Carbon dioxide methanogenesis from

The reactions catalyzed by B12 may be grouped into two classes those catalyzed by methylcobalamin and those catalyzed by cofactor B,2. The former reactions include formation of methionine from homocysteine, methanogenesis (formation of methylmercury is an important side reaction), and synthesis of acetate from carbon dioxide (82). The latter reactions include the ribonucleotide reductase reaction and a variety of isomerization reactions (82). Since dehydration and deamination have been studied quite extensively and very possibly proceed via [Pg.257]

The excreted end-products of fermentations, including hydrogen gas, can be used by other anaerobic bacteria, and what is known as an anaerobic food chain develops. At the bottom of the food chain are the archaeal methanogens, different species of which convert hydrogen gas, carbon dioxide, or acetate to CH4. The process of methanogenesis from H2 and C02 occurs as follows ... 

In contrast to Sec-tRNA and fMet-tRNA formation, Pyl-tRNA y formation can occur by direct acylation of pyrrolysine onto tRNA i Pyrrolysine (Pyl), the twenty-second amino acid, was discovered incorporated in methylamine methyltrans-ferases from Methanosarcinaceae, a branch of methanogenic archaea that has the ability to reduce a wide variety of compounds to methane including carbon dioxide, acetate, methanol, methylated thiols, and methylated amines (72). Methanogenesis... 

Methanogenesis from methanol occurs by its disproportionation to methane and carbon dioxide (Reactions (5) and (5A), Table 2). Reducing equivalents derived from the oxidation of one methanol to CO2 are used to reduce three methanols to CH4. 

Lovley D. R. and Klug M. J. (1983a) Methanogenesis from methanol and methylamines and acetogenesis from hydrogen and carbon dioxide in the sediments of a eutrophic lake. Appl. Environ. Microbiol. 45, 1310-1315. 

Finally, after all of the sulfate is consumed, organic material can be fermented into methane and carbon dioxide in the process of methanogenesis. Fermentative reactions, of which there are many examples in the environment, differ from respiration in that they do not change the average oxidation state of carbon. The reaction for methanogenesis is... 

There is another, very important and large repository of methane methane hydrates (also known as gas hydrates or clathrates Kvenvolden 1988).They comprise ice in which the interstices of the lattice house small molecules, such as methane, ethane, carbon dioxide and hydrogen sulphide. In fact, enough gas needs to be present to fill 90% of the interstices in order for the hydrate to form, and it has a different crystal structure from normal ice (Sloan 1990). If fully saturated, the most common crystalline structure can hold one molecule of methane for every 5.75 molecules of water, so lm3 of hydrate can contain 164 m3 of methane at STP (see Box 4.8).The solubility of methane in water is insufficient to account for hydrate formation, and a major nearby source is required, typically methanogenesis, based on the dominance of methane (99%) and its very light isotopic composition (813C generally <—60%o see Section 5.8.2). 

Methanogenic bacteria utilise the end products from the first phase and yield methane and carbon dioxide. This is also known as methanogenesis... 

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