Info

(0.42-0.84) 0.47Í0.29 '0.54 (0-0.68) 1.04±0.18 '1.35 0.69Í0.43 '0.99

Mha-Million hectares. Tg = 1012 g

16.4 Mechanism of Methane Formation

Methanogenesis is the microbial process for formation of CH4, as the terminal product of anaerobic breakdown of organic matter. Methane is exclusively produced by methanogenic bacteria that can metabolize only in the strict absence of free oxygen and at redox potential of less than -150mV (Wang et al. 1993). In contrast to this concept, Keppler et al. (2006) reported a startling finding that CH4 could be also produced by the living plants in aerobic conditions without the action of bacteria. This was proven by the laboratory experiments with 30 different kinds of tree leaves and grasses, but they could not explain the processes involved in it. This finding could explain many previous puzzling observations made by a group of physicists of Germany and Paul J. Crutzen, a 1995 Nobel Prize winner and his colleagues about the presence of methane clouds over the tropical forests and Venezuelan savanna, respectively. However, their observations have been questioned by many workers who still believe that CH4 is the terminal product of anaerobic digestion of organic matter in soils by methanogenic bacteria.

The methanogens are best at work at a pH of 6-8 and a temperature of 30-40°C. These bacteria produce methane (i) by transmethylation or decarboxylation of acetic acid, and (ii) by the reduction of carbon dioxide (Takai 1970). Before methanogens act, acidogens and acetogens break down lipids, proteins, polysaccharides, sugars and amino acids of the organic residues by their extra cellular enzymes in the sediments (Table 16.4).

Table 16.4 Bacteria involved at different stages of CH4 production

Group

Bacteria

Extra-cellular enzymes

Conversion

Acidogens

Clostridia,

Lipases

Lipids to long-chain fatty acids-further

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