Anaerobic respiration

Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen. In order for the electron transport chain to function, an exogenous final electron acceptor must be present to allow electrons to pass through the system. In aerobic organisms, this final electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and, therefore, is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulfate (SO₄^2-), nitrate (NO₃^-), sulfur (S), or fumarate are used. These terminal electron acceptors have smaller reduction potentials than O₂, meaning that less energy is released per oxidized molecule. Anaerobic respiration is, therefore, in general energetically less efficient than aerobic respiration.^{[citation needed]}

Anaerobic respiration is used mainly by prokaryotes that live in environments devoid of oxygen. Many anaerobic organisms are obligate anaerobes, meaning that they can respire only using anaerobic compounds and will die in the presence of oxygen.

Anaerobic respiration as compared to fermentation

Cellular respiration (both aerobic and anaerobic) utilizes highly reduced species such as NADH and FADH2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane, resulting in an electrical potential or ion concentration difference across the membrane. The reduced species are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials with the final electron acceptor being oxygen (in aerobic respiration) or another species (in anaerobic respiration). The membrane in question is the inner mitochondrial membrane in eukaryotes and the cell membrane in prokaryotes. A proton motive force or pmf drives protons down the gradient (across the membrane) through the proton channel of ATP synthase. The resulting current drives ATP synthesis from ADP and inorganic phosphate.

Fermentation in contrast, does not utilize an electrochemical gradient. Fermentation instead only uses substrate-level phosphorylation to produce ATP. The electron acceptor NAD+ is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3-phosphate is oxidized back to NAD+ by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast, acetaldehyde is reduced to ethanol.

Ecological importance

Anaerobic respiration plays a major role in the global nitrogen, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. Dissimilatory denitrification is the main route by which biologically fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. Hydrogen sulfide, a product of sulfate respiration, is a potent neurotoxin and responsible for the characteristic 'rotten egg' smell of brackish swamps. Along with volcanic hydrogen sulfide, biogenic sulfide has the capacity to precipitiate heavy metal ions from solution, leading to the deposition of sulfidic metal ores.

Economic relevance

Dissimiltory denitrification is widely used in the removal of nitrate and nitrite from municipal wastewater. An excess of nitrate can lead to eutrophication of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas.

Methanogenesis is a form of carbonate respiration that is exploited to produce methane gas by anaerobic digestion. Biogenic methane is used as a sustainable alternative to fossil fuels. On the negative side, uncontrolled methanogenesis in landfill sites releases large volumes of methane into the atmosphere, where it acts as a powerful greenhouse gas.

Specific types of anaerobic respiration are also used to convert toxic chemicals into less-harmful molecules. For example, toxic arsenate or selenate can be reduced to less toxic compounds by various bacteria.

Examples of respiration

examples of respiration types

type	lifestyle	electron acceptor	products	Eo' [V]	example organisms
aerobic respiration	obligate and facultative aerobes	oxygen O2	H2O + CO2	+ 0.82	eukaryotes and aerobic prokaryotes
iron reduction	facultative aerobes, obligate anaerobes	ferric iron Fe(III)	Fe(II)	+ 0.75	Geobacter, Geothermobacter, Geopsychrobacter, Pelobacter carbinolicus, P. acetylenicus, P. venetianus, Desulfuromonadales, Desulfovibrio
manganese	facultative or obligate anaerobes	Mn(IV)	Mn(II)		Desulfuromonadales, Desulfovibrio
cobalt reduction	facultative or obligate anaerobes	Co(III)	Co(II)		Geobacter sulfurreducens
uranium reduction	facultative or obligate anaerobes	U(VI)	U(IV)		Geobacter metallireducens, Shewanella putrefaciens, (Desulfovibrio)
nitrate reduction (denitrification)	facultative aerobes	nitrate NO3−	nitrite NO2–	+ 0.40	Paracoccus denitrificans, E. coli
fumarate respiration	facultative aerobes	fumarate	succinate	+ 0.03	Escherichia coli
sulfate respiration	obligate anaerobes	sulfate SO42−	sulfide HS−	- 0.22	Desulfobacter latus, Desulfovibrio' oxygen
methanogenesis (carbonate reduction)	methanogens	carbon dioxide CO2	methane CH4	- 0.25	Methanothrix thermophila
sulfur respiration (sulfur reduction)	facultative aerobes and obligate anaerobes	sulfur S0	sulfide HS−	- 0.27	Desulfuromonadales
acetogenesis (carbonate reduction)	acetogens	carbon dioxide CO2	acetate	- 0.30	Acetobacterium woodii
Dehalorespiration	facultative or obligate anaerobes	halogenated organic compoundss R-X	Halide ions and dehalogenated compound X- + R-H	+ 0.25–+ 0.60	Trichlorobacter (Geobacteraceae)

See also

• Hydrogenosomes and mitosomes

• Standard electrode potential (data page)

• Table of standard reduction potentials for half-reactions important in biochemistry

Bibliography

• Lawrence, Eleanor; Holmes, Sandra (1989), Henderson's dictionary of biological terms (10th ed.), University of Michigan: Wiley, ISBN 978-0-470-21446-6