Anaerobic respiration


Anaerobic respiration is respiration using electron acceptors other than molecular oxygen. Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.
In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Molecular oxygen is a high-energy oxidizing agent and, therefore, is an excellent electron acceptor. In anaerobes, other less-oxidizing substances such as nitrate, fumarate, sulphate, or sulphur are used. These terminal electron acceptors have smaller reduction potentials than O2, meaning that less energy is released per oxidized molecule. Therefore, anaerobic respiration is less efficient than aerobic.

As compared with fermentation

Anaerobic cellular respiration and fermentation generate ATP in very different ways, and the terms should not be treated as synonyms. Cellular respiration utilizes highly reduced chemical compounds such as NADH and FADH2 to establish an electrochemical gradient across a membrane. This results in an electrical potential or ion concentration difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials, with the final electron acceptor being oxygen or another chemical substance. A proton motive force drives protons down the gradient 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 to regenerate NAD+.
There are two important anaerobic microbial methane formation pathways, through carbon dioxide / bicarbonate reduction or acetate fermentation.

Ecological importance

Anaerobic respiration is a critical component of the global nitrogen, iron, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. The biogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts the carbon cycle and global warming. Anaerobic respiration occurs in many environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface environments, and biofilms. Even environments, such as soil, that contain oxygen also have micro-environments that lack oxygen due to the slow diffusion characteristics of oxygen gas.
An example of the ecological importance of anaerobic respiration is the use of nitrate as a terminal electron acceptor, or dissimilatory denitrification, which is the main route by which fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. Another example is methanogenesis, a form of carbon-dioxide respiration, that is used 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. Sulfate respiration produces hydrogen sulfide, which is responsible for the characteristic 'rotten egg' smell of coastal wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of sulfidic metal ores.

Economic relevance

Dissimilatory 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.
'', which uses nitrogen as the electron acceptor. NO3 goes through respiratory dehydrogenase and reduces through each step from the ubiquinose through the bc1 complex through the ATP synthase protein as well. Each reductase loses oxygen through each step so that the final product of anaerobic respiration is N2.
1. Cytoplasm
2. Periplasm
Compare to the aerobic electron transport chain.
Specific types of anaerobic respiration are also critical in bioremediation, which uses microorganisms to convert toxic chemicals into less-harmful molecules to clean up contaminated beaches, aquifers, lakes, and oceans. For example, toxic arsenate or selenate can be reduced to less toxic compounds by various anaerobic bacteria via anaerobic respiration. The reduction of chlorinated chemical pollutants, such as vinyl chloride and carbon tetrachloride, also occurs through anaerobic respiration.
Anaerobic respiration is useful in generating electricity in microbial fuel cells, which employ bacteria that respire solid electron acceptors to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity.

Examples of electron acceptors in respiration

TypeLifestyleElectron acceptorProductsReduction potential|Eo' Example organisms
aerobic respirationobligate aerobes and facultative anaerobesO2H2O, CO2+ 0.82eukaryotes and aerobic prokaryotes
iron reductionfacultative anaerobes and obligate anaerobesFeFe+ 0.75Organisms within the order Desulfuromonadales and Shewanella species
manganesefacultative anaerobes and obligate anaerobesMnMnDesulfuromonadales and Shewanella species
cobalt reductionfacultative anaerobes and obligate anaerobesCoCoGeobacter sulfurreducens
uranium reductionfacultative anaerobes and obligate anaerobesUUGeobacter metallireducens, Shewanella oneidensis
nitrate reduction facultative anaerobesnitrate NO3 N2+ 0.40Paracoccus denitrificans, Escherichia coli
fumarate respirationfacultative anaerobesfumaratesuccinate+ 0.03Escherichia coli
sulfate respirationobligate anaerobessulfate SO42−sulfide HS- 0.22Many Deltaproteobacteria species in the orders Desulfobacterales, Desulfovibrionales, and Syntrophobacterales
methanogenesis methanogenscarbon dioxide CO2methane CH4- 0.25Methanosarcina barkeri
sulfur respiration facultative anaerobes and obligate anaerobessulfur S0sulfide HS- 0.27Desulfuromonadales
acetogenesis obligate anaerobescarbon dioxide CO2acetate- 0.30Acetobacterium woodii
dehalorespirationfacultative anaerobes and obligate anaerobeshalogenated organic compounds R-XHalide ions and dehalogenated compound X + R-H+ 0.25–+ 0.60Dehalococcoides and Dehalobacter species