Hydrogen cycle

Hydrogen exchange between the living and non-living world

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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.[1] 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.[2]

## Abiotic cycles

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### Sources

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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.[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 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.[6]

### Sinks

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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.[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 by hydroxyl radicals (•OH), which forms water.[citation needed]

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.[citation needed]

## Biotic cycles

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Many microbial metabolisms produce or consume H2.

### Production

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Hydrogen is produced by hydrogenases and nitrogenases 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 three domains 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]

### Consumption

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Biological soil uptake is the dominant sink of atmospheric H2.[12] 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.[13]

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, SO2−
4 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.[11]

## Relevance for the global climate

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H2 can interfere with the removal of methane from the atmosphere, a greenhouse gas. Typically, atmospheric CH4 is oxidized by hydroxyl radicals (•OH), but H2 can also react with •OH to reduce it to H2O.[14]

CH4 \+ •OH → •CH3 \+ H2O
H2 \+ •OH → H• \+ H2O

## Implications for astrobiology

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Hydrothermal H2 may have played a major role in pre-biotic chemistry.[15] Production of H2 by serpentinization supported formation of the reactants proposed in the iron-sulfur world origin of life hypothesis.[16] The subsequent evolution of hydrogenotrophic methanogenesis is hypothesized as one of the earliest metabolisms on Earth.[17][2]

Serpentinization can occur on any planetary body with chondritic composition. The discovery of H2 on other ocean worlds, such as Enceladus,[18][19][20] suggests that similar processes are ongoing elsewhere in the Solar System, and potentially in other planetary systems as well.[13]

## See also

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* Biogeochemical cycle
* Carbon cycle
* Hydrogen
* Methane
* Serpentinization
* Interspecies hydrogen transfer
* Fermentation
* Hydrothermal vents
* Water cycle
* Ocean World Exploration Program

## References

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1. ^ Cameron AG (1973). "Abundances of the elements in the solar system". Space Science Reviews. 15 (1): 121. Bibcode:1973SSRv...15..121C. doi:10.1007/BF00172440. ISSN 0038-6308. S2CID 120201972.
2. ^ a b Colman DR, Poudel S, Stamps BW, Boyd ES, Spear JR (July 2017). "The deep, hot biosphere: Twenty-five years of retrospection". Proceedings of the National Academy of Sciences of the United States of America. 114 (27): 6895–6903. Bibcode:2017PNAS..114.6895C. doi:10.1073/pnas.1701266114. PMC 5502609. PMID 28674200.
3. ^ Russell MJ, Hall AJ, Martin W (December 2010). "Serpentinization as a source of energy at the origin of life". Geobiology. 8 (5): 355–71. Bibcode:2010Gbio....8..355R. doi:10.1111/j.1472-4669.2010.00249.x. PMID 20572872.
4. ^ a b Konn C, Charlou JL, Holm NG, Mousis O (May 2015). "The production of methane, hydrogen, and organic compounds in ultramafic-hosted hydrothermal vents of the Mid-Atlantic Ridge". Astrobiology. 15 (5): 381–99. Bibcode:2015AsBio..15..381K. doi:10.1089/ast.2014.1198. PMC 4442600. PMID 25984920.
5. ^ Petersen JM, Zielinski FU, Pape T, Seifert R, Moraru C, Amann R, et al. (August 2011). "Hydrogen is an energy source for hydrothermal vent symbioses". Nature. 476 (7359): 176–80. Bibcode:2011Natur.476..176P. doi:10.1038/nature10325. PMID 21833083. S2CID 25578.
6. ^ Kim JD, Yee N, Nanda V, Falkowski PG (June 2013). "Anoxic photochemical oxidation of siderite generates molecular hydrogen and iron oxides". Proceedings of the National Academy of Sciences of the United States of America. 110 (25): 10073–7. Bibcode:2013PNAS..11010073K. doi:10.1073/pnas.1308958110. PMC 3690895. PMID 23733945.
7. ^ Catling DC, Zahnle KJ, McKay C (August 2001). "Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth". Science. 293 (5531): 839–43. Bibcode:2001Sci...293..839C. doi:10.1126/science.1061976. PMID 11486082. S2CID 37386726.
8. ^ a b Khetkorn W, Rastogi RP, Incharoensakdi A, Lindblad P, Madamwar D, Pandey A, Larroche C (November 2017). "Microalgal hydrogen production - A review". Bioresource Technology. 243: 1194–1206. Bibcode:2017BiTec.243.1194K. doi:10.1016/j.biortech.2017.07.085. PMID 28774676.
9. ^ Das D (2001). "Hydrogen production by biological processes: a survey of literature". International Journal of Hydrogen Energy. 26 (1): 13–28. Bibcode:2001IJHE...26...13D. doi:10.1016/S0360-3199(00)00058-6.
10. ^ Peters JW, Schut GJ, Boyd ES, Mulder DW, Shepard EM, Broderick JB, King PW, Adams MW (June 2015). "[FeFe]\- and [NiFe]-hydrogenase diversity, mechanism, and maturation" (PDF). Biochimica et Biophysica Acta (BBA) - Molecular Cell Research. 1853 (6): 1350–69. doi:10.1016/j.bbamcr.2014.11.021. PMID 25461840.
11. ^ a b Kirchman DL (2011-02-02). Processes in Microbial Ecology. Oxford University Press. doi:10.1093/acprof:oso/9780199586936.001.0001. ISBN 9780199586936.
12. ^ 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. Bibcode:2006ACP.....6.1611R. doi:10.5194/acp-6-1611-2006.
13. ^ a b Seager S, Schrenk M, Bains W (January 2012). "An astrophysical view of Earth-based metabolic biosignature gases". Astrobiology. 12 (1): 61–82. Bibcode:2012AsBio..12...61S. doi:10.1089/ast.2010.0489. hdl:1721.1/73073. PMID 22269061.
14. ^ 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.
15. ^ 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.
16. ^ Wächtershäuser G. "Origin of life in an iron–sulfur world". The Molecular Origins of Life. Cambridge University Press. pp. 206–218. ISBN 9780511626180.
17. ^ 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.
18. ^ Seewald JS (April 2017). "Detecting molecular hydrogen on Enceladus". Science. 356 (6334): 132–133. Bibcode:2017Sci...356..132S. doi:10.1126/science.aan0444. PMID 28408557. S2CID 206658660.
19. ^ Hsu HW, Postberg F, Sekine Y, Shibuya T, Kempf S, Horányi M, et al. (March 2015). "Ongoing hydrothermal activities within Enceladus". Nature. 519 (7542): 207–10. Bibcode:2015Natur.519..207H. doi:10.1038/nature14262. PMID 25762281. S2CID 4466621.
20. ^ Glein CR, Baross JA, Waite Jr JH (2015). "The pH of Enceladus' ocean". Geochimica et Cosmochimica Acta. 162: 202–219. arXiv:1502.01946. Bibcode:2015GeCoA.162..202G. doi:10.1016/j.gca.2015.04.017. S2CID 119262254.

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Biogeochemical cycles

Cycles|

* Carbon cycle
* carbonate–silicate cycle
* deep carbon cycle
* oceanic carbon cycle
* Chlorine cycle
* Hydrogen cycle
* Iron cycle
* Mercury cycle
* Mineral cycle
* Nitrogen cycle
* Nutrient cycle
* Oxygen cycle
* ozone
* Phosphorus cycle
* Rock cycle
* Selenium cycle
* Silica cycle
* Sulfur cycle
* Water cycle
* deep water cycle
* Marine biogeochemical cycles

Research groups|

* DAAC
* GEOTRACES
* IMBER
* NOBM
* SOLAS

Related topics|

* Biogeochemistry
* geochemical cycle
* chemical cycling
* environmental chemistry
* Biosequestration
* carbon sequestration
* carbon sink
* soil carbon
* biological pump
* viral shunt
* mycorrhizal fungi
* Ocean acidification
* acid rain
* Methane clathrate
* clathrate gun hypothesis
* Arctic methane emissions
* Human impact on the nitrogen cycle
* Lichens and nitrogen cycling
* Nitrification
* Nitrogen fixation
* Nitrogen assimilation
* Phosphorus assimilation
* Sulfur assimilation
* Planetary boundaries

* Category

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