Beneath the green surface and rich, organic soil, life extends kilometers within the deep rocky crust of the Earth. The continental deep aquifer is possibly one of the largest reservoirs of bacteria and archaea on Earth, with many forming biofilms – such as the microbial layer of the surface of rocks. This microbial group lives without light or oxygen and with minimal sources of organic carbon, and they can obtain energy by eating or inhaling minerals. Distributed throughout the Earth’s interior, these biofilms can account for 20–80% of the total bacterial and archaeal biomass in the continental subsurface according to the latest estimates. But are these microbial groups spread evenly over rock surfaces, or do they prefer to colonize specific minerals in rocks?
To answer this question, researchers from Northwestern University in Evanston, Illinois, led a study analyzing the growth and distribution of microbial communities in deep continental subsurface environments. This work demonstrates that the host rock mineral composition drives the distribution of biofilms, producing “hot spots” for microbial life. The study was published in Frontiers in Microbiology.
Foci of microbial life
To achieve this study, researchers went 1.5 kilometers below the surface at the Deep Mining Microbial Observatory (DeMMO), located inside a former gold mine now known as the Sanford Underground Research Facility (SURF), located in Leed, South Dakota. There, underground, researchers have grown biofilms on native rocks rich in iron and sulfur-bearing minerals. After six months, the researchers analyzed the microbial composition and physical properties of the newly cultured biofilms, as well as their distributions using methods of microscopy, spectroscopy, and spatial modeling.
Spatial analyzes were performed by the researchers and revealed the hotspots where the biofilm was most intense. These hotspots are associated with iron-rich mineral grains in the rocks, highlighting some of the mineral preferences for colonizing biofilms. Our results demonstrate the strong spatial dependence of biofilm colonization on minerals in rock surfaces. We think this spatial dependence is due to the microbes obtaining their energy from the minerals that colonize them, ”explains Caitlin Cassar, first author of the study.
Altogether, these results demonstrate that host rock mineralogy is a major driver of biofilm distribution, which could help improve estimates of microbial distribution below the Earth’s deep continental surface. But leading studies within planet Earth could also help other topics. “Our results could inform the contribution of biofilms to global nutrient cycles, and also have astronomical biological implications, as these results provide insight into biomass distributions in a Mars analog system,” says Caitlin Casar.
In fact, extraterrestrial life can exist in similar subterranean environments where microorganisms are protected from both radiation and extreme temperatures. Mars, for example, has an iron and sulfur-rich composition similar to the DeMMO rock formations, which we now know can drive the formation of microbial hotspots underground.
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