The capacity for molecular hydrogen (H2) to hydrogenate oxygen and carbon is critical to the origin of life and represents the basis for all known life-forms. Major sources of H2 that strictly involve nonbiological processes and inorganic reactants include (1) the reduction of water during the oxidation of iron in minerals, (2) water splitting due to radioactive decay, (3) degassing of magma at low pressures, and (4) the reaction of water with surface radicals during mechanical breaking of silicate rocks. None of these processes seem to significantly affect the current global atmospheric budget of H2, yet there is substantial H2 cycling in a wide range of Earth’s subsurface environments, with multifaceted implications for microbial ecosystems.

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