Thehydrogen cycle consists ofhydrogen exchanges betweenbiotic (living) andabiotic (non-living) sources and sinks of hydrogen-containing compounds.
Hydrogen (H) is the most abundant element in the universe.[1] On Earth, common H-containing inorganic molecules include water (H2O), hydrogen gas (H2),hydrogen sulfide (H2S), andammonia (NH3). Many organic compounds also contain H atoms, such ashydrocarbons andorganic 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 inpre-biotic chemistry and the early evolution of life on Earth, and potentially elsewhere in theSolar System.[2]
Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermicserpentinization reactions between water and olivine minerals liberate H2 in the marine or terrestrial subsurface.[3][4] In the ocean,hydrothermal vents erupt magma and altered seawater fluids including abundant H2, depending on the temperature regime and host rock composition.[5][4] Molecular hydrogen can also be produced through photooxidation (via solarUV radiation) of some mineral species such assiderite in anoxic aqueous environments. This may have been an important process in the upper regions of early Earth'sArchaean oceans.[6]
Because H2 is the lightest element, atmospheric H2 can readily be lost to space viaJeans escape, an irreversible process that drivesEarth's net mass loss.[7]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 byhydroxyl radicals (•OH), which forms water.[citation needed]
Hydrogen is produced byhydrogenases andnitrogenases enzymes in many microorganisms, some of which are being studied for their potential for biofuel production.[8][9] These H2-metabolizing enzymes are found in all threedomains of life, and out of known genomes over 30% of microbial taxa contain hydrogenase genes.[10]Fermentation produces H2 from organic matter as part of the anaerobic microbial food chain[11] via light-dependent or light-independent pathways.[8]
Anaerobic H2 oxidation often occurs duringinterspecies hydrogen transfer in which H2 produced duringfermentation is transferred to another organism, which uses the H2 to reduce CO2 to CH4 oracetate,SO2− 4 to H2S, or Fe3+ to Fe2+. Interspecies hydrogen transfer keeps H2 concentrations very low in most environments because fermentation becomes lessthermodynamically favorable as thepartial pressure of H2 increases.[11]
Hydrogen typically acts as anelectron donor.[14] This quality has implications for globalatmospheric chemistry, possibly delaying the degradation and increasing the abundance ofgreenhouse gases. This makes hydrogen an indirect greenhouse gas.[15] For example, H2 can interfere with the removal ofmethane from theatmosphere. Typically, atmospheric CH4 isoxidized byhydroxyl radicals (•OH), but H2 can also react with•OH to reduce it to H2O.[16]
