Subsurface microbial communities make up a significant fraction of the global prokaryotic biomass, and what sustains them has been relatively unexplored. Three kilometers down in Precambrian continental crust there is no light and no organic rain from above. Something has to supply the carbon.
They build it from carbon dioxide, with carbon monoxide as a second option. In reached through a South African gold mine, the most abundant carbon fixation pathway in the was the energy-conserving reductive acetyl-CoA pathway. Alongside it sat genes that work in both directions.
Subsurface microbial communities comprise a significant fraction of the global prokaryotic biomass, and the carbon metabolisms that support the deep biosphere have been relatively unexplored. The question is not whether life is down there but what it runs on.
The sampling is opportunistic in the best sense. A 3-kilometer deep fracture fluid system was accessed through the Tau Tona gold mine in the Witwatersrand Basin, South Africa, and metagenomic and thermodynamic analyses were combined.
The answer is autotrophy. The energy-conserving reductive acetyl-CoA pathway, known as , was the most abundant carbon fixation pathway identified in the metagenome, meaning the community mostly builds its carbon from carbon dioxide.
The genes hold a second route. Carbon monoxide dehydrogenase genes have the potential to participate in both autotrophic and heterotrophic metabolisms, through the reversible oxidation of carbon monoxide, so the community can work in either direction.
wat. Three kilometers down, the commonest way to get carbon is to build it from carbon dioxide.
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A biosphere that runs on rock chemistry rather than on sunlight sets the terms for where life can exist at all.
How do we know?
Magnabosco and colleagues combined metagenomic sequencing of the fracture fluid with thermodynamic analysis of which reactions could yield energy under the conditions found there.
What is still unknown
One fracture fluid system in one basin, and a metagenome lists the genes present rather than the reactions actually running.
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