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Hydrogen exchange between the living and non living world

The hydrogen cycle consists of hydrogen exchanges between biotic (living) and abiotic (non living) sources and sinks of hydrogen containing compounds. Hydrogen (H) is the most abundant element in the universe. On Earth, common H containing inorganic molecules include water (H2O), hydrogen gas (H2), hydrogen sulfide (H2S), and ammonia (NH3). Many organic compounds also contain H atoms, such as hydrocarbons and organic matter. Given the ubiquity of hydrogen atoms in inorganic and organic chemical compounds, the hydrogen cycle is focused on molecular hydrogen, H2. As a consequence of microbial metabolisms or naturally occurring rock water interactions, hydrogen gas can be created. Other bacteria may then consume free H2, which may also be oxidised photochemically in the atmosphere or lost to space. Hydrogen is also thought to be an important reactant in pre biotic chemistry and the early evolution of life on Earth, and potentially elsewhere in the Solar System.

Because H2 is the lightest element, atmospheric H2 can readily be lost to space via Jeans escape, an irreversible process that drives Earth's net mass loss. Photolysis of heavier compounds not prone to escape, such as CH4 or H2O, can also liberate H2 from the upper atmosphere and contribute to this process. Another major sink of free atmospheric H2 is photochemical oxidation by hydroxyl radicals (•OH), which forms water. Anthropogenic sinks of H2 include synthetic fuel production through the Fischer Tropsch reaction and artificial nitrogen fixation through the Haber Bosch process to produce nitrogen fertilizers.

Many microbial metabolisms produce or consume H2.

Hydrothermal H2 may have played a major role in pre biotic chemistry. Production of H2 by serpentinization supported formation of the reactants proposed in the iron sulfur world origin of life hypothesis. The subsequent evolution of hydrogenotrophic methanogenesis is hypothesized as one of the earliest metabolisms on Earth. suggests that similar processes are ongoing elsewhere in the Solar System, and potentially in other planetary systems as well.