Inaerobic organisms, electrons are shuttled to an electron transport chain, and the final electron acceptor isoxygen. Molecular oxygen is an excellent electron acceptor.Anaerobes instead use less-oxidizing substances such asnitrate (NO− 3),fumarate (C 4H 2O2− 4),sulfate (SO2− 4), or elementalsulfur (S). These terminal electron acceptors have smallerreduction potentials than O2. Less energy per oxidized molecule is released. Therefore, anaerobic respiration is less efficient than aerobic.[citation needed]
Anaerobic cellular respiration andfermentation generateATP in very different ways, and the terms should not be treated as synonyms. Cellular respiration (bothaerobic and anaerobic) uses highly reduced chemical compounds such asNADH andFADH2 (for example produced duringglycolysis and thecitric acid cycle) to establish anelectrochemical gradient (often a proton gradient) across a membrane. This results in anelectrical potential or ionconcentration difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratoryintegral membrane proteins with sequentially increasingreduction potentials, with the final electron acceptor being oxygen (inaerobic respiration) or another chemical substance (in anaerobic respiration). Aproton motive force drivesprotons down the gradient (across the membrane) through the proton channel ofATP synthase. The resulting current drives ATP synthesis fromADP and inorganic phosphate.[citation needed]
Fermentation, in contrast, does not use an electrochemical gradient but instead uses onlysubstrate-level phosphorylation to produce ATP. The electron acceptorNAD+ is regenerated fromNADH 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 ofglyceraldehyde-3-phosphate is oxidized back to NAD+ by the reduction ofpyruvate tolactic acid at a later stage in the pathway. Inyeast,acetaldehyde is reduced toethanol to regenerate NAD+.[citation needed]
There are two important anaerobic microbial methane formation pathways, throughcarbon dioxide /bicarbonate (HCO− 3) reduction (respiration) or acetate fermentation.[2]
Anaerobic respiration is a critical component of the globalnitrogen,iron,sulfur, andcarbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. Thebiogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts thecarbon cycle andglobal 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 ofoxygen gas.[citation needed]
An example of the ecological importance of anaerobic respiration is the use of nitrate as aterminal electron acceptor, or dissimilatorydenitrification, which is the main route by which fixednitrogen 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 ismethanogenesis, a form of carbon-dioxide respiration, that is used to producemethane gas byanaerobic digestion. Biogenic methane can be a sustainable alternative to fossil fuels. However, uncontrolled methanogenesis in landfill sites releases large amounts of methane into the atmosphere, acting as a potentgreenhouse gas.[5]Sulfate respiration produceshydrogen 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 ofsulfidic metal ores.[6]
The model above shows the process of anaerobic respiration throughdenitrification, which uses nitrogen (in the form of nitrate,NO− 3) as the electron acceptor.NO− 3 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 removes oxygen step by step so that the final product of anaerobic respiration is N2.
Dissimilatorydenitrification is widely used in the removal ofnitrate andnitrite from municipal wastewater. An excess of nitrate can lead toeutrophication 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 inbioremediation, which uses microorganisms to convert toxic chemicals into less-harmful molecules to clean up contaminated beaches, aquifers, lakes, and oceans. For example, toxicarsenate orselenate can be reduced to less toxic compounds by various anaerobic bacteria via anaerobic respiration. The reduction ofchlorinated chemical pollutants, such asvinyl chloride andcarbon tetrachloride, also occurs through anaerobic respiration.[citation needed][8]
Anaerobic respiration is useful in generating electricity inmicrobial 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.
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Orange nodes:carbohydrate metabolism.
Violet nodes:photosynthesis.
Red nodes:cellular respiration.
Pink nodes:cell signaling.
Blue nodes:amino acid metabolism.
Grey nodes:vitamin andcofactor metabolism.
Brown nodes:nucleotide andprotein metabolism.
Green nodes:lipid metabolism.