The hydrogen cycle consists of hydrogen exchanges between Life (living) and abiotic (non-living) sources and sinks of hydrogen-containing compounds.
Hydrogen (H) is the most abundant element in the universe.
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 Abiogenesis and the early evolution of life on Earth, and potentially elsewhere in the Solar System.
Abiotic cycles
Sources
Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic
serpentinization reactions between water and olivine minerals liberate H
2 in the marine or terrestrial subsurface.
In the ocean, hydrothermal vents erupt magma and altered seawater fluids including abundant H
2, depending on the temperature regime and host rock composition.
Molecular hydrogen can also be produced through photooxidation (via solar
Ultraviolet) 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
Archean oceans.
Sinks
Because H
2 is the lightest element, atmospheric H
2 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 CH
4 or H
2O, can also liberate H
2 from the upper atmosphere and contribute to this process. Another major sink of free atmospheric H
2 is photochemical oxidation by
Hydroxyl radical radicals (•OH), which forms water.
Anthropogenic sinks of H2 include synthetic fuel production through the Fischer-Tropsch reaction and artificial nitrogen fixation through the Haber process to produce nitrogen .
Biotic cycles
Many microbial metabolisms produce or consume H
2.
Production
Hydrogen is produced by
and
enzymes in many microorganisms, some of which are being studied for their potential for biofuel production.
These H
2-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 H
2 from organic matter as part of the anaerobic microbial food chain
via light-dependent or light-independent pathways.
Consumption
Biological soil uptake is the dominant sink of atmospheric H
2.
Both
Aerobic organism and anaerobic
Microorganism metabolisms consume H
2 by
Redox it in order to
Redox other compounds during respiration. Aerobic H
2 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, 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.
This quality has implications for global atmospheric chemistry, possibly delaying the degradation and increasing the abundance of
Greenhouse gas. This makes hydrogen an indirect greenhouse gas.
For example, H
2 can interfere with the removal of
methane from the atmosphere. Typically, atmospheric CH
4 is
Redox by
Hydroxyl radical radicals (
•OH), but H
2 can also react with
•OH to reduce it to H
2O.
- CH4 + •OH → •CH3 + H2O
- H2 + •OH → H• + H2O
Implications for astrobiology
Hydrothermal H
2 may have played a major role in
Abiogenesis.
Liberation of H
2 by
serpentinization may have supported formation of the reactants proposed in the iron-sulfur world
Abiogenesis hypothesis.
The subsequent evolution of
methanogenesis is hypothesized as one of the earliest
on
Earth.
Serpentinization can occur on any planetary body with Chondrite composition. The discovery of H2 on other Ocean planet, such as Enceladus,
See also
