Never-tracked-before bacteria discovered in Greek caves

Normal (epigenic) caves form from surface water seeping down and dissolving rock, shaped by surface climate and topography. In contrast, hypogenic caves develop from groundwater rising from below, often under confined conditions, and are rich in dissolved minerals. This results in distinct morphologies and mineral formations.

Studying hypogenic caves is important to understand subsurface geological and hydrological processes and life in such harsh environments.

The Mavros Vrachos Quarry is a hypogenic cave in north-eastern Greece. The cave formed because of tectonic changes in the Krousovitis basin during the Neogene and Quaternary periods, as well as because of the presence of an ongoing hydrothermal system. These tectonic movements connected the surface and underground, allowing water to circulate and create caves. The sidewalls are almost fully covered by goethite (hydrated iron oxide).

“This is an uncharted cave, and so we thought it could potentially show different information to other hypogenic caves, and it did”, explains Georgios Lazaridis, field geologist at the Aristotle University of Thessaloniki and corresponding author of the article.

Biosignatures of unknown bacteria

Geologists dove into the confined, labyrinthine tunnels of the cave in order to extract samples for study. Getting access to the cave is not trivial: it requires skill to navigate the narrow tunnels as well as knowledge to extract the best samples. Their first aim was to give a geological analysis of the cave and analyse the composition of the goethites. However, they encountered with previously unknown bacteria that metabolised into iron. “We have the DNA of these microbes but we haven’t found a match in any database, which really surprised us”, explains Dimitrios Bessas, ESRF scientist.

Getting into the heart of the samples was not simple. The team used the technique of Mössbauer spectroscopy at the ESRF, which can detect minute changes in nanocrystalline goethites. “Our beamline offers a unique resolution of nanoelectron volts, which allows us to track small differences in samples that you couldn’t see using other methods”, explains Bessas. These quantitative and qualitative changes in distinct places of the goethite samples are potential biosignatures of the  unknown bacteria.

Dimitrios Bessas on the ID18 beamline. Credits: S. Candé.

The next step for the team is to investigate samples from parts of the cave that are covered with water, even though the access to these parts of the cave is extremely challenging. “We want to know what processes take place on the bacteria in a different environment and this may shed more light into their mechanism”, says Lazaridis.

Reference:

Lazaridis, G. et al,  CATENA, Volume 242, July 2024, 108113.  https://doi.org/10.1016/j.catena.2024.108113

Text by Montserrat Capellas Espuny

Top image: A view of the Mavros Vrachos cave in Greece. Credits: G. Lazaridis