A former gold mine in South Dakota is giving scientists clues as to how colonies of bacteria living in the continental deep subsurface and feeding out of minerals spread out and form biofilms, which are microbial coatings of the rock surface.
In detail, researchers from Northwestern University went 1.5 kilometers below the surface in the Deep Mine Microbial Observatory (DeMMO), housed within a past-producing mine now known as the Sanford Underground Research Facility (SURF) and located near the town of Lead.
Once below-ground, the experts cultivated biofilms on native rocks rich in iron and sulphur-bearing minerals. After six months, the team analyzed the microbial composition and physical characteristics of newly grown biofilms, as well as their distributions using microscopy, spectroscopy and spatial modelling approaches.
The analyses, which were published in the journal Frontiers in Microbiology, revealed hotspots where the biofilm was denser. These hotspots correlate with iron-rich mineral grains in the rocks, highlighting some mineral preferences for biofilm colonization.
“Our results demonstrate the strong spatial dependence of biofilm colonization on minerals in rock surfaces,” Caitlin Casar, first author of the study, said in a media statement. “We think that this spatial dependence is due to microbes getting their energy from the minerals they colonize.”
According to Casar, these results demonstrate that host rock mineralogy is a key driver of biofilm distribution, which could help improve estimates of the microbial distribution of the Earth’s deep continental subsurface.
“Our findings could inform the contribution of biofilms to global nutrient cycles, and also have astrobiological implications as these findings provide insight into biomass distributions in a Mars analog system,” Casar said.
In the researcher’s view, extraterrestrial life could exist in subsurface iron and sulphur-rich environments similar to that of DeMMO’s rock formations, where the microorganisms are protected from both radiation and extreme temperatures.