Anaerobic hydrogen metabolism in aquatic sediments

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.