Published online by Cambridge University Press: 05 September 2012
In the previous chapter, respiration was defined as an energy conservation process achieved through electron transport phosphorylation (ETP) using externally supplied electron acceptors. Electron acceptors used in anaerobic respiration include oxidized sulfur and nitrogen compounds, metal ions, organic halogens and carbon dioxide. Other oxidized compounds reduced under anaerobic conditions include iodate, (per)chlorate, and phosphate. There is evidence to suggest that these compounds are used as electron acceptors in anaerobic ecosystems but there are some exceptions. ATP synthesis mechanisms dependent on a proton motive force are known in some fermentative bacteria. These include Na+-dependent decarboxylation, fumarate reduction and product/proton symport, as described earlier (Section 5.8.6). Sulfidogenesis and methanogenesis are described as fermentations in some cases since a small amount of energy is conserved in these anaerobic processes. However, in these processes ATP is generated mainly through the proton motive force and they can therefore be classified as anaerobic respiration.
Many ecosystems become anaerobic when oxygen consumption is greater than its supply. Even under anaerobic conditions, natural organic compounds are continuously recycled. Anaerobic respiratory microbes convert organic materials to carbon dioxide and methane under anaerobic conditions in conjunction with fermentative microbes.
Energy is required for all forms of life. At any given conditions, those organisms utilizing energy sources more efficiently will become dominant over the others. Among the anaerobic respiratory prokaryotes, denitrifiers conserve more energy than other groups. For this reason sulfidogenesis and methanogenesis are inhibited in the presence of nitrate, and sulfate inhibits methanogenesis.
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