By Ralf Conrad
Director of the Department of Biogeochemistry
Max Planck Institute for Terrestrial Microbiology
13 November 2014, Thursday
Methane is an important greenhouse gas that is affected by anthropogenic activity. Seventy-five percent of the annual budget of atmospheric methane, about 600 million tons, is produced by methanogenic archaea. These archaea are the end-members of a microbial community that degrade organic matter under anaerobic conditions. Flooded rice fields constitute a major source (about 10%) of atmospheric methane. After soil flooding, anaerobic processes are initiated, finally resulting in the disproportionation of organic matter to carbon dioxide and methane. This process occurs in the bulk soil, on decaying organic debris, and in the rhizosphere. The methane produced is mostly ventilated through the plant vascular system into the atmosphere. This system also allows the diffusion of oxygen into the rhizosphere, where part of the produced methane is oxidized by aerobic methanotrophic bacteria. More than 50% of the methane produced is derived from plant photosynthetic products and is formed on the root surface. Methanocellales are an important group of methanogenic archaea colonizing rice roots. Soils lacking this group seem to result in reduced root colonization and methane production. In rice, soil methane is produced by two major paths of methanogenesis: the hydrogenotrophic one, which reduces carbon dioxide to methane, and the aceticlastic one, which disproportionates acetate to methane and carbon dioxide. Theoretically, at least two-thirds of the methane should be produced by aceticlastic, and the rest by hydrogenotrophic, methanogenesis. In nature, however, the exact contribution of the two paths can vary from 0 to 100%. Several environmental factors, such as temperature and quality of organic matter, affect the path of methane production. The impact of these factors on the composition and activity of the environmental methanogenic microbial community will be discussed.