic units (OTUs) specific to Dourioux and Montagne, mainly affiliated to Proteobacteria, Firmicutes, Acidobacteria, Actinobacteria and Bacteroidetes, could be involved in the biogeochemistry of radionuclides through different mechanisms (biosorption, biomineralisation, bioaccumulation and bioreduction), which would allow the development of other bacterial species sensitive to these metals/radioelements. In particular, the co-occurrence of sulphate and/or iron-reducing bacteria, capable of bioreducing uranium, with fermentative bacteria, releasing sources of organic carbons, reflects associations of bacteria with complementary functions that allow them to grow in this peculiar environment and maintain a high diversity in these extreme environments. This study has provided a better understanding of the structuring of bacterial communities exposed to ionising radiation for thousands of years in naturally radioactive environments.