What is "hydrogen cycle"

n. 1 (context biology English) the transmission of hydrogen from water to carbohydrates etc and back to water by living organisms 2 the electrolysis or solar photolysis of water to give hydrogen and oxygen which are then recombined in a fuel cell to produce electricity

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 (H). This hydrogen is then taken up by methanogens and converted to methane via methanogenesis. One important characteristic of interspecies hydrogen transfer is that the H concentration in the microbial environment is very low. Maintaining a low hydrogen concentration is important because the anaerobic fermentative process becomes 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.