Deep subsurface biospheres

Microbial life in the deep subsurface contributes significantly to overall biomass on Earth. Although the microbial communities inhabiting the deep subsurface are abundant, little is known about their diversity, activity, interactions and role in global biogeochemical cycles.

The deep biosphere studies provide a window to the past and to the other worlds. The deep subsurface may have been the only refuge for life during the early history of Earth. Additionally, understanding the richness, diversity and functionality of the microbial communities in extreme but still life-sustaining environments on Earth, such as the deep subsurface, facilitate our ability to imagine and interpret the possibilities of life on other celestial objects.

The Outokumpu Deep Drill Hole in Finland provides access to crystalline bedrock fluids of the Fennoscandian shield that are estimated to be tens of millions of years old. The indigenous bacterial and archaeal communities in addition to microbial communities with important functional properties in bedrock fluids have been characterized from a depth range of 180 m to 2300 m. Major part of this work has been done with molecular biological methods, such as fingerprinting, cloning and sequencing.

Low cell numbers but high diversity is characteristic to the microbial communities of the Outokumpu deep subsurface. The microbial communities resemble those in other deep subsurface sites and share similarity with those of serpentinization-driven ecosystems. Sulfate-reducing bacteria and methanogens have been detected at several depths, but the origin of methane that is abundant in the groundwater is still enigmatic. There are several proposed carbon and energy sources for these microbial communities, for example hydrogenotrophic metabolism, but interestingly detected usage of organic carbon compounds infers that heterotrophy is an important metabolic trait in deep bedrock subsurface.