Hydrogen cycle
{{Short description|Hydrogen exchange between the living and non-living world}}
{{Biogeochemical cycle sidebar|nutrient}}
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.{{cite journal| vauthors = Cameron AG |date=1973|title=Abundances of the elements in the solar system|journal=Space Science Reviews|language=en|volume=15|issue=1|pages=121|doi=10.1007/BF00172440|issn=0038-6308|bibcode=1973SSRv...15..121C|s2cid=120201972}} 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.{{cite journal | vauthors = Colman DR, Poudel S, Stamps BW, Boyd ES, Spear JR | title = The deep, hot biosphere: Twenty-five years of retrospection | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 27 | pages = 6895–6903 | date = July 2017 | pmid = 28674200 | pmc = 5502609 | doi = 10.1073/pnas.1701266114 | doi-access = free | bibcode = 2017PNAS..114.6895C }}
Abiotic cycles
= Sources =
Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic serpentinization reactions between water and olivine minerals liberate H2 in the marine or terrestrial subsurface.{{cite journal|vauthors=Russell MJ, Hall AJ, Martin W|date=December 2010|title=Serpentinization as a source of energy at the origin of life|journal=Geobiology|volume=8|issue=5|pages=355–71|doi=10.1111/j.1472-4669.2010.00249.x|pmid=20572872|bibcode=2010Gbio....8..355R }} In the ocean, hydrothermal vents erupt magma and altered seawater fluids including abundant H2, depending on the temperature regime and host rock composition.{{cite journal|display-authors=6|vauthors=Petersen JM, Zielinski FU, Pape T, Seifert R, Moraru C, Amann R, Hourdez S, Girguis PR, Wankel SD, Barbe V, Pelletier E, Fink D, Borowski C, Bach W, Dubilier N|date=August 2011|title=Hydrogen is an energy source for hydrothermal vent symbioses|journal=Nature|volume=476|issue=7359|pages=176–80|bibcode=2011Natur.476..176P|doi=10.1038/nature10325|pmid=21833083|s2cid=25578}}{{cite journal|vauthors=Konn C, Charlou JL, Holm NG, Mousis O|date=May 2015|title=The production of methane, hydrogen, and organic compounds in ultramafic-hosted hydrothermal vents of the Mid-Atlantic Ridge|journal=Astrobiology|volume=15|issue=5|pages=381–99|bibcode=2015AsBio..15..381K|doi=10.1089/ast.2014.1198|pmc=4442600|pmid=25984920}} 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.{{cite journal | vauthors = Kim JD, Yee N, Nanda V, Falkowski PG | title = Anoxic photochemical oxidation of siderite generates molecular hydrogen and iron oxides | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 25 | pages = 10073–7 | date = June 2013 | pmid = 23733945 | pmc = 3690895 | doi = 10.1073/pnas.1308958110 | bibcode = 2013PNAS..11010073K | doi-access = free }}
= Sinks =
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.{{cite journal | vauthors = Catling DC, Zahnle KJ, McKay C | title = Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth | journal = Science | volume = 293 | issue = 5531 | pages = 839–43 | date = August 2001 | pmid = 11486082 | doi = 10.1126/science.1061976 | bibcode = 2001Sci...293..839C | s2cid = 37386726 }} 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.{{cn|date=January 2024}}
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.{{cn|date=January 2024}}
Biotic cycles
Many microbial metabolisms produce or consume H2.
= Production =
Hydrogen is produced by hydrogenases and nitrogenases enzymes in many microorganisms, some of which are being studied for their potential for biofuel production.{{cite journal | vauthors = Khetkorn W, Rastogi RP, Incharoensakdi A, Lindblad P, Madamwar D, Pandey A, Larroche C | title = Microalgal hydrogen production - A review | journal = Bioresource Technology | volume = 243 | pages = 1194–1206 | date = November 2017 | pmid = 28774676 | doi = 10.1016/j.biortech.2017.07.085 | bibcode = 2017BiTec.243.1194K }}{{cite journal| vauthors = Das D |date=2001|title=Hydrogen production by biological processes: a survey of literature |journal=International Journal of Hydrogen Energy|language=en|volume=26|issue=1|pages=13–28|doi=10.1016/S0360-3199(00)00058-6 |bibcode=2001IJHE...26...13D }} 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.{{cite journal|vauthors=Peters JW, Schut GJ, Boyd ES, Mulder DW, Shepard EM, Broderick JB, King PW, Adams MW|author-link6=Joan B. Broderick|date=June 2015|title=[FeFe]- and [NiFe]-hydrogenase diversity, mechanism, and maturation|journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research|volume=1853|issue=6|pages=1350–69|doi=10.1016/j.bbamcr.2014.11.021|pmid=25461840|url=http://scholarworks.montana.edu/xmlui/bitstream/1/8928/1/Boyd_BBA_MCR_2014Postprint_A1b.pdf|doi-access=free}} Fermentation produces H2 from organic matter as part of the anaerobic microbial food chain{{cite book|title=Processes in Microbial Ecology|last=Kirchman|first=David L. |name-list-style=vanc |date=2011-02-02|publisher=Oxford University Press|isbn=9780199586936|doi=10.1093/acprof:oso/9780199586936.001.0001}} via light-dependent or light-independent pathways.
= Consumption =
Biological soil uptake is the dominant sink of atmospheric H2.{{cite journal|vauthors=Rhee TS, Brenninkmeijer CA, Röckmann T|date=2006-05-19|title=The overwhelming role of soils in the global atmospheric hydrogen cycle|journal=Atmospheric Chemistry and Physics|volume=6|issue=6|pages=1611–1625|doi=10.5194/acp-6-1611-2006|bibcode=2006ACP.....6.1611R|doi-access=free}} 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.{{cite journal|vauthors=Seager S, Schrenk M, Bains W|date=January 2012|title=An astrophysical view of Earth-based metabolic biosignature gases|journal=Astrobiology|volume=12|issue=1|pages=61–82|bibcode=2012AsBio..12...61S|doi=10.1089/ast.2010.0489|pmid=22269061|hdl=1721.1/73073|hdl-access=free}}
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, {{chem|SO|4|2-}} 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
Hydrogen typically acts as an electron donor.{{Cite web |date=2017-05-08 |title=5.4B: Electron Donors and Acceptors |url=https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Boundless)/05:_Microbial_Metabolism/5.04:_Glycolysis/5.4B:_Electron_Donors_and_Acceptors#:~:text=Inorganic%20electron%20donors%20include%20hydrogen,below%20the%20surface%20of%20Earth. |access-date=2025-03-20 |website=Biology LibreTexts |language=en}} This quality has implications for global atmospheric chemistry, possibly delaying the degradation and increasing the abundance of greenhouse gases. This makes hydrogen an indirect greenhouse gas.{{Cite journal |last=Sand |first=Maria |last2=Skeie |first2=Ragnhild Bieltvedt |last3=Sandstad |first3=Marit |last4=Krishnan |first4=Srinath |last5=Myhre |first5=Gunnar |last6=Bryant |first6=Hannah |last7=Derwent |first7=Richard |last8=Hauglustaine |first8=Didier |last9=Paulot |first9=Fabien |last10=Prather |first10=Michael |last11=Stevenson |first11=David |date=2023-06-07 |title=A multi-model assessment of the Global Warming Potential of hydrogen |url=https://www.nature.com/articles/s43247-023-00857-8 |journal=Communications Earth & Environment |language=en |volume=4 |issue=1 |pages=1–12 |doi=10.1038/s43247-023-00857-8 |issn=2662-4435|hdl=11250/3120918 |hdl-access=free }} For example, H2 can interfere with the removal of methane from the atmosphere. Typically, atmospheric CH4 is oxidized by hydroxyl radicals (•OH), but H2 can also react with •OH to reduce it to H2O.{{cite journal | vauthors = Novelli PC, Lang PM, Masarie KA, Hurst DF, Myers R, Elkins JW |date=1999-12-01|title=Molecular hydrogen in the troposphere: Global distribution and budget |journal=Journal of Geophysical Research: Atmospheres |volume=104 |issue=D23 |pages=30427–30444 |doi=10.1029/1999jd900788 |bibcode=1999JGR...10430427N|doi-access=free }}
: CH4 + •OH → •CH3 + H2O
: H2 + •OH → H• + H2O
Implications for astrobiology
Hydrothermal H2 may have played a major role in pre-biotic chemistry.{{cite journal|last=Colín-García|first=María | name-list-style = vanc |date=2016|title=Hydrothermal vents and prebiotic chemistry: a review|journal=Boletín de la Sociedad Geológica Mexicana|volume=68|issue=3|pages=599–620|doi=10.18268/BSGM2016v68n3a13|doi-access=free}} Liberation of H2 by serpentinization may have supported formation of the reactants proposed in the iron-sulfur world origin of life hypothesis.{{cite book |last=Wächtershäuser|first=Günter |name-list-style=vanc | chapter = Origin of life in an iron–sulfur world | title = The Molecular Origins of Life|pages=206–218|publisher=Cambridge University Press|isbn=9780511626180}} The subsequent evolution of hydrogenotrophic methanogenesis is hypothesized as one of the earliest metabolisms on Earth.{{cite journal| vauthors = Boyd ES, Schut GJ, Adams MW, Peters JW |date=2014-09-01|title=Hydrogen Metabolism and the Evolution of Biological Respiration |journal=Microbe Magazine|volume=9|issue=9|pages=361–367|doi=10.1128/microbe.9.361.1 |doi-access=}}
Serpentinization can occur on any planetary body with chondritic composition. The discovery of H2 on other ocean worlds, such as Enceladus,{{cite journal | vauthors = Seewald JS | title = Detecting molecular hydrogen on Enceladus | journal = Science | volume = 356 | issue = 6334 | pages = 132–133 | date = April 2017 | pmid = 28408557 | doi = 10.1126/science.aan0444 | bibcode = 2017Sci...356..132S | s2cid = 206658660 }}{{cite journal | vauthors = Hsu HW, Postberg F, Sekine Y, Shibuya T, Kempf S, Horányi M, Juhász A, Altobelli N, Suzuki K, Masaki Y, Kuwatani T, Tachibana S, Sirono SI, Moragas-Klostermeyer G, Srama R | display-authors = 6 | title = Ongoing hydrothermal activities within Enceladus | journal = Nature | volume = 519 | issue = 7542 | pages = 207–10 | date = March 2015 | pmid = 25762281 | doi = 10.1038/nature14262 | bibcode = 2015Natur.519..207H | s2cid = 4466621 }}{{cite journal| vauthors = Glein CR, Baross JA, Waite Jr JH |date=2015|title=The pH of Enceladus' ocean |journal=Geochimica et Cosmochimica Acta|language=en|volume=162|pages=202–219|doi=10.1016/j.gca.2015.04.017 |bibcode=2015GeCoA.162..202G|arxiv=1502.01946|s2cid=119262254}} suggests that similar processes are ongoing elsewhere in the Solar System, and potentially in other planetary systems as well.
See also
References
{{Reflist}}
{{Biogeochemical cycle}}