Get Hydrogen Cycle essential facts below. View Videos or join the Hydrogen Cycle discussion. Add Hydrogen Cycle to your PopFlock.com topic list for future reference or share this resource on social media.
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), methane (CH4), 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.
Hydrogen gas can be produced naturally through rock-water interactions or as a byproduct of microbial metabolisms. Free H2 can then be consumed by other microbes, oxidized 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 our solar system.
Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic serpentinization reactions between water and olivine minerals produce H2 in the marine or terrestrial subsurface. In the ocean, hydrothermal vents erupt magma and altered seawater fluids including abundant H2, 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.
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 (oOH), which forms water.
Hydrogen is produced by hydrogenases and nitrogenases enzymes in many microorganisms, some of which are being studied for their potential for biofuel production. These H2-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 H2 from organic matter as part of the anaerobic microbial food chain via light-dependent or light-independent pathways.
Biological soil uptake is the dominant sink of atmospheric H2. Both aerobic and anaerobic microbial metabolisms consume H2 by oxidizing it in order to reduce other compounds during respiration. Aerobic H2 oxidation is known as the Knallgas reaction.
Anaerobic H2 oxidation often occurs during interspecies hydrogen transfer in which H2 produced during fermentation is transferred to another organism, which uses the H2 to reduce CO2 to CH4 or acetate, SO42- to H2S, or Fe3+ to Fe2+. Interspecies hydrogen transfer keeps H2 concentrations very low in most environments because fermentation becomes less thermodynamically favorable as the partial pressure of H2 increases.
Relevance for the global climate
H2 can interfere with the removal of methane from the atmosphere, a greenhouse gas. Typically, atmospheric CH4 is oxidized by hydroxyl radicals (oOH), but H2 can also react with oOH to reduce it to H2O.
Serpentinization can occur on any planetary body with chondritic composition. The discovery of H2 on other ocean worlds, such as Enceladus, suggests that similar processes are ongoing elsewhere in our solar system, and potentially in other solar systems as well.
^Rhee TS, Brenninkmeijer CA, Röckmann T (2006-05-19). "The overwhelming role of soils in the global atmospheric hydrogen cycle". Atmospheric Chemistry and Physics. 6 (6): 1611-1625. doi:10.5194/acp-6-1611-2006.
^Novelli PC, Lang PM, Masarie KA, Hurst DF, Myers R, Elkins JW (1999-12-01). "Molecular hydrogen in the troposphere: Global distribution and budget". Journal of Geophysical Research: Atmospheres. 104 (D23): 30427-30444. Bibcode:1999JGR...10430427N. doi:10.1029/1999jd900788.
^Colín-García M (2016). "Hydrothermal vents and prebiotic chemistry: a review". Boletín de la Sociedad Geológica Mexicana. 68 (3): 599-620. doi:10.18268/BSGM2016v68n3a13.
^Wächtershäuser G. "Origin of life in an iron-sulfur world". =The Molecular Origins of Life. Cambridge University Press. pp. 206-218. ISBN9780511626180.
^Boyd ES, Schut GJ, Adams MW, Peters JW (2014-09-01). "Hydrogen Metabolism and the Evolution of Biological Respiration". Microbe Magazine. 9 (9): 361-367. doi:10.1128/microbe.9.361.1.