Anaerobes

Aharon Oren, The Hebrew University of Jerusalem, Jerusalem, Israel

Published online: June 2011

DOI: 10.1002/9780470015902.a0020369.pub2

Full Article on Wiley Online Library

A diverse world of microorganisms inhabits anaerobic environments on Earth, and these obtain their energy by fermentation or by anaerobic respiration, in which electron acceptors other than molecular oxygen drive the oxidation of organic compounds. We also know phototrophic and chemoautotrophic processes that function in the absence of oxygen. Collaboration of different groups of microorganisms can lead to degradation of complex organic matter also under anaerobic conditions, with carbon dioxide or a mixture of carbon dioxide and methane as the main products, depending on the availability of electron acceptors other than molecular oxygen, such as nitrate or sulfate. The different groups of anaerobes are essential for the functioning of the biogeochemical cycles of carbon, nitrogen, sulfur and other elements, and also play important roles in the production of fermented food products, treatment of organic wastes and other biotechnological processes.

Key Concepts:

Life developed on planet Earth in the absence of molecular oxygen (O₂).
The anaerobic way of life is widespread in the prokaryotic world – in Bacteria as well as Archaea.
Permanently anaerobic environments include marine and freshwater lake bottom sediments, digestive systems, the deep subsurface and others.
Some protozoa can live anaerobically.
Recently a community of multicellular meiozoa that lives in permanently anoxic conditions was discovered in L'Atalante basin, 3.5 km below the surface of the Mediterranean Sea.
In the absence of molecular oxygen, many prokaryotes can use alternative electron acceptors, such as nitrate, sulfate, Fe(III) and others, for respiration (‘anaerobic respiration’).
When no suitable electron acceptor is available, energy can be gained by fermentation processes, in which ATP is gained by substrate-level phosphorylation.
Complete anaerobic breakdown of organic matter is a cooperative process in which fermentative microorganisms, bacteria performing anaerobic respiration and methanogenic Archaea cooperate.
Hydrogen and acetate are key intermediates in many anaerobic degradation processes.

Keywords: oxygen; anaerobic; denitrification; sulfate reduction; fermentation; methanogenesis

	Figure 1. Comparison of two modes of anaerobic respiration – denitrification and dissimilatory sulfate reduction – and aerobic respiration. In each case reduced organic substrates are oxidised, and the electrons released are transferred through membrane-bound electron carriers with conservation of energy in the form of ATP. Molecular oxygen, the terminal electron acceptor in aerobic respiration, is replaced by electron acceptors such as nitrate and sulfate in anaerobic respiration processes. Note that most bacteria that perform denitrification are capable of aerobic respiration as well.
	Figure 2. The principle of fermentation, as exemplified by the homolactic fermentation of Lactobacillus and Streptococcus species and by the alcohol fermentation of yeasts. Organic substrates are degraded to simpler compounds without involvement of an external electron acceptor. Part of the energy released is conserved in the form of ATP.
	Figure 3. Different scenarios of degradation of complex organic material: in the presence of external electron acceptors such as sulfate and possibly nitrate (left panel), complete breakdown to CO₂+CH₄ in the absence of such electron acceptors (middle panel), and partial breakdown occurs in the rumen of ruminant animals, with production of the organic acids acetate, propionate and butyrate which are used by the animal, in addition to minor amounts of carbon dioxide and CH₄. In all cases, the degradation is effected by combination of fermentative processes and other types of metabolism, including anaerobic respiration, methanogenesis and syntrophy. The schemes show the principal steps in the degradation processes only.
	Figure 4. An example of syntrophic growth in which two bacteria cooperate to degrade a substrate. The conversion of ethanol into acetate and molecular hydrogen by the ‘S-organism’ is energetically favourable only when methanogenic Archaea in close physical contact maintain the concentration of hydrogen at very low levels by interspecies hydrogen transfer.

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Further Reading
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	book Zinder SH and Dworkin M (2006) "Morphological and physiological diversity". In: Dworkin M, Falkow S, Rosenberg E et al. (eds) The Prokaryotes. A Handbook on the Biology of Bacteria: Ecophysiology and Biochemistry, vol. 1, pp. 185–220. New York: Springer.

Article Title:	Anaerobes
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