Contribution of hydrogen to methane production and control of hydrogen concentrations in methanogenic soils and sediments
Hydrogen is, with acetate, one of the most important intermediates in the methanogenic degradation of organic matter and serves as substrate for methanogenic archaea. Hydrogen should theoretically account for 33% of total methanogenesis when carbohydrates or similar forms of organic matter are degraded. Many methanogenic environments show both much lower and much higher contributions of H2 to CH4 production than is considered normal. While the lower contributions are relatively easily explained (e.g. by the contribution of homoacetogenesis), the mechanisms behind higher contributions are mostly unclear. In methanogenic environments H2 is rapidly turned over, its concentration being the result of simultaneous production by fermenting plus syntrophic bacteria and consumption by methanogenic archaea. The steady-state concentration observed in most methanogenic environments is close to the thermodynamic equilibrium of H2-dependent methanogenesis. The threshold is usually equivalent to a Gibbs free energy of −23 kJ mol−1 CH4 that is necessary to couple CH4 production to the generation of 1/3 ATP. Methanogenesis from H2 is inhibited if the H2 concentration decreases below this threshold. Concentrations of H2 can only be decreased below this threshold if a H2-consuming reaction with a lower H2 threshold (e.g. sulfate reduction) takes over at a rate that is equal to or higher than that of methanogenesis. The instantaneous and complete inhibition of H2-dependent CH4 production that is often observed upon addition of sulfate can only be explained if a comparably high sulfate reduction potential is cryptically present in the methanogenic environment.