A competing process with methanogenesis in anaerobic environments is acetogenesis. Acetogens are obligate anaerobic bacteria that (1) use chemolithoautotrophic substrates (H2/CO2, CO/CO2, or H2/CO) as the sole sources of carbon and energy or (2) make a living by converting certain sugars or aromatic compounds to acetate. This heterogeneous group of bacteria was initially characterized based on its ability to produce acetic acid as the sole metabolic end-product from a variety of substrates. Later, investigations by many researchers led to a more specific biochemical definition of acetogens: bacteria that generate ATP and the acetic acid end product via the carbon monoxide dehydrogenase (CODH) pathway (often referred to as the acetyl-CoA Wood-Ljungdahl pathway; named for its discoverers). While the biochemistry of this pathway was being elucidated, the ecological importance of this form of autotrophic carbon assimilation began to be appreciated.
The study of acetogenesis is important because of the role that acetogens play in carbon cycling in many anaerobic communities. Acetate is a trophically important metabolite for microbial communities in many ecosystems. The flux of organic carbon into the acetate pool and its metabolic turnover can be enormous in some natural habitats. Approximately 1013 kg of acetate are formed and metabolized annually in anaerobic habitats globally. Of this amount, 10% is derived from CO2 fixation through the Wood-Ljungdahl pathway, and 1.22 x 1012 kg of acetate per year is estimated to be produced microbiologically in the hindgut of termites alone. This mass of acetate greatly exceeds the amount of methane produced annually through microbial reduction of CO2. Acetogenesis also occurs in the human gastrointestinal tract. Reports have estimated that 90% of the carbohydrates ingested by the human population are processed anaerobically through the homoacetate fermentation biochemical pathway. Approximately 1.25 x 10 kg of acetate per year is made from H2/CO2. Clearly, the impact of acetogenesis and the Wood-Ljungdahl biochemical pathway is of great importance to the global carbon cycle as well as to the understanding of how different bacteriological groups interact to influence ecosystem processes.
Although factors that regulate and control the relative importance of methanogenesis and acetogenesis in anaerobic habitats are not fully understood, based on simple energetic consideration, methanogenesis from H2 is a more favorable process than acetogenesis (— 136kJmol—1 vs. — 105 kJmol—1, respectively). However, in certain environments (e.g., termite gut; microaerophilic zones), acetogens can compete with methanogens by positioning themselves closer to the H2 source or by supplementing their nutrition with fixed organic compounds. Regardless of which is the dominant process, the metabolic activities of both methano-gens and acetogens are clearly very important parts of the global carbon cycling in virtually all ecosystems.
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