Methanogens 243

Methanogens (methane-producing bacteria) oxidize hydrogen gas (H2) with carbon dioxide (C02) to produce methane (CH4) (Fig. 1.15). Some methanogens utilize 

Recently, Pseudomonas sp. HD-1 has been found, which grows on carbon dioxide and hydrogen gas and decomposes anaerobically n-alkane, benzene, and toluene (Morikawa and Imanaka, 1993). 

The energy source of the archaebacterium Methanobacterium thermoautot-rophicum is hydrogen and is supplied by the gaseous phase, as well as the 

Species Carbon (C-mmol) Available electrons (mmol) Enthalpy (kJ) 

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Methanogen (Section 2 5) An organism that produces methane... 

Methane has also been used in aerobic bioreactors that are part of a pump-and-treat operation, and toluene and phenol have also been used as co-substrates at the pilot scale (29). Anaerobic reactors have also been developed for treating trichloroethylene. Eor example, Wu and co-workers (30) have developed a successful upflow anaerobic methanogenic bioreactor that converts trichloroethylene and several other halogenated compounds to ethylene. 

Carbamates such as Aldicarb undergo degradation under both aerobic and anaerobic conditions. Indeed the oxidation of the sulfur moiety to the sulfoxide and sulfone is part of the activation of the compound to its most potent form. Subsequent aerobic metaboHsm can completely mineralize the compound, although this process is usually relatively slow so that it is an effective iasecticide, acaricide and nematocide. Anaerobically these compounds are hydrolyzed, and then mineralized by methanogens (61). 

Environmental effects, including effect on methanogenic and nitrifying bacteria, persistence in the environment, and projections of possible HabiUty to relevant ecosystems. 

Hackstein et al ° have suggested that there may be a phylogenetic basis for the occurrence of significant methane formation in the gut of animals. It was argued that this refiected the presence of a methanogen receptor in the gut of animals which support large populations of these bacteria, since the production of... 

Figure 1. Methanogenic decomposition of compounds in nature (Adapted from ref. 46)...

Figure 3. Reactions of methanogenic bacteria. (Adapted from ret 47)...

In relation to PVC additives, the study concluded that heavy metal additives are more likely to be released under acidogenic conditions (compared with plasticizers, for example, which are released mainly during the anaerobic and methanogenic phases of landfill development). With respect to landfill emissions, it was concluded that organotin compormds carmot be directly attributed to the presence of PVC in landfills. 

C. thermoaceticum contains a gene cluster encoding at least five genes in the Wood-Ljungdahl pathway (130). These genes are apparently not regulated by CO. The CODH activity does increase about fourfold when methanogenic cells are exposed to CO (131). There is... 

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