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 m ethanogenic degradation of organic matter and serves as substrate for metha nogenic archaea. Hydrogen should theoretically account for 33% of total met hanogenesis when carbohydrates or similar forms of organic matter are degra ded. Many methanogenic environments show both much lower and much higher co ntributions of H-2 to CH4 production than is considered normal. While the l ower contributions are relatively easily explained (e.g. by the contributio n of homoacetogenesis), the mechanisms behind higher contributions are most ly unclear. In methanogenic environments H-2 is rapidly turned over, its co ncentration being the result of simultaneous production by fermenting plus syntrophic bacteria and consumption by methanogenic archaea. The steady-sta te concentration observed in most methanogenic environments is close to the thermodynamic equilibrium of H-2-dependent methanogenesis. The threshold i s usually equivalent to a Gibbs free energy of -23 kJ mol(-1) CH4 that is n ecessary to couple CH4 production to the generation of 1/3 ATP. Methanogene sis from Hp is inhibited if the H-2 concentration decreases below this thre shold. Concentrations of H-2 can only be decreased below this threshold if a H-2-consuming reaction with a lower H-2 threshold (e.g. sulfate reduction ) lakes over at a rate that is equal to or higher than that of methanogenes is. The instantaneous and complete inhibition of H-2-dependent CH4 producti on that is often observed upon addition of sulfate can only be explained if a comparably high sulfate reduction potential is cryptically present in th e methanogenic environment. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.