Abstract
Flooding and desiccation of soil environments mainly affect the availability of water and oxygen. While water is necessary for all life, oxygen is required for aerobic microorganisms. In the absence of O2, anaerobic processes such as CH4 production prevail. There is a substantial theoretical knowledge of the biogeochemistry and microbiology of processes in the absence of O2. Noteworthy are processes involved in the sequential degradation of organic matter coupled with the sequential reduction of electron acceptors, and, finally, the formation of CH4. These processes follow basic thermodynamic and kinetic principles, but also require the presence of microorganisms as catalysts. Meanwhile, there is a lot of empirical data that combines the observation of process function with the structure of microbial communities. While most of these observations confirmed existing theoretical knowledge, some resulted in new information. One important example was the observation that methanogens, which have been believed to be strictly anaerobic, can tolerate O2 to quite some extent and thus survive desiccation of flooded soil environments amazingly well. Another example is the strong indication of the importance of redox-active soil organic carbon compounds, which may affect the rates and pathways of CH4 production. It is noteworthy that drainage and aeration turns flooded soils, not generally, into sinks for atmospheric CH4, probably due to the peculiarities of the resident methanotrophic bacteria.
Highlights
Environmental methanogenesis is the degradation of organic matter under anaerobic conditions to the gaseous products CH4 and CO2
Microorganisms catalyzing the methanogenic degradation of organic matter are believed to be facultatively or obligately anaerobic, meaning that growth and/or activity of these microorganisms requires the absence of O2 and sometimes even a low (
From a microbiological point of view, it cannot be predicted that all the necessary functional types of microorganisms are present in all the different soil environments. This is true for methanogenic microbial communities, which are expected to consist of anaerobic microorganisms that are sensitive to O2 and, should not survive desiccation events
Summary
Environmental methanogenesis is the degradation of organic matter under anaerobic conditions to the gaseous products CH4 and CO2. Once inorganic oxidants are no longer available, anaerobic degradation of organic matter can only proceed by disproportionation resulting in the production of CO2 (oxidized compound) and CH4 (reduced compound). In soils with the potential for methanogenesis, important microbial functions include hydrolysis of complex organic matter, various fermentation processes, methanogenic processes and processes involved in the sequential reduction of inorganic compounds. Hydrolytic and fermenting bacteria degrade complex organic matter to H2, CO2 and simple organic compounds, which are subsequently further fermented to acetate, H2 and CO2 These compounds are the substrates for methanogenic archaea, which convert them to CH4 and CO2 [3,14,16,17]. Methanogenic archaea in particular are unable to proliferate in the presence of O2, and their activity is strongly inhibited [3] Another important theoretical microbiological background concerns the concentrations of the growth substrates required for energy generation. There are two groups of acetotrophic methanogens (Methanothrix spp. versus Methanosarcina spp.) having different enzyme systems for acetate activation and which dominate at different acetate concentrations in the environment [30,31,32]
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