Microbial signature of groundwater mixing in geothermal areas: insights from the Cimino-Vico volcanic system (central Italy).

The complex structure and dynamics of geothermal ecosystems strongly affect the spatial distribution and activity of aquatic microbial communities. The interactions between groundwaters and thermal waters represent an additional selective factor. A deeper understanding of microbial diversity, metabolic potential, and ecological interactions in groundwater mixing zones is essential for evaluating their impact on biogeochemical cycles (such as sulfur, nitrogen, and carbon) and predicting the ecological consequences of water mixing on ecosystem functioning. In this study, the taxonomic diversity and metabolic potentialities of microbial communities in groundwater and thermal waters revealed the occurrence of novel thermophiles able to cope with extreme physical-chemical conditions and high concentrations of toxic elements, such as arsenic, characteristics of the studied area. Furthermore, a core microbiome composed of the families Burkholderiaceae, Caulobacteraceae, Halothiobacillaceae, and Sulfurovaceae was identified as markers of the interaction between the two water compartments. Our findings emphasize the key role of microbial communities in S-, As-, and N-related biogeochemical cycles of geothermal areas.