# Hydrogen cycle

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 (H$_2$O), hydrogen gas (H$_2$), hydrogen sulfide (H$_2$S), and ammonia (NH$_3$). 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, H$_2$.
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.

## Abiotic cycles
### Sources

Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic serpentinization reactions between water and olivine minerals liberate H$_2$ in the marine or terrestrial subsurface. In the ocean, hydrothermal vents erupt magma and altered seawater fluids including abundant H$_2$, depending on the temperature regime and host rock composition. Molecular hydrogen can also be produced through photooxidation (via solar UV radiation) of some mineral species such as siderite in anoxic aqueous environments. This may have been an important process in the upper regions of early Earth's Archaean oceans.

### Sinks

Because H$_2$ is the lightest element, atmospheric H$_2$ 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 CH$_4$ or H$_2$O, can also liberate H$_2$ from the upper atmosphere and contribute to this process. Another major sink of free atmospheric H$_2$ is photochemical oxidation by hydroxyl radicals (•OH), which forms water.
Anthropogenic sinks of H$_2$ include synthetic fuel production through the Fischer-Tropsch reaction and artificial nitrogen fixation through the Haber-Bosch process to produce nitrogen fertilizers.

## Biotic cycles

Many microbial metabolisms produce or consume H$_2$.

### Production

Hydrogen is produced by hydrogenases and nitrogenases enzymes in many microorganisms, some of which are being studied for their potential for biofuel production. These H$_2$-metabolizing enzymes are found in all three domains of life, and out of known genomes over 30% of microbial taxa contain hydrogenase genes. Fermentation produces H$_2$ from organic matter as part of the anaerobic microbial food chain via light-dependent or light-independent pathways.

### Consumption

Biological soil uptake is the dominant sink of atmospheric H$_2$. Both aerobic and anaerobic microbial metabolisms consume H$_2$ by oxidizing it in order to reduce other compounds during respiration. Aerobic H$_2$ oxidation is known as the Knallgas reaction.
Anaerobic H$_2$ oxidation often occurs during interspecies hydrogen transfer in which H$_2$ produced during fermentation is transferred to another organism, which uses the H$_2$ to reduce CO$_2$ to CH$_4$ or acetate, SO$^{2−}$ $_4$ to H$_2$S, or Fe$^{3+}$ to Fe$^{2+}$. Interspecies hydrogen transfer keeps H$_2$ concentrations very low in most environments because fermentation becomes less thermodynamically favorable as the partial pressure of H$_2$ increases.

## Relevance for the global climate

H$_2$ can interfere with the removal of methane from the atmosphere, a greenhouse gas. Typically, atmospheric CH$_4$ is oxidized by hydroxyl radicals ($^•$OH), but H$_2$ can also react with $^•$OH to reduce it to H$_2$O.
    CH$_4$ + $^•$OH → $^•$CH$_3$ + H$_2$O
    H$_2$ + $^•$OH → H$^•$ + H$_2$O

## Implications for astrobiology

Hydrothermal H$_2$ may have played a major role in pre-biotic chemistry. Liberation of H$_2$ by serpentinization may have 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.
Serpentinization can occur on any planetary body with chondritic composition. The discovery of H$_2$ on other ocean worlds, such as Enceladus, suggests that similar processes are ongoing elsewhere in the Solar System, and potentially in other planetary systems as well.