Ralf Conrad and co-workers

Max-Planck-Institute for Terrestrial Microbiology, 35043 Marburg, Germany

　　Methane and carbon dioxide are the end products of the degradation of organic matter in anoxic environments, when oxidants such as nitrate, ferric iron and sulfate have been depleted. Then, organic matter is degraded by a complex community consisting of hydrolytic, fermenting, syntrophic, acetogenic and methanogenic microorganisms. In order to elucidate function and structure of methanogenic microbial communities, we have studied different aquatic environments, such as flooded rice field soil, lake sediments, periodically flooded river sediments and peat bogs. Using most-probable number counting combined with isolation and cultivation-independent molecular techniques we have identified in rice field soil, for example, that Clostridia, Verrucomicrobia and bacteria of the Cytophaga-Flavobacterium-Bacteroides cluster are the most important bacteria that ferment polysaccharides to acetate, propionate and H₂ as primary products. Using stable isotope probing of rRNA, we identified Syntrophobacter, Pelotomaculum and Smithella to be the most important syntrophic bacteria that degrade propionate to acetate, H₂ and CO₂. Acetate and H₂/CO₂ are the substrates that are finally converted to CH₄. This conversion is catalyzed by various methanogenic archaea belonging to the orders Methanobacteriales, Methanomicrobiales, Methanosarcinales and the so far uncultured methanogenic Rice Cluster I. The latter group was found to be prevalent in rice field soils but also in other environments, especially at elevated temperatures. At 50°C, for example, Rice cluster I was the almost exclusive methanogen surviving in Italian rice field soil, which then produced CH₄ exclusively from H₂/CO₂. At other temperatures, however, acetate was the more important methanogenic substrate consumed by Methanosarcina sp. or Methanosaeta sp.. These two acetoclastic methanogenic genera were also found in other aquatic environments. As a rule, however, Methanosaeta versus Methanosarcina seemed to be dominant when acetate concentrations were low and vice versa. This as well as other examples show that particular ecosystem functions demand a particular methanogenic community.