Methanogens Often Grow In Association With

Methanogens are found in anaerobic environments where hydrogen gas and carbon dioxide are available. Because these gases are generated by chemoorganotrophs during the fermentation of organic material, methanogens often grow in association with these microorganisms. Methanogens, however, are generally not found in environments that contain high levels of sulfate, nitrate, or other inorganic electron acceptors. This is because microorganisms that oxidize hydrogen gas using these electron acceptors have a competitive advantage; the use of CO2 as an electron acceptor releases comparatively little energy (see figure 6.25). Environments from which methanogens are commonly isolated include sewage, swamps, marine sediments, rice paddies, and the digestive tracts of humans and other animals. The methane produced can present itself as bubbles rising in swamp waters or the 10 cubic feet of gas discharged from a cow's digestive system each day. As a by-product of sewage treatment plants, methane gas can be collected and used for heating, cooking, and even the generation of electricity (see Perspective 31.1).

The study of methanogens is quite challenging because they are exquisitely sensitive to O2, as are many of their enzymes. Special techniques, including anaerobe jars and anaerobic chambers, are used for their cultivation. ■ culturing anaerobes, p. 95

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