Conrad 1999

Conrad R. 1999. Contribution of hydrogen to methane production and control of hydrogen concentrations in methanogenic soils and sediments. FEMS Microbiology Ecology 28: 193-202.

This author reviews the chemistry behind methane production by Archaea in anaerobic environments, focusing on the contribution of H2 rather than acetate to methanogenesis. The thermodynamics and kinetics of H2-driven CH4 production are distinct from those of acetate-driven, and the stoichiometry of the situation indicates that H2 should contribute 33% of the CH4 from a given ecosystem.

Methanogenesis in anaerobic sediments and soils is the end of a short chain of microbial interactions. First, organic matter is broken down by fermenting bacteria. The products of fermentation includes H2, and the other components such as alcohols and fatty acids, are further decomposed by syntrophic bacteria, also supplying some acetate to the environment. Finally, methanogens consume either H2 and CO2 or acetate (CH3CO2-) to produce methane.

There are many studies that show this expected pattern of methanogenesis, but many other that show either over- or under-representation of H2. Where H2 contributes less CH4 than expected, the most likely explanations involve sulfate reducers, microbes capable of outcompeting H2-consuming methanogens by more efficient use of H2 and faster population growth, based on the thermodynamics of the two guilds respective metabolisms. Such situations are common in marine and acidic freshwater sediments.

Where H2 contributes more CH4 than expected, including an Antarctic soil where H2 is the basis of 100% of CH4 production, the explanation is not as well established. The explanations that have been proposed, by this and other authors, include additional sinks of acetate such as scavenging by other organisms, additional sources of H2 including geological sources, or measurements of the system taken when it was far from equilibrium. The models of H2 and CH4 dynamics are mostly based on equilibrium conditions.

That addition of sulfate inhibits methanogenesis is well established. Competition explains this observation in sediments and soils where the biological community has had time to reach something like equilibrium, with methanogens outcompeted by sulfate reducers. However, addition of sulfate to sediment immediately and completely inhibits H2-driven CH4 production, which cannot be explained by ecosystem dynamics. A model involving a threshold H2 concentration, in which H2 levels lower than some critical level determined by the thermodynamics of the situation shut down that pathway, does explain these observations.