The deep subsurface biosphere underlying the continents, or terrestrial deep biosphere, has emerged in recent years as a dynamic, populated, metabolically active environment, one offering clues to the extremes of microbial survival.
A former goldmine located in South Dakota’s Black Hills has been found to provides a portal to microbial life deep into the Earth’s crust. This was one of six experimental sites, collectively called the Deep Mine Microbial Observatory.
In investigations, Northwestern University scientists have characterised nearly 600 microbial genomes collected from goldmine and this analysis illuminates the so-termed ‘microbial dark matter’ residing below the surface.
By boring holes into rocks inside the mine the researchers sought to capture fracture fluids, composed of water and dissolved gases. Some of these fluids were up to 10,000 years old and many were found to be teeming with microbial life.
In some cases these spanned depths from the surface all the way to about 1.5 kilometres deep. By assessing samples in sequence, the data provides a window into a gradient of microbial life with depth.
The microbes represented 50 distinct phyla and 18 candidate phyla (including Proteobacteria and Nitrospira, and others in terms of a deep phylogenetic diversity including Eisenbacteria, Omnitrophota, and Edwardsbacteria). In total, the researchers characterized nearly 600 microbial genomes. The assessment of microbial life enabled the scientists to divide the organisms into two groups: minimalists that have one, specialized job or maximalists that are prepared to use any available resource.
As to the significance of the research, the study has implications for how underground activities, like mining and carbon storage, could affect or be affected by microbial life. As industry looks for locations for long-term carbon storage, for example, many companies are exploring the possibilities for injecting carbon dioxide deep into the ground. However, if we store carbon dioxide underground, there are microbes that could metabolise it to make methane, for example. Consequently modifications to the biosphere underground could affect the surface.
Northwestern geoscientist Magdalena Osburn states: “We used the mine as a conduit to access that biosphere, which is difficult to reach no matter how you approach it. The power of our study is that we ended up with a lot of genomes, and many from understudied groups. From that DNA, we can understand which organisms live underground and learn what they could be doing. These are organisms that we often can’t grow in the lab or study in more traditional contexts. They are often called ‘microbial dark matter’ because we know so little about them.”
In addition, the lifestyle of these ‘intraterrestrial’ microbes also provides clues to what organisms could be living on other planets, especially Mars.
Given that the goldmine microbes live on resources found within rocks and water that are physically separate from the surface, these organisms could also potentially could survive buried within Mars’ dusty red depths.
The research appears in the journal Environmental Microbiology, titled “A metagenomic view of novel microbial and metabolic diversity found within the deep terrestrial biosphere at DeMMO: A microbial observatory in South Dakota, USA”.
