Hydrogen Generation in the Deep, Hot Biosphere

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The discovery of the Subsurface LithoAutotrophic Microbial Ecosystem (SLiME) in basalt formations in 1995, seemingly using hydrogen formed from water rock interactions, was of great significance as this anaerobic community could be independent of surface photosynthesis, both organic matter and oxygen. This potentially significant energy supply might also explain the surprisingly large numbers of prokaryotes found in subsurface terrestrial environment (at least to 3 km depth), despite extreme conditions and lack of obvious energy supply. It also has profound astrobiological significance as a mechanism for subsurface life in planets even with surface conditions that are unsuitable for life. However, the significance of this hydrogen generation is controversial having being criticized as being a negligible reaction in the environment (conditions too alkaline, restricted by limited reduced iron concentrations in minerals and by its dependence on the production of fresh reactive surfaces). However, hydrogen formation has also been detected at depth in earthquake fault zones and there is indirect evidence that this is used by subsurface prokaryotes to produce methane. The mechanism of hydrogen formation in this case is thought to be due to mechanochemistry as a result of subsurface fracturing of rocks in earthquake zones. If this is true then with some greater than 20,000 earthquakes a year any rock type could potentially produce hydrogen making a substantial SLiME community distinctly more possible. In addition, we have demonstrated that some prokaryotes may actually speed-up hydrogen formation from minerals in sediment slurries, including hydrogen generation from pure silica sand. As silicates make up ~95% of the Earth's crust this could potentially be a significant source of hydrogen. We intend to investigate further these mechanisms of hydrogen formation by testing a range of common minerals and conditions for hydrogen generation, including at increasing temperatures to simulate the heating that occurs due to sediment burial. We will determine whether microbial processes are stimulated by hydrogen formation and identify and culture the microbes involved. These enriched microbes will then be used with pure minerals to investigate their involvement and ability to use the mineral as an energy source in more detail. Some high pressure experiments will enable temperatures up to 150oC to be investigated. This is too high for microbes (max ~120oC) but may produce hydrogen and other compounds which can diffuse upwards to feed the base of the biosphere. Novel sealed rock crushing experiments will also be conducted (30 - 120oC) to test whether just cracking of rocks can produce enough hydrogen to feed a microbial population.