Anaerobic respiration is
respiration using
electron acceptors other than
molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.[1]
In
aerobic organisms undergoing respiration, electrons are shuttled to an
electron transport chain, and the final electron acceptor is
oxygen. Molecular oxygen is an excellent electron acceptor.
Anaerobes instead use less-oxidizing substances such as
nitrate (NO− 3),
fumarate (C 4H 2O2− 4),
sulfate (SO2− 4), or elemental
sulfur (S). These terminal electron acceptors have smaller
reduction potentials than O2. Less energy per oxidized molecule is released. 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 (both
aerobic and anaerobic) uses highly reduced chemical compounds 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. 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 (in
aerobic respiration) or another chemical substance (in anaerobic respiration). A
proton motive force 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.[citation needed]
Fermentation, in contrast, does not use an electrochemical gradient but instead uses only
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+.[citation needed]
There are two important anaerobic microbial methane formation pathways, through
carbon dioxide /
bicarbonate (HCO− 3) reduction (respiration) or acetate fermentation.[2]
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 that contain oxygen, such as soil, have micro-environments that lack oxygen due to the slow diffusion characteristics of
oxygen gas.[citation needed]
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.[3] The denitrification process is also very important in host-microbe interactions. Like mitochondria in oxygen-respiring microorganisms, some single-cellular anaerobic ciliates use denitrifying endosymbionts to gain energy.[4] 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, however, uncontrolled methanogenesis in landfill sites releases large volumes of methane into the atmosphere, where it acts as a powerful
greenhouse gas.[5]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.[6]
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.[7]
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.[citation needed][8]
Anaerobic respiration is useful in generating electricity in
microbial fuel cells, which employ bacteria that respire solid electron acceptors (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity.[9]
^Bogner, Jean; Pipatti, Riitta; Hashimoto, Seiji; Diaz, Cristobal; Mareckova, Katarina; Diaz, Luis; Kjeldsen, Peter; Monni, Suvi; Faaij, Andre (2008-02-01). "Mitigation of global greenhouse gas emissions from waste: conclusions and strategies from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. Working Group III (Mitigation)". Waste Management & Research. 26 (1): 11–32.
Bibcode:
2008WMR....26...11B.
doi:
10.1177/0734242x07088433.
ISSN0734-242X.
PMID18338699.
S2CID29740189.