Deep Microbial Life Beneath Lavey-les-Bains Reveals Surprising Resilience

Beneath the warm, mineral-rich waters of Switzerland’s renowned Lavey-les-Bains Thermal Spa, the microbial world reveals surprising stability in the face of seasonal change. A new study published in Proceedings of the National Academy of Sciences (PNAS) shows that, despite the seasonal shifts in water sources driven by temperature and precipitation in the Swiss Alps, deep underground microbial communities remain consistent throughout the year.

The research, conducted in part by Dr. Mark Lever, Associate Professor at The University of Texas Marine Science Institute (UTMSI), offers new insight into how life thrives hundreds of meters underground in geothermal systems. Their findings could offer important implications for understanding long-term carbon cycling and the stability of biogeochemical processes in Earth’s subsurface.

Thermal groundwater systems, like those feeding Lavey-les-Bains, are naturally heated underground waters that often emerge from Earth’s surface as hot springs. These systems are rich in gases and minerals, but also house ecosystems where microbes survive without sunlight by fueling their metabolism with sulfur, iron, or hydrogen.

To investigate how these microbial ecosystems respond to seasonal changes, the researchers conducted year-round sampling of groundwater at depths between 200 and 500 meters. Using stable water isotopes, noble gases, electrical conductivity measurements, and DNA sequencing, they tracked changes in both water chemistry and microbial composition.

The results revealed something unexpected: although indicators like electrical conductivity and isotopic composition showed clear seasonal mixing of shallow and deep groundwater—reflecting inputs from snowmelt and rainfall—the microbial communities remained stable and distinct at each depth. At 200 meters, sulfur-cycling bacteria such as Dissulfurispira and members of the Micrarchaeota group were prevalent, while at 500 meters, the community was dominated by heat-tolerant microbes that reduce sulfate and iron or oxidize hydrogen, including Thermales, Thermodesulfobacteriota, and Bathyarchaeota.

This discovery illustrates microbial communities in a geothermal system remaining largely stable over time despite changing water sources. It suggests that environmental conditions like temperature, rather than fluctuating water sources, are the primary drivers of microbial structure in deep aquifers.

The findings highlight the remarkable resilience of deep subsurface life in extreme environments and help fill a critical gap in our understanding of how microbial ecosystems function in dynamic, interconnected groundwater systems with implications for deep carbon cycling, geothermal resource potential, and biosignature detection in subsurface ecosystems.

Lever is joined by lead author Dr. Sébastien Giroud and co-authors Drs. Longhui Deng, Oliver S. Schilling (OSS), and Rolf Kipfer. Funding was provided by Eawag – Swiss Federal Institute of Aquatic Science and Technology, with additional financial support from the Canton of Valais. OSS gratefully acknowledges support from the SNSF-JSPS SJSSTP grant 214048.