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Hydrogen cycle
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<h2 id="abiotic-cycles">Abiotic cycles</h2> <h3>Sources</h3> <p>Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic <a href="/facts/Serpentinization/CDfNaGF3">serpentinization</a> reactions between water and olivine minerals liberate H2 in the marine or terrestrial subsurface.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> In the ocean, <a href="/facts/Hydrothermal_vent/0xPqNUkY">hydrothermal vents</a> erupt magma and altered seawater fluids including abundant H2, depending on the temperature regime and host rock composition.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> Molecular hydrogen can also be produced through photooxidation (via solar <a href="/facts/Ultraviolet/kncBUqn5">UV radiation</a>) of some mineral species such as <a href="/facts/Siderite/2Vzd8MDE">siderite</a> in anoxic aqueous environments. This may have been an important process in the upper regions of early Earth's <a href="/facts/Archean/PHt8Sm8n">Archaean</a> oceans.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h3>Sinks</h3> <p>Because H2 is the lightest element, atmospheric H2 can readily be lost to space via <a href="/facts/Atmospheric_escape/lgjeWLCg">Jeans escape</a>, an irreversible process that drives <a href="/facts/Earth_mass/G74DaKFv">Earth's net mass loss</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> <a href="/facts/Photodissociation/nzRyzh8I">Photolysis</a> 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 <a href="/facts/Hydroxyl_radical/XafpvqK5">hydroxyl</a> radicals (•OH), which forms water. </p><p>Anthropogenic sinks of H2 include synthetic fuel production through the <a href="/facts/Fischer%25E2%2580%2593Tropsch_process/qx8noLlC">Fischer-Tropsch</a> reaction and artificial nitrogen fixation through the <a href="/facts/Haber_process/DMlVT2WP">Haber-Bosch process</a> to produce nitrogen <a href="/facts/Fertilizer/ysh3r6NJ">fertilizers</a>. </p> <h2 id="biotic-cycles">Biotic cycles</h2> <p>Many microbial metabolisms produce or consume H2. </p> <h3>Production</h3> <p>Hydrogen is produced by <a href="/facts/Hydrogenase/seztU9x0">hydrogenases</a> and <a href="/facts/Nitrogenase/gIhxgCaL">nitrogenases</a> enzymes in many microorganisms, some of which are being studied for their potential for biofuel production.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a><a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> These H2-metabolizing enzymes are found in all three <a href="/facts/Domain_(biology)/RpP6V7n6">domains of life</a>, and out of known genomes over 30% of microbial taxa contain hydrogenase genes.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> <a href="/facts/Fermentative_hydrogen_production/KDXHxLSM">Fermentation</a> produces H2 from organic matter as part of the anaerobic microbial food chain<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> via light-dependent or light-independent pathways.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h3>Consumption</h3> <p>Biological soil uptake is the dominant sink of atmospheric H2.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Both <a href="/facts/Aerobic_organism/a629EQD7">aerobic</a> and <a href="/facts/Anaerobic_organism/bhQnlqCq">anaerobic</a> <a href="/facts/Microorganism/pysJYcE2">microbial</a> metabolisms consume H2 by <a href="/facts/Redox/Pyd5RVcj">oxidizing</a> it in order to <a href="/facts/Redox/Pyd5RVcj">reduce</a> other compounds during <a href="/facts/Respiration_(physiology)/KKWAJIbd">respiration</a>. Aerobic H2 oxidation is known as the <a href="/facts/Knallgas-bacteria/nVRTdgEP">Knallgas</a> reaction.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p><p>Anaerobic H2 oxidation often occurs during <a href="/facts/Interspecies_hydrogen_transfer/6S2T5FJN">interspecies hydrogen transfer</a> in which H2 produced during <a href="/facts/Fermentation/rxaDr28b">fermentation</a> is transferred to another organism, which uses the H2 to reduce CO2 to CH4 or <a href="/facts/Acetate/U90VVczy">acetate</a>, SO2−4 to H2S, or Fe3+ to Fe2+. Interspecies hydrogen transfer keeps H2 concentrations very low in most environments because fermentation becomes less <a href="/facts/Chemical_stability/j0YIgc6F">thermodynamically</a> favorable as the <a href="/facts/Partial_pressure/sRw2hj4G">partial pressure</a> of H2 increases.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> </p> <h2 id="relevance-for-the-global-climate">Relevance for the global climate</h2> <p>Hydrogen typically acts as an <a href="/facts/Electron_donor/7JxwwW43">electron donor</a>.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> This quality has implications for global <a href="/facts/Atmospheric_chemistry/EbmISm33">atmospheric chemistry</a>, possibly delaying the degradation and increasing the abundance of <a href="/facts/Greenhouse_gas/HXaHqrT8">greenhouse gases</a>. This makes hydrogen an indirect greenhouse gas.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> For example, H2 can interfere with the removal of <a href="/facts/Methane/kBHneYnh">methane</a> from the <a href="/facts/Atmosphere_of_Earth/FV97Am25">atmosphere</a>. Typically, atmospheric CH4 is <a href="/facts/Redox/Pyd5RVcj">oxidized</a> by <a href="/facts/Hydroxyl_radical/XafpvqK5">hydroxyl</a> radicals (•OH), but H2 can also react with •OH to reduce it to H2O.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> CH4 + •OH → •CH3 + H2O H2 + •OH → H• + H2O <h2 id="implications-for-astrobiology">Implications for astrobiology</h2> <p><a href="/facts/Hydrothermal_vent/0xPqNUkY">Hydrothermal</a> H2 may have played a major role in <a href="/facts/Abiogenesis/BvAmLPQP">pre-biotic chemistry</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Liberation of H2 by <a href="/facts/Serpentinization/CDfNaGF3">serpentinization</a> may have supported formation of the reactants proposed in the <a href="/facts/Iron%25E2%2580%2593sulfur_world_hypothesis/UqEfdSW9">iron-sulfur world</a> <a href="/facts/Abiogenesis/BvAmLPQP">origin of life</a> hypothesis.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> The subsequent evolution of <a href="/facts/Hydrogenotroph/o0L9wUKG">hydrogenotrophic</a> <a href="/facts/Methanogenesis/5sA0no5A">methanogenesis</a> is hypothesized as one of the earliest <a href="/facts/Metabolism/WxDrm8k5">metabolisms</a> on <a href="/facts/Earth/HLLmDfj0">Earth</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> </p><p>Serpentinization can occur on any <a href="/facts/Planetary-mass_object/rbPjE6Bx">planetary body</a> with <a href="/facts/Chondrite/mbzbGm5A">chondritic</a> composition. The discovery of H2 on other <a href="/facts/Ocean_planet/Ml33anp6">ocean worlds</a>, such as <a href="/facts/Enceladus/DyWXz9jl">Enceladus</a>,<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a><a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a><a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> suggests that similar processes are ongoing elsewhere in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>, and potentially in other <a href="/facts/Planetary_system/6mcVtrTC">planetary systems</a> as well.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Biogeochemical_cycle/3vG5KBu4">Biogeochemical cycle</a></li> <li><a href="/facts/Carbon_cycle/gVusRaPF">Carbon cycle</a></li> <li><a href="/facts/Hydrogen/BoYHjl0J">Hydrogen</a></li> <li><a href="/facts/Methane/kBHneYnh">Methane</a></li> <li><a href="/facts/Serpentinite/CDfNaGF3">Serpentinization</a></li> <li><a href="/facts/Interspecies_hydrogen_transfer/6S2T5FJN">Interspecies hydrogen transfer</a></li> <li><a href="/facts/Fermentation/rxaDr28b">Fermentation</a></li> <li><a href="/facts/Hydrothermal_vent/0xPqNUkY">Hydrothermal vents</a></li> <li><a href="/facts/Water_cycle/wALHdKjJ">Water cycle</a></li> <li><a href="/facts/Ocean_Worlds_Exploration_Program/KnrtL01x">Ocean World Exploration Program</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Cameron AG (1973). 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