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 archaea (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.
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.
- Methane and non methane hydrocarbon oxidation
- Industries and fossil fuels
- Biomass burning
- Nitrogen fixation
- Oxidation by hydroxyl radicals
- Microbial soil uptake
Both biological and abiological processes contribute significantly to the global H2 cycle. The largest sources of environmental H2 are the atmospheric photochemical process of hydrocarbon dissociation and microbial H2 production. Biological processes are the dominant sink for atmospheric H2 illustrating that overall the most important physiological role of H2 is as a biological fuel. Within microbial environments cellular processes which result in H2 production are nearly always linked with either inter or intracellular H2 uptake.
- Microbe-mediated soil uptake
- Nitrogen-fixation by-products
- Biomass Burning
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- Fossil Fuel & Industrial Output
Human Impact on H2 Cycle
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- Biogeochemical cycle
- Hydrogen fuel cell
- Carbon cycle
- Sulfur-iodine cycle
- 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
- Parkin, Alison (2014). "Chapter 5. Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts". In Peter M.H. Kroneck and Martha E. Sosa Torres. The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Metal Ions in Life Sciences 14. Springer. pp. 99–124. doi:10.1007/978-94-017-9269-1_5.
- "Microbiology and Biochemistry of Strict Anaerobes Involved in Interspecies Hydrogen Transfer" by Jean-Pierre Bélaich; Mireille Bruschi; Jean-Louis Garcia; Federation of European Microbiological Societies. Published Nov 1990. ISBN 0-306-43517-9
-  F.A.M. de Bok, C.M. Plugge, and A.J.M. Stams; "Interspecies electron transfer in methanogenic propionate degrading consortia". Water Research 38 (2004): 1368-1375
-  A.J.M. Stams et al., "Exocellular electron transfer in anaerobic microbial communities", Environmental Microbiology, 8 (2006):371-382