# Hydrogen cycle

Anaerobic fermentation of organic substances to carbon dioxide and methane is a collaborative effort involving many different biochemical reactions, processes and species of microorganisms. One of these many processes that occur is termed "interspecies hydrogen transfer". This process has been described as integral to the symbiosis between certain methane-producing bacteria (methanogens) and nonmethanogenic anaerobes. In this symbiosis, the nonmethanogenic anaerobes degrade the organic substance and produce -among other things- molecular hydrogen (H2). This hydrogen is then taken up by methanogens and converted to methane via methanogenesis. One important characteristic of interspecies hydrogen transfer is that the H2 concentration in the microbial environment is very low. Maintaining a low hydrogen concentration is important because the anaerobic fermentative process become increasingly thermodynamically unfavorable as the partial pressure of hydrogen increases. A key difference compared to other biogeochemical cycles is that because of its low molecular weight hydrogen can leave Earth's atmosphere. It has been suggested that this occurred on a grand scale in the past and that this is why today the Earth is mostly irreversibly oxidised.[1]

## Relevance for the Global Climate

H2 is a trace, secondary greenhouse gas that interferes with the removal of methane. H2 interacts with hydroxyl radicals (•OH), reducing them to H2O (water). •OH radicals that would typically oxidize methane in the following reaction are removed if they first interact with H2 in the atmosphere.

$\mathrm{H_2 + OH \longrightarrow H + H_2O}$

$\mathrm{CH_4 + OH \longrightarrow CH_3 + H_2O}$

## Main Components

• Sources
• Methane and non methane hydrocarbon oxidation
• Industries and fossil fuels
• Biomass burning
• Nitrogen fixation
• Oceans
• Sinks
• Microbial soil uptake

### Atmosphere

• Photochemical Reactions

### Terrestrial Biosphere

• Microbe-mediated soil uptake
• Nitrogen-fixation by-products
• Biomass Burning

### Anthropogenic

• Fossil Fuel & Industrial Output

## References

1. ^ http://www.sciencemag.org/cgi/content/abstract/293/5531/839 Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth David C. Catling, Kevin J. Zahnle, and Christopher McKay (3 August 2001) Science 293 (5531), 839. doi:10.1126/science.1061976