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.
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.
"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 proprionate 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