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Abstract

The role and fate of H2 in anaerobic aquatic sediments is reviewed with emphasis on methanogenic freshwater environments. H2 is an intermediate in anaerobic degradation of organic matter and is a substrate for methanogenesis. Its contribution of CH4 production can vary between 4 and 96%. H2 is formed by various reactions. Energetics of glucose fermentation, for example, allow for stoichiometric H2 formation from low redox potential intermediates. This is also the case for other fermentative reactions if endergonic H2 production is coupled to exergonic H2 consumption. The latter requires syntrophic partner organisms such as methanogens. In situ H, concentrations are constrained by the energetics of both H2-producing and H2-consuming microorganisms. However, H2 can also be cycled within microenvironments such as H2-syntrophic methanogenic bacterial associations: traces of H2 can be produced without energetic constraint by various anaerobic bacteria, and H2 formation can be bypassed by homoacetogenic bacteria. Despite steep gradients, H2 usually does not escape from sediments into the overlying waters because of efficient recycling at the sediment water interface by yet poorly known processes.