The methanogens are known to possess new coenzymes and pathway which use hydrogen to reduce carbon dioxide or in some cases acetate to produce methane. Decomposition of cellulosic plant residues in swamps, bogs and digestive tracts of animals is brought about in nature by methanogenic bacteria. The main metabolic reaction is the reduction of carbon dioxide (CO2 ) with hydrogen (H2 ) to methane (CH4). The CO2 and H2 are produced in fermentation reactions. The methane and residual CO2 are released as biogas also called marsh gas.

However, because of narrow substrate range of methanogens. They depend on anaerobic cellulose decomposers to form glucose and other fermentable carbohydrates and on mixed acid fermenters to make a range of short-chain fatty acids. Subsequently these are fermented further to give H2 and CO2 and other products by bacteria very closely syntrophic with methanogens, taking part in the phenomenon called syntrophism, in which extent of growth of an organism depends on the preparation of one or more metabolic product or nutrients given out by the organism growing in the immediate surrounding or viscinity. Archaeal populations know for methane production have fascinating synergistic relationships with bacterial and other microbial populations, e.g. Syntrophomonas species oxiside bytyrice acid caproic acid to acetate and H2 . Syntrophomonas also oxidise valeric acid and enanthic acid to acetate CO2 and H2 (hydrogen). On the other hand Syntrophobacter can oxidise propionic acid to acetate, CO2 and H2 . The acetate and H2 produced by these bacteria are utilized by methanogenic archaea to produce methane or the biogas.

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