The hydrogen cycle consists of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
exchanges between
biotic (living) and
abiotic
In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...
(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 (H
2O), hydrogen gas (H
2),
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
(H
2S), and
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
(NH
3). Many organic compounds also contain H atoms, such as
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
s and
organic matter
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
. Given the ubiquity of hydrogen atoms in inorganic and organic chemical compounds, the hydrogen cycle is focused on molecular hydrogen, H
2.
Hydrogen gas can be produced naturally through rock-water interactions or as a byproduct of microbial metabolisms. Free H
2 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 the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
.
Abiotic cycles
Sources
Abiotic sources of hydrogen gas include water-rock and photochemical reactions. Exothermic serpentinization reactions between water and olivine minerals produce H
2 in the marine or terrestrial subsurface.
In the ocean,
hydrothermal vent
A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
s 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
UV radiation
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
) of some mineral species such as
siderite
Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
in anoxic aqueous environments. This may have been an important process in the upper regions of early Earth's
Archaean oceans.
Sinks
Because H
2 is the lightest element, atmospheric H
2 can readily be lost to space via
Jeans escape
Atmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and ...
, an irreversible process that drives
Earth's net mass loss.
Photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
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
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
radicals (•OH), which forms water.
Anthropogenic sinks of H
2 include synthetic fuel production through the
Fischer-Tropsch reaction and artificial nitrogen fixation through the
Haber-Bosch process
The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today. It is named after its inventors, the German chemists Fritz Haber and C ...
to produce nitrogen
fertilizer
A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
s.
Biotic cycles
Many microbial metabolisms produce or consume H
2.
Production
Hydrogen is produced by
hydrogenase A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below:
Hydrogen uptake () is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide (), and fumara ...
s and
nitrogenase
Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrog ...
s 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
In Biology, biological Taxonomy (biology), taxonomy, a domain ( or ) (Latin: ''regio''), also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of ...
, and out of known genomes over 30% of microbial taxa contain hydrogenase genes.
Fermentation
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
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 and anaerobic microbial metabolisms consume H
2 by oxidizing it in order to reduce other compounds during respiration. Aerobic H
2 oxidation is known as the
Knallgas
Oxyhydrogen is a mixture of hydrogen (H2) and oxygen (O2) gases. This gaseous mixture is used for torches to process refractory materials and was the first
gaseous mixture used for welding. Theoretically, a ratio of 2:1 hydrogen:oxygen is enough ...
reaction.
Anaerobic H
2 oxidation often occurs during
interspecies hydrogen transfer Interspecies hydrogen transfer (IHT) is a form of interspecies electron transfer. It is a syntrophic process by which H2 is transferred from one organism to another, particularly in the rumen and other anaerobic environments.
IHT was discovered be ...
in which H
2 produced during fermentation is transferred to another organism, which uses the H
2 to reduce CO
2 to CH
4 or acetate, to H
2S, or Fe
3+ to Fe
2+. Interspecies hydrogen transfer keeps H
2 concentrations very low in most environments because fermentation becomes less thermodynamically favorable as the partial pressure of H
2 increases.
Relevance for the global climate
H
2 can interfere with the removal of methane from the atmosphere, a
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
. Typically, atmospheric CH
4 is oxidized by
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
radicals (
•OH), but H
2 can also react with
•OH to reduce it to H
2O.
# CH
4 +
•OH →
•CH
3 + H
2O
# H
2 +
•OH → H
• + H
2O
Implications for astrobiology
Hydrothermal H
2 may have played a major role in
pre-biotic chemistry. Production of H
2 by serpentinization supported formation of the reactants proposed in the
iron-sulfur world origin of life hypothesis. The subsequent evolution of
hydrogenotroph Hydrogenotrophs are organisms that are able to metabolize molecular hydrogen as a source of energy.
An example of hydrogenotrophy is performed by carbon dioxide-reducing organismsStams, J.M., and Plugge, C.M. (2010) The microbiology of methanogene ...
ic
methanogenesis
Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...
is hypothesized as one of the earliest metabolisms on Earth.
Serpentinization can occur on any planetary body with
chondritic
A chondrite is a stony (non-metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form prim ...
composition. The discovery of H
2 on other
ocean worlds, such as
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refl ...
,
suggests that similar processes are ongoing elsewhere in the Solar System, and potentially in other planetary systems as well.
See also
*
Biogeochemical cycle
A biogeochemical cycle (or more generally a cycle of matter) is the pathway by which a chemical substance cycles (is turned over or moves through) the biotic and the abiotic compartments of Earth. The biotic compartment is the biosphere and the ...
*
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
*
Hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
*
Methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
*
Serpentinization
Serpentinization is a hydration and Metamorphic rock, metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the Serp ...
*
Interspecies hydrogen transfer Interspecies hydrogen transfer (IHT) is a form of interspecies electron transfer. It is a syntrophic process by which H2 is transferred from one organism to another, particularly in the rumen and other anaerobic environments.
IHT was discovered be ...
*
Fermentation
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
*
Hydrothermal vents
A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
*
Water cycle
The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
*
Ocean World Exploration Program
References
{{Biogeochemical cycle
Biogeochemical cycle
Metabolism
Cycle
Cycle, cycles, or cyclic may refer to:
Anthropology and social sciences
* Cyclic history, a theory of history
* Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr.
* Social cycle, various cycles in soc ...
Hydrogen biology