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.
In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and, therefore, is an excellent electron acceptor. In anaerobes, other less-oxidizing substances such as sulphate (SO42−), nitrate (NO3−), sulphur (S), or fumarate are used. These terminal electron acceptors have smaller reduction potentials than O2, meaning that less energy is released per oxidized molecule. Therefore, generally speaking, anaerobic respiration is less efficient than aerobic.
There are two important microbial methane formation pathways, through carbonate reduction (respiration), and acetate fermentation.
Cellular respiration (both aerobic and anaerobic) utilizes 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, resulting 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.
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+. The two processes thus generate ATP in very different ways, and the terms should not be treated as synonyms.
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 carbonate 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 coast wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of sulfidic metal ores.
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.
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 (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity.
|Type||Lifestyle||Electron acceptor||Products||Eo' [V]||Example organisms|
|aerobic respiration||obligate aerobes and facultative anaerobes||O2||H2O, CO2||+ 0.82||eukaryotes and aerobic prokaryotes|
|iron reduction||facultative anaerobes and obligate anaerobes||Fe(III)||Fe(II)||+ 0.75||Organisms within the order Desulfuromonadales (such as Geobacter, Geothermobacter, Geopsychrobacter, Pelobacter) and Shewanella species |
|manganese||facultative anaerobes and obligate anaerobes||Mn(IV)||Mn(II)||Desulfuromonadales and Shewanella species |
|cobalt reduction||facultative anaerobes and obligate anaerobes||Co(III)||Co(II)||Geobacter sulfurreducens|
|uranium reduction||facultative anaerobes and obligate anaerobes||U(VI)||U(IV)||Geobacter metallireducens, Shewanella oneidensis|
|nitrate reduction (denitrification)||facultative anaerobes||nitrate NO3−||nitrite NO2−||+ 0.40||Paracoccus denitrificans, Escherichia coli|
|fumarate respiration||facultative anaerobes||fumarate||succinate||+ 0.03||Escherichia coli|
|sulfate respiration||obligate anaerobes||sulfate SO42−||sulfide HS−||- 0.22||Many Deltaproteobacteria species in the orders Desulfobacterales, Desulfovibrionales, and Syntrophobacterales|
|methanogenesis (carbonate reduction)||methanogens||carbon dioxide CO2||methane CH4||- 0.25||Methanosarcina barkeri|
|sulfur respiration (sulfur reduction)||facultative anaerobes and obligate anaerobes||sulfur S0||sulfide HS−||- 0.27||Desulfuromonadales|
|acetogenesis (carbonate reduction)||obligate anaerobes||carbon dioxide CO2||acetate||- 0.30||Acetobacterium woodii|
|dehalorespiration||facultative anaerobes and obligate anaerobes||halogenated organic compounds R-X||Halide ions and dehalogenated compound X− + R-H||+ 0.25–+ 0.60||Dehalococcoides and Dehalobacter species|
An acetogen is a microorganism that generates acetate (CH3COO−) as an end product of anaerobic respiration or fermentation. However, this term is usually employed in a more narrow sense only to those bacteria and archaea that perform anaerobic respiration and carbon fixation simultaneously through the reductive acetyl coenzyme A (acetyl-CoA) pathway (also known as the Wood-Ljungdahl pathway). These genuine acetogens are also known as "homoacetogens" and they can produce acetyl-CoA (and from that, in most cases, acetate as the end product) from two molecules of carbon dioxide (CO2) and four molecules of molecular hydrogen (H2). This process is known as acetogenesis, and is different from acetate fermentation, although both occur in the absence of molecular oxygen (O2) and produce acetate. Although previously thought that only bacteria are acetogens, some archaea can be considered to be acetogens.Acetogens are found in a variety of habitats, generally those that are anaerobic (lack oxygen). Acetogens can use a variety of compounds as sources of energy and carbon; the best studied form of acetogenic metabolism involves the use of carbon dioxide as a carbon source and hydrogen as an energy source. Carbon dioxide reduction is carried out by the key enzyme acetyl-CoA synthase. Together with methane-forming archaea, acetogens constitute the last limbs in the anaerobic food web that leads to the production of methane from polymers in the absence of oxygen. Acetogens may represent ancestors of the first bioenergetically active cells in evolution.Alcaligenes
Alcaligenes is a genus of Gram-negative, aerobic, rod-shaped bacteria. The species are motile with amphitrichous flagella and rarely nonmotile. It is a genus of nonfermenting bacteria (in the family Alcaligenaceae). Additionally, some strains of Alcaligenes are capable of anaerobic respiration, but they must be in the presence of nitrate or nitrite; otherwise, their metabolism is respiratory and never fermentative; The genus does not use carbohydrates. Strains of Alcaligenes (such as A. faecalis) are found mostly in the intestinal tracts of vertebrates, decaying materials, dairy products, water, and soil; they can be isolated from human respiratory and gastrointestinal tracts and wounds in hospitalized patients with compromised immune systems. They are occasionally the cause of opportunistic infections, including nosocomial sepsis.Alcaligenes faecalis causes nosocomial sepsis, arising from contaminated hemodialysis or intravenous fluid, in immunocompromised patients.Alcaligenes species have been used for the industrial production of nonstandard amino acids; A. eutrophus also produces the biopolymer polyhydroxybutyrate.
They are rods, coccal rods, or cocci, sized at about 0.5-1.0 x 0.5-2.6 μm. They are obligately aerobic, but some can undergo anaerobic respiration if nitrate is present. They tend to be colorless. They typically occur in the soil and water, and some live in the intestinal tracts of vertebrates. Samples from blood, urine, feces, discharge from ears, spinal fluid, and wounds have produced this type of bacteria. Zoonotic infections from ferrets have been recorded.Alcaligenes species have been increasingly recovered over the past decade from patients with cystic fibrosis (CF). An experiment recently examined the frequency of correct identification of Alcaligenes species by microbiology labs affiliated with American CF patient care centers. Most (89%) strains of microbial agents in these tests were correctly identified by the referring laboratories as Alcaligenes xylosoxidans.A. faecalis was isolated in 1896 by Petruschky from stale beer. Several strains of the organism have been found since then. This species is motile, flagellated, slender, slightly curved, not spore-forming, slowly growing, nonfermenting, capsule forming, Gram-negative aerobe of the family Alcaligenaceae. This species is most commonly found in the alimentary tract as a harmless saprophyte in 5% – 19% of the normal population. Systemic infection with this organism is very uncommon. It has been reported to cause sepsis, meningitis, peritonitis, enteric fever, appendicitis, cystitis, chronic suppurative otitis media, abscesses, arthritis, pneumonitis, and endocarditis. It has been associated with fatal outcomes because these organisms are resistant to commonly used antibiotics.Anaerobic
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
Anaerobic adhesive, a bonding agent that does not cure in the presence of air
Anaerobic clarigester, an anaerobic digester that treats dilute biodegradable feedstocks and allows different retention times for solids and liquids
Anaerobic contact process, an anaerobic digester with a set of reactors in series
Anaerobic digestion, the use of anaerobic bacteria to break down waste, with biogas as a valuable byproduct
Hypoxia (environmental) (anaerobic environment), an environment with little or no available oxygen
Anaerobic exercise, exercise intense enough to cause lactate to form, used in non-endurance sports
Anaerobic filter, an anaerobic digester with a tank containing a filter medium where anaerobic microbes can establish themselves
Anaerobic lagoon, used to dispose of animal waste, particularly that of cows and pigs
Anaerobic organism, any organism whose redox metabolism does not depend on free oxygen
Anaerobic respiration, respiration in the absence of oxygen, using some other molecule as the final electron acceptor
Anammox, anaerobic ammonium oxidation, a globally important microbial process of the nitrogen cycleCapnophile
Capnophiles are microorganisms that thrive in the presence of high concentrations of carbon dioxide (CO2).
Some capnophiles may have a metabolic requirement for carbon dioxide, while others merely compete more successfully for resources under these conditions. The term is a generally descriptive one and has less relevance as a means of establishing a taxonomic or evolutionary relationship among organisms with this characteristic.For example, the ability of capnophiles to tolerate (or utilize) the amount of oxygen that is also in their environment may vary widely and may be far more critical to their survival. Species of Campylobacter are bacterial capnophiles that are more easily identified because they are also microaerophiles, organisms that can grow in high carbon dioxide as long as a small amount of free oxygen is present, but at a dramatically reduced concentration. (In the earth's atmosphere carbon dioxide levels are approximately five hundred times lower than that of oxygen, 0.04% and 21% of the total, respectively.) Obligate anaerobes are microbes that will die in the presence of oxygen without respect to the concentration of carbon dioxide in their environment, and typically acquire energy through anaerobic respiration or fermentation.
In 2004, a capnophilic bacterium was characterized that appears to require carbon dioxide. This organism, Mannheimia succiniciproducens, has a unique metabolism involving carbon fixation. While carbon fixation is common to most plant life on earth since it is the key initial step in the biosynthesis of complex carbon compounds during photosynthesis (the Calvin cycle), it is found in relatively few microorganisms and not found in animals. M. succiniciproducens can attach carbon dioxide to the three-carbon backbone of phosphoenolpyruvate, an endproduct in glycolysis, to generate the four-carbon compound, oxaloacetic acid, an intermediate in the Krebs cycle. Although M. succiniciproducens has most of the intermediates in the Krebs cycle, it appears incapable of aerobic respiration, instead using fumarate as a final electron acceptor.Carbohydrate catabolism
Digestion is the breakdown of carbohydrates to yield an energy rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic.
In aerobic respiration, oxygen is required. Oxygen plays a key role as it increases ATP production from 4 ATP molecules to about 30 ATP molecules.
In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation.There are two types of fermentation: alcohol fermentation and lactic acid fermentation.
There are several different types of carbohydrates: polysaccharides (e.g., starch, amylopectin, glycogen, cellulose), monosaccharides (e.g., glucose, galactose, fructose, ribose) and the disaccharides (e.g., sucrose, maltose, lactose).
Glucose reacts with oxygen in the following redox reaction, C6H12O6 + 6O2 → 6CO2 + 6H2O, Carbon dioxide and water are waste products, and the overall reaction is exothermic.
The breakdown of glucose into energy in the form of molecules of ATP is therefore one of the most important biochemical pathways found in living organisms.Cardiovascular physiology
Cardiovascular physiology is the study of the cardiovascular system, specifically addressing the physiology of the heart ("cardio") and blood vessels ("vascular").
These subjects are sometimes addressed separately, under the names cardiac physiology and circulatory physiology.Although the different aspects of cardiovascular physiology are closely interrelated, the subject is still usually divided into several subtopics.Cellular respiration
Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process, as weak so-called "high-energy" bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions.
Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and the most common oxidizing agent (electron acceptor) is molecular oxygen (O2). The chemical energy stored in ATP (its third phosphate group is weakly bonded to the rest of the molecule and is cheaply broken allowing stronger bonds to form, thereby transferring energy for use by the cell) can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes.Cellular waste product
Cellular waste products are formed as a by-product of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP. One example of cellular respiration creating cellular waste products are aerobic respiration and anaerobic respiration.
Each pathway generates different waste products.Deferribacteraceae
The Deferribacteraceae are a family of gram-negative bacteria which make energy by anaerobic respiration.Desulfuromonadales
The Desulfuromonadales are an order within the Proteobacteria. Various members of the Desulfomonadales are capable of anaerobic respiration utilizing a variety of compounds as electron acceptors, including sulfur, Mn(IV), Fe(III), nitrate, Co(III), Tc(VII), U(VI) and trichloroacetic acidThe order Desulfuromonadales contains the following families and genera:
Desulfuromonadaceae corrig. Kuever et al. 2006
Desulfuromonas Pfennig & Biebl 1977
Desulfuromusa Liesack & Finster 1994
Geobacteraceae Holmes et al. 2004
Geoalkalibacter Zavarzina et al. 2007
Geobacter Lovley et al. 1995
Geopsychrobacter Holmes et al. 2005
Geothermobacter Kashefi et al. 2005
Malonomonas Dehning & Schink 1990
Pelobacter Schink & Pfennig 1983Dissimilatory sulfate reduction
Dissimilatory sulfate reduction is a form of anaerobic respiration that uses sulfate as the terminal electron acceptor. This metabolism is found in some types of bacteria and archaea which are often termed sulfate-reducing organisms.
Dissimilatory sulfate reduction occurs in three steps:
Conversion (activation) of sulfate to Adenosine 5’-phosphosulfate (APS)
reduction of APS to sulfite
reduction of sulfite to sulfideWhich requires the consumption of a single ATP molecule and the input of 8 electrons (e−).The protein complexes responsible for these chemical conversions — Sat, Apr and Dsr — are found in all currently known organisms that perform dissimilatory sulfate reduction. Energetically, sulfate is a poor electron acceptor for microorganisms as the sulfate-sulfite redox couple is E0' -516 mV, which is too negative to allow reduction by NADH or ferrodoxin that are the primary intracellular electron mediators. To overcome this issue, sulfate is first converted into APS by the enzyme ATP sulfurylase (Sat), at the cost of a single ATP molecule. The APS-sulfite redox couple has a E0' of -60 mV, which allows APS to be reduced by either NADH or reduced ferrodoxin using the enzyme adenylyl-sulfate reductase (Apr), which requires the input of 2 electrons. In the final step, sulfite is reduced by the dissimilatory sulfite reductase (Dsr) to form sulfide, requiring the input of 6 electrons.Note. The term "dissimilatory" is used when hydrogen sulfide is produced in an anaerobic respiration process. By contrast, the term "assimilatory" would be used in relation to the biosynthesis of organo-sulfur compounds.Fumarate reductase
Fumarate reductase is the enzyme that converts fumarate to succinate, and is important in microbial metabolism as a part of anaerobic respiration.Succinate + acceptor <=> fumarate + reduced acceptor
Fumarate reductases can be divided into two classes depending on the electron acceptor:
Fumarate reductase (quinol) (EC 18.104.22.168)
The membrane-bound enzyme covalently linked to flavin cofactors, which is composed of 3 or 4 subunits, transfers electrons from a quinol to fumarate. This class of enzyme is thus involved in the production of ATP by oxidative phosphorylation.
Fumarate reductase (NADH) (EC 22.214.171.124)
The enzyme is monomeric and soluble, and can reduce fumarate independently from the electron transport chain. Fumarate reductase is absent from all mammalian cells.Gemmatimonadetes
The Gemmatimonadetes are a phylum of bacteria created for the type species Gemmatimonas aurantiaca. This bacterium makes up about 2% of soil bacterial communities and has been identified as one of the top nine phyla found in soils; yet, there are currently only six cultured isolates. Gemmatimonadetes have been found in a variety of arid soils, such as grassland, prairie, and pasture soil, as well as eutrophic lake sediments and alpine soils. This wide range of environments where Gemmatimonadetes have been found suggests an adaptation to low soil moisture. A study conducted showed that the distribution of the Gemmatimonadetes in soil tends to be more dependent on the moisture availability than aggregation, reinforcing the belief that the members of this phylum prefer dryer soils. The phylum Gemmatimonadetes is distinct from the phylum Cyanobacteria and may have diverged in early microbial evolution at least 3 billion years ago.The first member of this phylum was discovered in 2003 in activated sludge in a sewage treatment system. The bacterium was named Gemmatimonas aurantiaca. This bacterium is identified as strain T-27T, is Gram-negative, and is the only member of this phylum that has been studied in depth. The metabolic pathways and enzymes of this bacterium are unique and it is able to grow by both aerobic and anaerobic respiration.Hydrogen hypothesis
The hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within an archaeon (without doubts classified as prokaryote at then times), giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have arisen (symbiogenesis).
According to the hydrogen hypothesis:
The host that acquired the mitochondrion was a hydrogen-dependent archaeon, possibly similar in physiology to a modern methanogenic archaea, which use hydrogen and carbon dioxide to produce methane;
The future mitochondrion was a facultatively anaerobic eubacterium which produced hydrogen and carbon dioxide as byproducts of anaerobic respiration;
A symbiotic relationship between the two started, based on the host's hydrogen dependence (anaerobic syntrophy).O-succinylbenzoate—CoA ligase
O-succinylbenzoate CoA ligase (EC 126.96.36.199), encoded from the menE gene in Escherichia coli, catalyzes the fifth reaction in the synthesis of menaquinone (vitamin K2). This pathway is called 1, 4-dihydroxy-2-naphthoate biosynthesis I. Vitamin K is a quinone that serves as an electron transporter during anaerobic respiration. This process of anaerobic respiration allows the bacteria to generate the energy required to survive.Obligate aerobe
An obligate aerobe is an organism that requires oxygen to grow. Through cellular respiration, these organisms use oxygen to metabolise substances, like sugars or fats, to obtain energy. In this type of respiration, oxygen serves as the terminal electron acceptor for the electron transport chain. Aerobic respiration has the advantage of yielding more energy (adenosine triphosphate or ATP) than fermentation or anaerobic respiration, but obligate aerobes are subject to high levels of oxidative stress.Examples of obligately aerobic bacteria include and Mycobacterium tuberculosis and Nocardia asteroides. With the exception of the yeasts, most fungi are obligate aerobes. Also, almost all algae are obligate aerobes.Obligate anaerobe
Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O2). Oxygen tolerance varies between species, some capable of surviving in up to 8% oxygen, others losing viability unless the oxygen concentration is less than 0.5%. An important distinction needs to be made here between the obligate anaerobes and the microaerophiles. Microaerophiles, like the obligate anaerobes, are damaged by normal atmospheric concentrations of oxygen. However, microaerophiles metabolise energy aerobically, and obligate anaerobes metabolise energy anaerobically. Microaerophiles therefore require oxygen (typically 2–10% O2) for growth. Obligate anaerobes do not.Primary nutritional groups
Primary nutritional groups are groups of organisms, divided in relation to the nutrition mode according to the sources of energy and carbon, needed for living, growth and reproduction. The sources of energy can be light and organic or inorganic compounds; the sources of carbon can be of organic or inorganic origin.
The terms aerobic respiration, anaerobic respiration and fermentation do not refer to primary nutritional groups, but simply reflect the different use of possible electron acceptors in particular organisms, such as O2 in aerobic respiration, or nitrate (NO3−), sulfate (SO42−) or fumarate in anaerobic respiration, or various metabolic intermediates in fermentation. Because all ATP-generating steps in fermentation involve modifications of metabolic intermediates instead of the use of an electron transport chain fermentation is often referred to as substrate-level phosphorylation.Shewanella
Shewanella is the sole genus included in the marine bacteria family Shewanellaceae. Some species within it were formerly classed as Alteromonas. Shewanella consists of facultatively anaerobic Gram-negative rods, most of which are found in extreme aquatic habitats where the temperature is very low and the pressure is very high. Shewanella bacteria are a normal component of the surface flora of fish and are implicated in fish spoilage.Shewanella oneidensis MR-1 is a widely used laboratory model to study anaerobic respiration of metals and other anaerobic extracellular electron acceptors, and for teaching about microbial electrogenesis and microbial fuel cells.