Nitrate anaerobic respiration

Nitrate anaerobic respiration DEFAULT

Electron Donors and Acceptors in Anaerobic Respiration

In anaerobic respiration, a molecule other than oxygen is used as the terminal electron acceptor in the electron transport chain.

Learning Objectives

Describe various types of electron acceptors and donors including: nitrate, sulfate, hydrgoen, carbon dioxide and ferric iron

Key Takeaways

Key Points

  • Both inorganic and organic compounds may be used as electron acceptors in anaerobic respiration. Inorganic compounds include sulfate (SO42-), nitrate (NO3), and ferric iron (Fe3+). Organic compounds include DMSO.
  • These molecules have a lower reduction potential than oxygen. Therefore, less energy is formed per molecule of glucose in anaerobic versus aerobic conditions.
  • The reduction of certain inorganic compounds by anaerobic microbes is often ecologically significant.

Key Terms

  • anaerobic: Without oxygen; especially of an environment or organism.
  • reduction: A reaction in which electrons are gained and valence is reduced; often by the removal of oxygen or the addition of hydrogen.
  • anaerobic respiration: metabolic reactions and processes that take place in the cells of organisms that use electron acceptors other than oxygen

Anaerobic respiration is the formation of ATP without oxygen. This method still incorporates the respiratory electron transport chain, but without using oxygen as the terminal electron acceptor. Instead, molecules such as sulfate (SO42-), nitrate (NO3), or sulfur (S) are used as electron acceptors. These molecules have a lower reduction potential than oxygen; thus, less energy is formed per molecule of glucose in anaerobic versus aerobic conditions.

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Anaerobic Respiration: A molecule other than oxygen is used as the terminal electron acceptor in anaerobic respiration.

Many different types of electron acceptors may be used for anaerobic respiration. Denitrification is the utilization of nitrate (NO3) as the terminal electron acceptor. Nitrate, like oxygen, has a high reduction potential. This process is widespread, and used by many members of Proteobacteria. Many denitrifying bacteria can also use ferric iron (Fe3+) and different organic electron acceptors.

Sulfate reduction uses sulfate (SO2−4) as the electron acceptor, producing hydrogen sulfide (H2S) as a metabolic end product. Sulfate reduction is a relatively energetically poor process, and is used by many Gram negative bacteria found within the δ-Proteobacteria. It is also used in Gram-positive organisms related to Desulfotomaculum or the archaeon Archaeoglobus.

Sulfate reduction requires the use of electron donors, such as the carbon compounds lactate and pyruvate (organotrophic reducers), or hydrogen gas (lithotrophic reducers). Some unusual autotrophic sulfate-reducing bacteria, such as Desulfotignum phosphitoxidans, can use phosphite (HPO3) as an electron donor. Others, such as certain Desulfovibrio species, are capable of sulfur disproportionation (splitting one compound into an electron donor and an electron acceptor) using elemental sulfur (S0), sulfite (SO3−2), and thiosulfate (S2O32-) to produce both hydrogen sulfide (H2S) and sulfate (SO2−).

Acetogenesis is a type of microbial metabolism that uses hydrogen (H2) as an electron donor and carbon dioxide (CO2) as an electron acceptor to produce acetate, the same electron donors and acceptors used in methanogenesis.

Ferric iron (Fe3+) is a widespread anaerobic terminal electron acceptor used by both autotrophic and heterotrophic organisms. Electron flow in these organisms is similar to those in electron transport, ending in oxygen or nitrate, except that in ferric iron-reducing organisms the final enzyme in this system is a ferric iron reductase. Since some ferric iron-reducing bacteria (e.g.G. metallireducens) can use toxic hydrocarbons (e.g. toluene) as a carbon source, there is significant interest in using these organisms as bioremediation agents in ferric iron contaminated aquifers.

Other inorganic electron acceptors include the reduction of Manganic ion (Mn4+) to manganous (Mn2+), Selenate (SeO42−) to selenite (SeO32−) to selenium (Se), Arsenate (AsO43−) to arsenite (AsO33-), and Uranyl (UO22+) to uranium dioxide (UO2)

Organic compounds may also be used as electron acceptors in anaerobic respiration. These include the reduction of fumarate to succinate, Trimethylamine N-oxide (TMAO) to trimethylamine (TMA), and Dimethyl sulfoxide (DMSO) to Dimethyl sulfide (DMS).

Nitrate Reduction and Denitrification

Denitrification is a type of anaerobic respiration that uses nitrate as an electron acceptor.

Learning Objectives

Outline the processes of nitrate reduction and denitrification and the organisms that utilize it

Key Takeaways

Key Points

  • Denitrification generally proceeds through a stepwise reduction of some combination of the following intermediate forms: NO3 → NO2 → NO + N2O → N2.
  • Generally, several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway has been identified in the reduction process.
  • Complete denitrification is an environmentally significant process as some intermediates of denitrification (nitric oxide and nitrous oxide) are significant greenhouse gases that react with sunlight and ozone to produce nitric acid, a component of acid rain.

Key Terms

  • electron acceptor: An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process.
  • eutrophication: The process of becoming eutrophic.
  • facultative: Not obligate; optional, discretionary or elective

In anaerobic respiration, denitrification utilizes nitrate (NO3) as a terminal electron acceptor in the respiratory electron transport chain. Denitrification is a widely used process; many facultative anaerobes use denitrification because nitrate, like oxygen, has a high reduction potential

Denitrification is a microbially facilitated process involving the stepwise reduction of nitrate to nitrite (NO2) nitric oxide (NO), nitrous oxide (N2O), and, eventually, to dinitrogen (N2) by the enzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase. The complete denitrification process can be expressed as a redox reaction: 2 NO3− + 10 e + 12 H+ → N2 + 6 H2O.

Protons are transported across the membrane by the initial NADH reductase, quinones and nitrous oxide reductase to produce the electrochemical gradient critical for respiration. Some organisms (e.g. E. coli) only produce nitrate reductase and therefore can accomplish only the first reduction leading to the accumulation of nitrite. Others (e.g. Paracoccus denitrificans or Pseudomonas stutzeri) reduce nitrate completely. Complete denitrification is an environmentally significant process because some intermediates of denitrification (nitric oxide and nitrous oxide) are significant greenhouse gases that react with sunlight and ozone to produce nitric acid, a component of acid rain. Denitrification is also important in biological wastewater treatment, where it can be used to reduce the amount of nitrogen released into the environment, thereby reducing eutrophication.

Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen is depleted and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in anaerobic environments where oxygen consumption exceeds the oxygen supply and where sufficient quantities of nitrate are present. These environments may include certain soils and groundwater, wetlands, oil reservoirs, poorly ventilated corners of the ocean, and in sea floor sediments.

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The role of soil bacteria in the Nitrogen cycle: Denitrification is an important process in maintaining ecosystems. Generally, denitrification takes place in environments depleted of oxygen.

Denitrification is performed primarily by heterotrophic bacteria (e.g. Paracoccus denitrificans), although autotrophic denitrifiers have also been identified (e.g., Thiobacillus denitrificans). Generally, several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.

Rhizobia are soil bacteria with the unique ability to establish a N2-fixing symbiosis on legume roots. When faced with a shortage of oxygen, some rhizobia species are able to switch from O2-respiration to using nitrates to support respiration.

The direct reduction of nitrate to ammonium (dissimilatory nitrate reduction) can be performed by organisms with the nrf- gene. This is a less common method of nitrate reduction than denitrification in most ecosystems. Other genes involved in denitrification include nir (nitrite reductase) and nos (nitrous oxide reductase), which are possessed by such organisms as Alcaligenes faecalis, Alcaligenes xylosoxidans, Pseudomonas spp, Bradyrhizobium japonicum, and Blastobacter denitrificans.

Sulfate and Sulfur Reduction

Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain.

Learning Objectives

Outline the process of sulfate and sulfur reduction including its various purposes

Key Takeaways

Key Points

  • Sulfate reduction is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulfate-rich environments.
  • Sulfate reducers may be organotrophic, using carbon compounds, such as lactate and pyruvate as electron donors, or lithotrophic, and use hydrogen gas (H2) as an electron donor.
  • Before sulfate can be used as an electron acceptor, it must be activated by ATP -sulfurylase, which uses ATP and sulfate to create adenosine 5′-phosphosulfate (APS).
  • Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon after life emerged on Earth.
  • Toxic hydrogen sulfide is one waste product of sulfate-reducing bactera, and is the source of the rotten egg odor.
  • Sulfate-reducing bacteria may be utilized for cleaning up contaminated soils.

Key Terms

  • lithotrophic: Obtains electrons for respiration from inorganic substrates.
  • organotrophic: Obtains electrons for respiration from organic substrates.

Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain. Compared to aerobic respiration, sulfate reduction is a relatively energetically poor process, though it is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulfate-rich environments.

Many sulfate reducers are organotrophic, using carbon compounds, such as lactate and pyruvate (among many others) as electron donors, while others are lithotrophic, and use hydrogen gas (H2) as an electron donor. Some unusual autotrophic sulfate-reducing bacteria (e.g., Desulfotignum phosphitoxidans) can use phosphite (HPO3-) as an electron donor, whereas others (e.g., Desulfovibrio sulfodismutans, Desulfocapsa thiozymogenes, and Desulfocapsa sulfoexigens) are capable of sulfur disproportionation (splitting one compound into two different compounds, in this case an electron donor and an electron acceptor) using elemental sulfur (S0), sulfite (SO32−), and thiosulfate (S2O32−) to produce both hydrogen sulfide (H2S) and sulfate (SO42−).

Before sulfate can be used as an electron acceptor, it must be activated. This is done by the enzyme ATP-sulfurylase, which uses ATP and sulfate to create adenosine 5′-phosphosulfate (APS). APS is subsequently reduced to sulfite and AMP. Sulfite is then further reduced to sulfide, while AMP is turned into ADP using another molecule of ATP. The overall process, thus, involves an investment of two molecules of the energy carrier ATP, which must to be regained from the reduction.

All sulfate-reducing organisms are strict anaerobes. Because sulfate is energetically stable, it must be activated by adenylation to form APS (adenosine 5′-phosphosulfate) to form APS before it can be metabolized, thereby consuming ATP. The APS is then reduced by the enzyme APS reductase to form sulfite (SO32−) and AMP. In organisms that use carbon compounds as electron donors, the ATP consumed is accounted for by fermentation of the carbon substrate. The hydrogen produced during fermentation is actually what drives respiration during sulfate reduction.

Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon after life emerged on Earth. Sulfate-reducing bacteria are common in anaerobic environments (such as seawater, sediment, and water rich in decaying organic material) where they aid in the degradation of organic materials. In these anaerobic environments, fermenting bacteria extract energy from large organic molecules; the resulting smaller compounds (such as organic acids and alcohols) are further oxidized by acetogens, methanogens, and the competing sulfate-reducing bacteria.

Many bacteria reduce small amounts of sulfates in order to synthesize sulfur-containing cell components; this is known as assimilatory sulfate reduction. By contrast, sulfate-reducing bacteria reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste; this is known as “dissimilatory sulfate reduction. ” Most sulfate-reducing bacteria can also reduce other oxidized inorganic sulfur compounds, such as sulfite, thiosulfate, or elemental sulfur (which is reduced to sulfide as hydrogen sulfide).

Toxic hydrogen sulfide is one waste product of sulfate-reducing bacteria; its rotten egg odor is often a marker for the presence of sulfate-reducing bacteria in nature. Sulfate-reducing bacteria are responsible for the sulfurous odors of salt marshes and mud flats. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides. These metal sulfides, such as ferrous sulfide (FeS), are insoluble and often black or brown, leading to the dark color of sludge. Thus, the black color of sludge on a pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.

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Black sludge: The black color of this pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.

Some sulfate-reducing bacteria play a role in the anaerobic oxidation of methane (CH4 + SO42- → HCO3– + HS– + H2O). An important fraction of the methane formed by methanogens below the seabed is oxidized by sulfate-reducing bacteria in the transition zone separating the methanogenesis from the sulfate reduction activity in the sediments.This process is also considered a major sink for sulfate in marine sediments. In hydrofracturing fluids used to frack shale formations to recover methane (shale gas), biocide compounds are often added to water to inhibit the microbial activity of sulfate-reducing bacteria in order to avoid anaerobic methane oxidation and to minimize potential production loss.

Sulfate-reducing bacteria often create problems when metal structures are exposed to sulfate-containing water. The interaction of water and metal creates a layer of molecular hydrogen on the metal surface. Sulfate-reducing bacteria oxidize this hydrogen, creating hydrogen sulfide, which contributes to corrosion. Hydrogen sulfide from sulfate-reducing bacteria also plays a role in the biogenic sulfide corrosion of concrete, and sours crude oil.

Sulfate-reducing bacteria may be utilized for cleaning up contaminated soils; some species are able to reduce hydrocarbons, such as benzene, toluene, ethylbenzene, and xylene. Sulfate-reducing bacteria may also be a way to deal with acid mine waters.

Methanogenesis

Methanogenesis is a form of anaerobic respiration that uses carbon as a electron acceptor and results in the production of methane.

Learning Objectives

Recognize the characteristics associated with methanogenesis

Key Takeaways

Key Points

  • Carbon dioxide or acetic acid are the most commonly used electron acceptor in methanogenesis.
  • Microbes capable of producing methane are called methanogens. They have been identified only from the domain Archaea – a group that is phylogenetically distinct from eukaryotes and bacteria.
  • The production of methane is an important and widespread form of microbial metabolism. In most environments, it is the final step in the decomposition of biomass.
  • Methane is a major greenhouse gas. The average cow emits around 250 liters of methane a day as a result of the breakdown of cellulose by methanogens. Therefore, the large scale raising of cattle for meat is a considerable contributor to global warming.

Key Terms

  • methanethiol: A colourless gas, a thiol with a smell like rotten cabbage, found naturally in plants and animals.
  • cofactor: A substance, especially a coenzyme or a metal, that must be present for an enzyme to function.
  • fermentation: Any of many anaerobic biochemical reactions in which an enzyme (or several enzymes produced by a microorganism) catalyses the conversion of one substance into another; especially the conversion (using yeast) of sugars to alcohol or acetic acid with the evolution of carbon dioxide.

Methanogenesis, or biomethanation, is a form of anaerobic respiration that uses carbon as the terminal electron acceptor, resulting in the production of methane. The carbon is sourced from a small number of low molecular weight organic compounds, such as carbon dioxide, acetic acid, formic acid (formate), methanol, methylamines, dimethyl sulfide, and methanethiol. The two best described pathways of methanogenesis use carbon dioxide or acetic acid as the terminal electron acceptor:

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Methanogenesis of acetate: Acetate is broken down to methane by methanogenesis, a type of anaerobic respiration.

CO2 + 4 H2 → CH4 + 2H2O

CH3COOH → CH4 + CO2

The biochemistry of methanogenesis is relatively complex. It involves the coenzymes and cofactors F420, coenzyme B, coenzyme M, methanofuran, and methanopterin.

Microbes capable of producing methane are called methanogens. They have been identified only from the domain Archaea – a group that is phylogenetically distinct from eukaryotes and bacteria – though many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism, and in most environments, it is the final step in the decomposition of biomass.

During the decay process, electron acceptors (such as oxygen, ferric iron, sulfate, and nitrate) become depleted, while hydrogen (H2), carbon dioxide, and light organics produced by fermentation accumulate. During advanced stages of organic decay, all electron acceptors become depleted except carbon dioxide, which is a product of most catabolic processes. It is not depleted like other potential electron acceptors.

Only methanogenesis and fermentation can occur in the absence of electron acceptors other than carbon. Fermentation only allows the breakdown of larger organic compounds, and produces small organic compounds. Methanogenesis effectively removes the semi-final products of decay: hydrogen, small organics, and carbon dioxide. Without methanogenesis, a great deal of carbon (in the form of fermentation products) would accumulate in anaerobic environments.

Methanogenesis also occurs in the guts of humans and other animals, especially ruminants. In the rumen, anaerobic organisms, including methanogens, digest cellulose into forms usable by the animal. Without these microorganisms, animals such as cattle would not be able to consume grass. The useful products of methanogenesis are absorbed by the gut. Methane is released from the animal mainly by belching (eructation). The average cow emits around 250 liters of methane per day. Some, but not all, humans emit methane in their flatus!

Some experiments even suggest that leaf tissues of living plants emit methane, although other research indicates that the plants themselves do not actually generate methane; they are just absorbing methane from the soil and then emitting it through their leaf tissues. There may still be some unknown mechanism by which plants produce methane, but that is by no means certain.

Methane is one of the earth’s most important greenhouse gases, with a global warming potential 25 times greater than carbon dioxide (averaged over 100 years). Therefore, the methane produced by methanogenesis in livestock is a considerable contributor to global warming.

Methanogenesis can also be beneficially exploited. It is the primary pathway that breaks down organic matter in landfills (which can release large volumes of methane into the atmosphere if left uncontrolled), and can be used to treat organic waste and to produce useful compounds. Biogenic methane can be collected and used as a sustainable alternative to fossil fuels.

Proton Reduction

Anaerobic respiration utilizes highly reduced species – such as a proton gradient – to establish electrochemical membrane gradients.

Learning Objectives

Outline the role of the proton motive force in metabolism

Key Takeaways

Key Points

  • In denitrification, protons are transported across the membrane by the initial NADH reductase, quinones, and nitrous oxide reductase to produce the electrochemical gradient critical for respiration.
  • An electrochemical gradient represents one of the many interchangeable forms of potential energy through which energy may be conserved. In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient.
  • In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force.

Key Terms

  • phosphorylation: The process of transferring a phosphate group from a donor to an acceptor; often catalysed by enzymes

Proton Gradients in Reductive Metabolism

Biological energy is frequently stored and released by means of redox reactions, or the transfer of electrons. Reduction occurs when an oxidant gains an electron. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. The reverse reaction, respiration, oxidizes sugars (loses an electron) to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce nicotinamide adenine dinucleotide (NAD+), which then contributes to the creation of a proton gradient. This then drives the synthesis of adenosine triphosphate ( ATP ) and is maintained by the reduction of oxygen, or alternative receptors for anaerobic respiration. In animal cells, the mitochondria performs similar functions.

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The Basics of Redox: In every redox reaction you have two halves: reduction and oxidation.

An electrochemical gradient represents one of the many interchangeable forms of potential energy through which energy may be conserved. In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient. In the mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by phosphorylation. An electrochemical gradient has two components. First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The combination of these two factors determines the thermodynamically favorable direction for an ion’s movement across a membrane. The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force.

In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction, creating ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. The same process takes place in the mitochondria, where ATP synthase is located in the inner mitochondrial membrane, so that F1 part sticks into the mitochondrial matrix where ATP synthesis takes place.

Cellular respiration (both aerobic and anaerobic) utilizes highly reduced species such as NADH and FADH2 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, 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.

Proton reduction is important for setting up electrochemical gradients for anaerobic respiration. For example, in denitrification, protons are transported across the membrane by the initial NADH reductase, quinones, and nitrous oxide reductase to produce the electrochemical gradient critical for respiration. In organisms that use hydrogen as an energy source, hydrogen is oxidized by a membrane-bound hydrogenase causing proton pumping via electron transfer to various quinones and cytochromes. Sulfur oxidation is a two step process that occurs because energetically sulfide is a better electron donor than inorganic sulfur or thiosulfate, allowing for a greater number of protons to be translocated across the membrane.

In contrast, fermentation does not utilize an electrochemical gradient. Instead, it 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.

Anoxic Hydrocarbon Oxidation

Anoxic hydrocarbon oxidation can be used to degrade toxic hydrocarbons, such as crude oil, in anaerobic environments.

Learning Objectives

Describe the process of anoxic hydrocarbon oxidation in regards to marine environments

Key Takeaways

Key Points

  • Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon.
  • The majority of hydrocarbons occur naturally in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen. The combustion of hydrocarbons is the primary energy source for current civilizations.
  • Anaerobic oxidation of methane (AOM) is a microbial process that occurs in anoxic marine sediments. AOM is considered to be a very important process, reducing the emission of methane (a greenhouse gas) from the ocean into the atmosphere by up to 90%.

Key Terms

  • methanotrophic: The ability to metabolize methane as an only source of carbon and energy.
  • syntrophic: When one species lives off the products of another species.
  • anoxic: Lacking oxygen.

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. The majority of hydrocarbons occur naturally in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen. The combustion of hydrocarbons is the primary energy source for current civilizations.

Crude oil contains aromatic compounds that are toxic to most forms of life. Their release into the environment by human spills and natural seepages can have detrimental effects. Marine environments are especially vulnerable. Despite its toxicity, a considerable fraction of crude oil entering marine systems is eliminated by the hydrocarbon-degrading activities of microbial communities. Although it was once thought that hydrocarbon compounds could only be degraded in the presence of oxygen, the discovery of anaerobic hydrocarbon-degrading bacteria and pathways show that the anaerobic degradation of hydrocarbons occurs naturally.

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Contaminated soil: Microbes may be used to degrade toxic hydrocarbons in anaerobic environments.

The facultative denitrifying proteobacteria Aromatoleum aromaticum strain EbN1 was the first to be determined as an anaerobic hydrocarbon degrader, using toluene or ethylbenzene as substrates. Some sulfate-reducing bacteria can reduce hydrocarbons such as benzene, toluene, ethylbenzene, and xylene, and have been used to clean up contaminated soils. The genome of the iron-reducing and hydrocarbon degrading species Geobacter metallireducens was recently determined.

Anaerobic oxidation of methane (AOM) is a microbial process that occurs in anoxic marine sediments. During this process, the hydrocarbon methane is oxidized with sulfate as the terminal electron acceptor: CH4 + SO42- → HCO3- + HS + H2O. It is believed that AOM is mediated by a syntrophic aggregation of methanotrophic archaea and sulfate-reducing bacteria, although the exact mechanisms of this syntrophic relationship are still poorly understood. AOM is considered to be a very important process in reducing the emission of methane (a greenhouse gas) from the ocean into the atmosphere. It is estimated that almost 90% of all the methane that arises from marine sediments is oxidized anaerobically by this process. Recent investigations have shown that some syntrophic pairings are able to oxidize methane with nitrate instead of sulfate.

Sours: https://courses.lumenlearning.com/boundless-microbiology/chapter/anaerobic-respiration/

Anaerobic Respiration

Anaerobic respiration is a process that generates cell energy by coupling membrane-associated electron transfer reactions using an electron acceptor other than O2. The process creates a membrane potential across the cytoplasmic membrane called the proton motive force (pmf). The cell then uses this energy to drive ATP synthesis using the membrane-bound ATP synthase (electron transport phosphorylation).

 

Key Concepts

  • Anaerobic respiratory chains are located in the cytoplasmic membranes and generate a proton motive force (pmf).
  • The electron transport chains always consist of an electron donating dehydrogenase and an electron accepting terminal reductase.
  • Menaquinone (MQ) is the electron mediator between the enzyme complexes.
  • The proton motive force drives ATP synthesis via the membrane-bound ATP synthase.
  • Anaerobic respiration generates the majority of the cell energy under anaerobic growth conditions.
  • There are many compounds that can support anaerobic respiration in E. coli.

 

Principles of Anaerobic Respiration

Anaerobic respiration supports growth of E. coli cells under conditions when suitable electron donors (DH) and acceptors (A) are present. There are a variety of different inorganic and organic donors and acceptors that can be used, and each respiratory substrate requires a specific membrane bound enzyme for its utilization.

Together, these donor and acceptor enzymes form modular electron transport chains that consist of a membrane-associated dehydrogenase enzyme that transfers electrons to an anaerobic terminal reductase enzyme. The overall reaction is represented by:

DH + A → D + AH

Example 1: Anaerobic electron transfer from formate to nitrate

In the following example of anaerobic respiration, formate (HCOO-) serves as the electron donor and nitrate (NO3-) is the electron acceptor. Formate is first oxidized by formate dehydrogenase N and electrons are then transferred to nitrate reductase which in turn reduces nitrate to nitrite (NO2-). The two enzymes form a modular electron transport chain.

This overall reaction is:
formate + NO3-→ CO2 + NO2- + H2O

The individual reactions catalyzed by each enzyme in the above electron transport chain are:

Formate dehydrogenase N:
formate + MQ  →  CO2 + MQH2

Nitrate reductase:
MQH2  + NO3-→ NO2- + H2O + MQ

Under anaerobic conditions, the lipid soluble cofactor menaquinone (MQ) mediates (i.e., transfers) electrons between the dehydrogenase and the reductase enzymes.

 

 

How E. coli Respires under anaerobic conditions

The E. coli genome encodes a variety of distinct dehydrogenase and terminal reductase enzymes that accomplish anaerobic respiration. Their synthesis usually requires the absence of O2 (anaerobiosis) and the presence of the respective enzyme substrate. Regardless of which enzymes are used, the resulting electron transport chain forms a proton motive force (pmf) that is then used for ATP synthesis and for other energy-requiring processes. 

 

Anaerobic Electron Donors

There are at least six compounds that E. coli can use as anaerobic electron donors: they include formate, hydrogen, NADH, lactate, glycerol-3-phosphate, and ethanol.

Electron donors and their dehydrogenases:

E. coli produces one or more specific dehydrogenase enzymes to oxidize each electron donor. Any of these dehydrogenases can donate electrons to any of the electron acceptor enzymes (i.e., terminal reductases) described below to form an electron transport chain. The synthesis of the individual enzyme is usually controlled by oxygen and the availability of the enzyme’s substrate.

  • Formate and formate dehydrogenase

    E. coli contains three formate dehydrogenase enzymes.

          

    Two enzymes, formate dehydrogenase-N (FdnGHI)and formate dehydrogenase-O (FdoGHI) participate in anaerobic respiration byoxidizing formate in the periplasm to carbonate. They transfer electrons to the menaquinone pool which supplies nitrate reductase or another terminal reductase with these electrons. The enzymes share extensive sequence similarity and immunological properties: both contain molybdenum and selenium cofactors, iron-sulfur centers and heme b. The third enzyme, formate dehydrogenase-H (FdhF) is membrane bound and interacts with a hydrogenase to form the formate-hydrogen lyase complex.

    Expression of formate dehydrogenase-N (FdnGHI)is induced by nitrate and anaerobiosis, while expression of formate dehydrogenase-O (FdoGHI) occurs under both aerobic and anaerobic conditions where nitrate weakly stimulates gene expression. Expression of formate dehydrogenase-H (FdhF) is induced by formate and repressed by nitrate, nitrite or TMAO.

  • NADH and NADH dehydrogenase

    E. coli contains two NADH dehydrogenases.

          

    Only one, NADH dehydrogenase I (Ndh-1, also called NuoABCDEFGHIJKLMN) is present under both aerobic and anaerobic conditions. NADH dehydrogenase II (Ndh-2), encoded by the ndh gene is synthesized only aerobically. Ndh-1 catalyzes the transfer of electrons from NADH to the quinone pool and is able to generate a proton electrochemical gradient by pumping protons from the cytoplasm to the cell periplasm. In contrast, Ndh-2 does not pump protons.

    The purified Ndh-1 enzyme can be separated into three components: a soluble fragment composed of the NuoE, F and G subunits which catalyze the oxidation of NADH. It represents the electron input section of the enzyme and contains all of the iron-sulfur clusters and the FMN cofactor. An amphipathic connecting fragment is composed of the NuoB, CD and I subunits and is linked to a hydrophobic membrane fragment composed of the remaining NuoA, H, J, K, L, M and N subunits. These latter two parts are responsible for reducing the MQ cofactor to MQH2 (or Q cofactor to QH2), and for pumping protons across the cytoplasmic membrane.

    Expression of the 14 gene nuo operon is regulated by oxygen and nitrate availability, and by other factors including C4 dicarboxylic acids.

Other electron donor enzymes:
Several other dehydrogenases may also function anaerobically to provide electrons to the terminal reductases. These include:

  • Hydrogen and hydrogenase      

    E. coli contains 4 hydrogenases, of which hydrogenase 1 (HyaABC) and hydrogenase 2 (HybABOC) are proposed to be involved in electron transfer to anaerobic respiratory reductases. A third hydrogenase, hydrogenase 3 (HycBCDEFG) interacts with formate dehydrogenase-H (FdhF) to form the formate-hydrogen lyase complex.

 

Anaerobic Electron Acceptors

There are at least five compounds that can function as anaerobic electron acceptors in E. coli. They include nitrate, nitrite, trimethylamine-N-oxide, dimemethyl-sufloxide, and fumarate. To catalyze their reduction, the cell must synthesize one or more substrate specific terminal reductase enzymes.

Electron Acceptors and their terminal reductase enzymes:

  • Nitrate and nitrate reductase

    E. coli contains genes for three distinct nitrate reductase enzymes that reduce nitrate to nitrite.

          

    Two of these, nitrate reductase A (NRA or NarGHI) and nitrate reductase Z (NRZ or NarZYV), are membrane bound and are biochemically nearly identical. The third nitrate reductase, Nap (NapAGHBC), is located in the periplasm.

    NarGHI is the preferred terminal reductase under anaerobic conditions when nitrate is abundant. It often couples with formate dehydrogenase-N to form a respiratory chain. The pmf is generated by a menaquinone (MQ) loop where MQ is reduced on the cytoplasmic face of the membrane when formate dehydrogenase oxidizes formate. This allows MQ to pick up its protons from the cytoplasm to form MQH2. MQH2 is then oxidized by nitrate reductase on the periplasmic side of the membrane and protons are deposited in the periplasm. Concurrently, the electrons are passed to the active site of nitrate reductase to reduce nitrate to nitrite.

    Formate + NO3-→ CO2 + NO2- + H2O

    The NapA nitrate reductase (NapAGHBC) also participates under anaerobic conditions to form electron transfer chains. Because the enzyme is located in the periplasmic space, it does not require nitrate uptake into the cytoplasm as do the NarGHI and NarZYV enzymes. NapA receives electrons from the quinone pool via other Nap subunits.

    The nap operon is expressed under low nitrate conditions while the narGHI operon is expressed only when environmental nitrate levels are high. Both are induced by anaerobiosis and nitrate. The expression of the narZYV operon is constitutive and further induced during stationary phase: however, it is independent of nitrate availability or anaerobiosis.

  • Nitrite and nitrite reductase      

    Nitrite is toxic to the cell and is transported out or detoxified when concentrations are high. However, nitrite at low concentrations can serve as terminal electron acceptor once nitrate is depleted from the environment. Nitrite reductase serves as the terminal reductase.

    E. coli has two distinct nitrite reductases called NrfA and NirB. The NrfA enzyme is encoded by the nrfA gene, which is expressed optimally at low environmental nitrate conditions. In contrast, nirB gene expression is maximal only at high nitrate concentrations. At intermediate concentration of nitrate, both nitrite reductases are made. Expression of the genes encoding both enzymes is regulated by nitrate. Nitrite has only a minor affect on nrfA and nirB gene expression.

    The two nitrite reductases differ in cellular location and in metabolic function. NrfA is associated with the cytoplasmic membrane with its cytochrome components facing the periplasm where it reduces nitrite to ammonium. The enzyme received electrons from formate dehydrogenase-N or other dehydrogenases which donate electrons from their respective substrates. Menaquinone acts as electron mediator between enzymes. Formate-dependent nitrite reduction via NrfA generates a proton motive force.

    Formate + 7 H+ + NO2-→ CO2 + NH4+ + 2 H2O

    In contrast, NirB is located in the cytoplasm and does not generate a proton gradient. Its probable metabolic role is to detoxify nitrite.

  • TMAO and TMAO reductase      

    E. coli possesses three membrane bound trimethylamine N-oxide (TMAO) reductase enzymes. Reduction of TMAO by each enzyme occurs in the periplasmic space.

    TMAO reductase I (encoded by torAC) can accept electrons from various physiological donors via either menaquinone or ubiquinone. Unlike other anaerobic respiratory systems, which are only synthesized under anaerobic conditions, the torAC operon is expressed under both anaerobic and aerobic conditions; however, the operon is inducible by TMAO. TMAO reductase I contains molybdenum, iron, zinc and acid-labile sulfur cofactors.

    No genes have been identified for trimethylamine N-oxide (TMAO) reductase II and the enzyme is uncharacterized. It has been suggested that dimethylsulphoxide reductase, coded for by the dmsABC genes, may in fact be TMAO reductase II.

    The torYZ-encoded trimethylamine N-oxide (TMAO) reductase III complex represents the third TMAO respiratory system in E. coli. TorZ is the catalytic subunit and TorY is the pentahemic c-type cytochrome subunit. The enzyme has broad substrate specificity: it is able to reduce N- and S-oxide compounds. TMAO is the best substrate for the enzyme. Expression of torYZ is low and not induced by TMAO, DMSO or BSO.

    Reduction of TMAO by all three TMAO reductases may be coupled with formate oxidation via formate dehydrogenase-N or coupled to other dehydrogenases which results in formation of a proton motive force.

    Formate + trimethylamine N-oxide → CO2 + trimethylamine + H2O

  • DMSO and DMSO reductase      

    Dimethyl sulfoxide (DMSO) reductase (DmsABC) is a membrane-associated terminal electron transfer enzyme with structural similarity to formate dehydrogenase and not to TMAO reductase. Like other anaerobic reductases, it forms an energy-transducing anaerobic electron transport chain. DMSO reductase has a broad substrate range: it reduces DMSO plus other amine-N-oxides and methyl-sulfoxides, including trimethylamine N-oxide (TMAO). The enzyme contains a molybdenum cofactor, and four iron-sulfur clusters, which are oriented towards the periplasmic space, where the reduction of DMSO occurs. The dmsABC operon is expressed optimally under anaerobic conditions and when nitrate is absent.

    Formate + dimethyl sulfoxide → CO2 + dimethysulfide + H2O

  • Fumarate and fumarate reductase      

    The membrane-bound fumarate reductase (FrdABCD) catalyzes the reduction of fumarate to succinate. It can couple with a variety of dehydrogenases in the cell including NADH dehydrogenase to form an anaerobic electron transfer chain. Fumarate reductase contains covalently bound FAD and three iron-sulfur centers. This membrane bound enzyme is synthesized optimally under anaerobic conditions and in the presence of fumarate.

    Formate + fumarate → CO2 + succinate

Example 2: Anaerobic electron transfer from NADH to nitrate

In the following example of anaerobic respiration, NADH serves as the electron donor and nitrate is the electron acceptor. NADH is first oxidized by NADH dehydrogenase and electrons are then transferred to nitrate reductase which in turn reduces nitrate to nitrite. The two enzymes form a modular electron transport chain.

This overall reaction is:
NADH + H+ + NO3-→ NAD+ + NO2- + H2O

The individual reactions catalyzed by each enzyme in the above electron transport chain are:

NADH dehydrogenase:
NADH + H+ + Q → NAD+ + QH2

Nitrate reductase:
QH2  + NO3-→  NO2- + H2O + Q

Under anaerobic conditions, the lipid soluble cofactor menaquinone (MQ) usually mediates (i.e., transfers) electrons between the dehydrogenase and the reductase enzyme. This is one of the few exceptions where ubiquinone (Q) is used in place of MQ.

 

 

 

Credits:

Authored by Robert Gunsalus and Imke Schröder
©The Escherichia coli Student Portal

This project acknowledges support from:
NIH Grant Award GM077678 to SRI, International
Peter Karp and coworkers at EcoCyc.org
The UCLA Department of MIMG

 

 

Sours: http://ecolistudentportal.org/article_anaerobic_respiration
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Anaerobic respiration

Respiration using electron acceptors other than oxygen

Not to be confused with Fermentation.

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 a high-energy [2]oxidizing agent and, therefore, is an excellent electron acceptor. In anaerobes, other less-oxidizing substances such as nitrate (NO3), fumarate, sulfate (SO42−), or sulfur (S) 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[edit]

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.

Fermentation, in contrast, does not use 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 (HCO3) reduction (respiration) or acetate fermentation.[3]

Ecological importance[edit]

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.[4] The denitrification process is also very important in host-microbe interactions. Similar to mitochondria in oxygen-respiring microorganisms, some single-cellular anaerobic ciliates use denitrifying endosymbionts to gain energy.[5] 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.[6]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.[7]

Economic relevance[edit]

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.[8]

Anaerobic Denitrification (ETC System)

English: The model above shows the process of anaerobic respiration through denitrification, which uses nitrogen (in the form of nitrate, NO3) as the electron acceptor. NO3goes 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 (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]

Examples of electron acceptors in respiration[edit]

Type Lifestyle Electron acceptor Products Eo' [V]Example organisms
aerobic respirationobligate aerobes and facultative anaerobesO2H2O, CO2+ 0.82 aerobicprokaryotes
perchloraterespirationfacultative anaerobesClO4, ClO3H2O, O2, Cl+ 0.797 Azospira Suillum, Sedimenticola selenatireducens, Sedimenticola thiotaurini, and other gram negativeprokaryotes[10]
iodaterespirationfacultative anaerobesIO3H2O, H2O2, I+ 0.72 Denitromonas,[11]Azoarcus, Pseudomonas, and other prokaryotes[12]
iron reductionfacultative anaerobes and obligate anaerobesFe(III) Fe(II) + 0.75 Organisms within the order Desulfuromonadales(such as Geobacter, Geothermobacter, Geopsychrobacter, Pelobacter) and Shewanella species [13]
manganesefacultative anaerobes and obligate anaerobesMn(IV) Mn(II) Desulfuromonadales and Shewanella species [13]
cobalt reduction facultative anaerobes and obligate anaerobesCo(III) Co(II) Geobacter sulfurreducens
uranium reduction facultative anaerobes and obligate anaerobesU(VI) U(IV) Geobacter metallireducens, Shewanella oneidensis[14]
nitrate reduction (denitrification) facultative anaerobesnitrate NO3(ultimately) N2+ 0.40 Paracoccus denitrificans, Escherichia coli
fumarate respirationfacultative anaerobesfumaratesuccinate+ 0.03 Escherichia coli
sulfate respirationobligate anaerobessulfate SO42−sulfide HS- 0.22 Many Deltaproteobacteria species in the orders Desulfobacterales, Desulfovibrionales, and Syntrophobacterales
methanogenesis (carbon dioxide reduction) methanogenscarbon dioxide CO2methane CH4- 0.25 Methanosarcina barkeri
sulfur respiration (sulfur reduction) facultative anaerobes and obligate anaerobessulfur S0sulfide HS- 0.27 Desulfuromonadales
acetogenesis (carbon dioxide reduction) obligate anaerobescarbon dioxide CO2acetate- 0.30 Acetobacterium woodii
dehalorespirationfacultative anaerobes and obligate anaerobeshalogenated organic compounds R-X Halide ions and dehalogenated compound X + R-H + 0.25–+ 0.60[15]Dehalococcoides and Dehalobacter species

See also[edit]

Further reading[edit]

References[edit]

  1. ^Slonczewski, Joan L.; Foster, John W. (2011). Microbiology : An Evolving Science (2nd ed.). New York: W.W. Norton. p. 166. ISBN .
  2. ^Schmidt-Rohr, K. (2020). "Oxygen Is the High-Energy Molecule Powering Complex Multicellular Life: Fundamental Corrections to Traditional Bioenergetics” ACS Omega5: 2221-2233. doi:10.1021/acsomega.9b03352
  3. ^Sapart; et al. (2017). "The origin of methane in the East Siberian Arctic Shelf unraveled with triple isotope analysis". Biogeosciences. 14 (9): 2283–2292. Bibcode:2017BGeo...14.2283S. doi:10.5194/bg-14-2283-2017.
  4. ^Simon, Jörg; Klotz, Martin G. (2013-02-01). "Diversity and evolution of bioenergetic systems involved in microbial nitrogen compound transformations". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1827 (2): 114–135. doi:10.1016/j.bbabio.2012.07.005. PMID 22842521.
  5. ^Graf, Jon S.; Schorn, Sina; Kitzinger, Katharina; Ahmerkamp, Soeren; Woehle, Christian; Huettel, Bruno; Schubert, Carsten J.; Kuypers, Marcel M. M.; Milucka, Jana (3 March 2021). "Anaerobic endosymbiont generates energy for ciliate host by denitrification". Nature. 591 (7850): 445–450. Bibcode:2021Natur.591..445G. doi:10.1038/s41586-021-03297-6. PMC 7969357. PMID 33658719.
  6. ^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. doi:10.1177/0734242x07088433. ISSN 0734-242X. PMID 18338699. S2CID 29740189.
  7. ^Pester, Michael; Knorr, Klaus-Holger; Friedrich, Michael W.; Wagner, Michael; Loy, Alexander (2012-01-01). "Sulfate-reducing microorganisms in wetlands - fameless actors in carbon cycling and climate change". Frontiers in Microbiology. 3: 72. doi:10.3389/fmicb.2012.00072. ISSN 1664-302X. PMC 3289269. PMID 22403575.
  8. ^Nancharaiah, Y. V.; Venkata Mohan, S.; Lens, P. N. L. (2016-09-01). "Recent advances in nutrient removal and recovery in biological and bioelectrochemical systems". Bioresource Technology. 215: 173–185. doi:10.1016/j.biortech.2016.03.129. ISSN 1873-2976. PMID 27053446.
  9. ^Xu, Bojun; Ge, Zheng; He, Zhen (2015-05-15). "Sediment microbial fuel cells for wastewater treatment: challenges and opportunities". Environmental Science: Water Research & Technology. 1 (3): 279–284. doi:10.1039/c5ew00020c. ISSN 2053-1419.
  10. ^Melnyk, Ryan A.; Engelbrektson, Anna; Clark, Iain C.; Carlson, Hans K.; Byrne-Bailey, Kathy; Coates, John D. (2011). "Identification of a Perchlorate Reduction Genomic Island with Novel Regulatory and Metabolic Genes". Applied and Environmental Microbiology. 77 (20): 7401–7404. doi:10.1128/AEM.05758-11. PMC 3194888. PMID 21856823.
  11. ^Reyes-Umana, Victor; Henning, Zachary; Lee, Kristina; Barnum, Tyler P.; Coates, John D. (2021-07-02). "Genetic and phylogenetic analysis of dissimilatory iodate-reducing bacteria identifies potential niches across the world's oceans". The ISME Journal: 1–12. doi:10.1038/s41396-021-01034-5. ISSN 1751-7370.
  12. ^Reyes-Umana, Victor; Henning, Zachary; Lee, Kristina; Barnum, Tyler; Coates, John (2020). "Genetic and phylogenetic analysis of dissimilatory iodate-reducing bacteria identifies potential niches across the world's oceans". bioRxiv 10.1101/2020.12.28.424624.
  13. ^ abRichter, Katrin; Schicklberger, Marcus; Gescher, Johannes (2012-02-01). "Dissimilatory reduction of extracellular electron acceptors in anaerobic respiration". Applied and Environmental Microbiology. 78 (4): 913–921. doi:10.1128/AEM.06803-11. ISSN 1098-5336. PMC 3273014. PMID 22179232.
  14. ^Wall, Judy D.; Krumholz, Lee R. (13 October 2006). "Uranium Reduction". Annual Review of Microbiology. 60: 149–166. doi:10.1146/annurev.micro.59.030804.121357. PMID 16704344.
  15. ^Holliger, C.; Wohlfarth, G.; Diekert, G. (1998). "Reductive dechlorination in the energy metabolism of anaerobic bacteria"(PDF). FEMS Microbiology Reviews. 22 (5): 383. doi:10.1111/j.1574-6976.1998.tb00377.x.
  16. ^Lovley, Derek R.; Fraga, Jocelyn L.; Coates, John D.; Blunt‐Harris, Elizabeth L. (1999). "Humics as an electron donor for anaerobic respiration". Environmental Microbiology. 1 (1): 89–98. doi:10.1046/j.1462-2920.1999.00009.x. PMID 11207721.
Sours: https://en.wikipedia.org/wiki/Anaerobic_respiration
Anaerobic respiration - anaerobic metabolism

5.9B: Nitrate Reduction and Denitrification

Key Terms

  • electron acceptor: An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process.
  • eutrophication: The process of becoming eutrophic.
  • facultative: Not obligate; optional, discretionary or elective

In anaerobic respiration, denitrification utilizes nitrate (NO3) as a terminal electron acceptor in the respiratory electron transport chain. Denitrification is a widely used process; many facultative anaerobes use denitrification because nitrate, like oxygen, has a high reduction potential

Denitrification is a microbially facilitated process involving the stepwise reduction of nitrate to nitrite (NO2) nitric oxide (NO), nitrous oxide (N2O), and, eventually, to dinitrogen (N2) by the enzymes nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase. The complete denitrification process can be expressed as a redox reaction: 2 NO3− + 10 e + 12 H+ → N2 + 6 H2O.

Protons are transported across the membrane by the initial NADH reductase, quinones and nitrous oxide reductase to produce the electrochemical gradient critical for respiration. Some organisms (e.g. E. coli) only produce nitrate reductase and therefore can accomplish only the first reduction leading to the accumulation of nitrite. Others (e.g. Paracoccus denitrificans or Pseudomonas stutzeri) reduce nitrate completely. Complete denitrification is an environmentally significant process because some intermediates of denitrification (nitric oxide and nitrous oxide) are significant greenhouse gases that react with sunlight and ozone to produce nitric acid, a component of acid rain. Denitrification is also important in biological wastewater treatment, where it can be used to reduce the amount of nitrogen released into the environment, thereby reducing eutrophication.

Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen is depleted and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in anaerobic environments where oxygen consumption exceeds the oxygen supply and where sufficient quantities of nitrate are present. These environments may include certain soils and groundwater, wetlands, oil reservoirs, poorly ventilated corners of the ocean, and in sea floor sediments.

image

Denitrification is performed primarily by heterotrophic bacteria (e.g. Paracoccus denitrificans), although autotrophic denitrifiers have also been identified (e.g., Thiobacillus denitrificans). Generally, several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.

Rhizobia are soil bacteria with the unique ability to establish a N2-fixing symbiosis on legume roots. When faced with a shortage of oxygen, some rhizobia species are able to switch from O2-respiration to using nitrates to support respiration.

The direct reduction of nitrate to ammonium (dissimilatory nitrate reduction) can be performed by organisms with the nrf- gene. This is a less common method of nitrate reduction than denitrification in most ecosystems. Other genes involved in denitrification include nir (nitrite reductase) and nos (nitrous oxide reductase), which are possessed by such organisms as Alcaligenes faecalis, Alcaligenes xylosoxidans, Pseudomonas spp, Bradyrhizobium japonicum, and Blastobacter denitrificans.

Sours: https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/5%3A_Microbial_Metabolism/5.09%3A_Anaerobic_Respiration/5.9B%3A_Nitrate_Reduction_and_Denitrification

Respiration nitrate anaerobic

Respiration of Nitrate and Nitrite

Nitrate reduction to ammonia via nitrite occurs widely as an anabolic process through which bacteria, archaea, and plants can assimilate nitrate into cellular biomass. Escherichia coli and related enteric bacteria can couple the eight-electron reduction of nitrate to ammonium to growth by coupling the nitrate and nitrite reductases involved to energy-conserving respiratory electron transport systems. In global terms, the respiratory reduction of nitrate to ammonium dominates nitrate and nitrite reduction in many electron-rich environments such as anoxic marine sediments and sulfide-rich thermal vents, the human gastrointestinal tract, and the bodies of warm-blooded animals. This review reviews the regulation and enzymology of this process in E. coli and, where relevant detail is available, also in Salmonella and draws comparisons with and implications for the process in other bacteria where it is pertinent to do so. Fatty acids may be present in high levels in many of the natural environments of E. coli and Salmonella in which oxygen is limited but nitrate is available to support respiration. In E. coli, nitrate reduction in the periplasm involves the products of two seven-gene operons, napFDAGHBC, encoding the periplasmic nitrate reductase, and nrfABCDEFG, encoding the periplasmic nitrite reductase. No bacterium has yet been shown to couple a periplasmic nitrate reductase solely to the cytoplasmic nitrite reductase NirB. The cytoplasmic pathway for nitrate reduction to ammonia is restricted almost exclusively to a few groups of facultative anaerobic bacteria that encounter high concentrations of environmental nitrate.

Sours: https://pubmed.ncbi.nlm.nih.gov/26443731/
Aerobic Vs Anaerobic Respiration

5.9A: Electron Donors and Acceptors in Anaerobic Respiration

Key Terms

  • anaerobic: Without oxygen; especially of an environment or organism.
  • reduction: A reaction in which electrons are gained and valence is reduced; often by the removal of oxygen or the addition of hydrogen.
  • anaerobic respiration: metabolic reactions and processes that take place in the cells of organisms that use electron acceptors other than oxygen

Anaerobic respiration is the formation of ATP without oxygen. This method still incorporates the respiratory electron transport chain, but without using oxygen as the terminal electron acceptor. Instead, molecules such as sulfate (SO42-), nitrate (NO3), or sulfur (S) are used as electron acceptors. These molecules have a lower reduction potential than oxygen; thus, less energy is formed per molecule of glucose in anaerobic versus aerobic conditions.

image

Many different types of electron acceptors may be used for anaerobic respiration. Denitrification is the utilization of nitrate (NO3) as the terminal electron acceptor. Nitrate, like oxygen, has a high reduction potential. This process is widespread, and used by many members of Proteobacteria. Many denitrifying bacteria can also use ferric iron (Fe3+) and different organic electron acceptors.

Sulfate reduction uses sulfate (SO2−4) as the electron acceptor, producing hydrogen sulfide (H2S) as a metabolic end product. Sulfate reduction is a relatively energetically poor process, and is used by many Gram negative bacteria found within the δ-Proteobacteria. It is also used in Gram-positive organisms related to Desulfotomaculum or the archaeon Archaeoglobus.

Sulfate reduction requires the use of electron donors, such as the carbon compounds lactate and pyruvate (organotrophic reducers), or hydrogen gas (lithotrophic reducers). Some unusual autotrophic sulfate-reducing bacteria, such as Desulfotignum phosphitoxidans, can use phosphite (HPO3) as an electron donor. Others, such as certain Desulfovibrio species, are capable of sulfur disproportionation (splitting one compound into an electron donor and an electron acceptor) using elemental sulfur (S0), sulfite (SO3−2), and thiosulfate (S2O32-) to produce both hydrogen sulfide (H2S) and sulfate (SO2−).

Acetogenesis is a type of microbial metabolism that uses hydrogen (H2) as an electron donor and carbon dioxide (CO2) as an electron acceptor to produce acetate, the same electron donors and acceptors used in methanogenesis.

Ferric iron (Fe3+) is a widespread anaerobic terminal electron acceptor used by both autotrophic and heterotrophic organisms. Electron flow in these organisms is similar to those in electron transport, ending in oxygen or nitrate, except that in ferric iron-reducing organisms the final enzyme in this system is a ferric iron reductase. Since some ferric iron-reducing bacteria (e.g.G. metallireducens) can use toxic hydrocarbons (e.g. toluene) as a carbon source, there is significant interest in using these organisms as bioremediation agents in ferric iron contaminated aquifers.

Other inorganic electron acceptors include the reduction of Manganic ion (Mn4+) to manganous (Mn2+), Selenate (SeO42−) to selenite (SeO32−) to selenium (Se), Arsenate (AsO43−) to arsenite (AsO33-), and Uranyl (UO22+) to uranium dioxide (UO2)

Organic compounds may also be used as electron acceptors in anaerobic respiration. These include the reduction of fumarate to succinate, Trimethylamine N-oxide (TMAO) to trimethylamine (TMA), and Dimethyl sulfoxide (DMSO) to Dimethyl sulfide (DMS).

Sours: https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/5%3A_Microbial_Metabolism/5.09%3A_Anaerobic_Respiration/5.9A%3A_Electron_Donors_and_Acceptors_in_Anaerobic_Respiration

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Growth Yields in Bacterial Denitrification and Nitrate Ammonification

1. Bak, F., and H. Cypionka. 1987. A novel type of energy metabolism involving fermentation of inorganic sulphur compounds. Nature326:891-892. [PubMed] [Google Scholar]

2. Bokranz, M., J. Katz, I. Schröder, A. M. Roberton, and A. Kröger. 1983. Energy metabolism and biosynthesis of Vibrio succinogenes growing with nitrate or nitrite as terminal electron acceptor. Arch. Microbiol.135:36-41. [Google Scholar]

3. Dimroth, P. 2000. Operation of the F0 motor of the ATP synthase. Biochim. Biophys. Acta1458:374-386. [PubMed] [Google Scholar]

4. Einsle, O., and P. M. Kroneck. 2004. Structural basis of denitrification. Biol. Chem.385:875-883. [PubMed] [Google Scholar]

5. Engelbrecht, S., and W. Junge. 1997. ATP synthase: a tentative structural model. FEBS Lett.414:485-491. [PubMed] [Google Scholar]

6. Ferguson, S. J. 1994. Denitrification and its control. Antonie Leeuwenhoek66:89-110. [PubMed] [Google Scholar]

7. Heldt, H. W. 2003. Pflanzenbiochemie. Spektrum Akademischer Verlag, Heidelberg, Germany.

8. John, P., and F. R. Whatley. 1970. Oxidative phosphorylation coupled to oxygen uptake and nitrate reduction in Micrococcus denitrificans. Biochim. Biophys. Acta216:342-352. [PubMed] [Google Scholar]

9. Koike, I., and A. Hattori. 1975. Growth yield of a denitrifying bacterium, Pseudomonas denitrificans, under aerobic and denitrifying conditions. J. Gen. Microbiol.88:1-10. [PubMed] [Google Scholar]

10. Koike, I., and A. Hattori. 1975. Energy yield of denitrification: an estimate from growth yield in continuous cultures of Pseudomonas denitrificans under nitrate-, nitrite- and oxide-limited conditions. J. Gen. Microbiol.88:11-19. [PubMed] [Google Scholar]

11. Naik, M. S., and D. J. D. Nicholas. 1966. Phosphorylation associated with nitrate and nitrite reduction in Micrococcus denitrificans and Pseudomonas denitrificans. Biochim. Biophys. Acta113:490-497. [PubMed] [Google Scholar]

12. Otte, S., J. G. Kuenen, L. P. Nielsen, H. W. Paerl, J. Zopfi, H. N. Schulz, A. Teske, B. Strotmann, V. A. Gallardo, and B. B. Jørgensen. 1999. Nitrogen, carbon, and sulfur metabolism in natural Thioploca samples. Appl. Environ. Microbiol.65:3148-3157. [PMC free article] [PubMed] [Google Scholar]

13. Platen, H., and B. Schink. 1989. Anaerobic degradation of acetone and higher ketones via carboxylation by newly isolated denitrifying bacteria. J. Gen. Microbiol.135:883-891. [PubMed] [Google Scholar]

14. Schink, B. 1984. Fermentation of 2.3-butanediol by Pelobacter carbinolicus sp. nov. and Pelobacter propionicus sp. nov., and evidence for propionate formation from C2 compounds. Arch. Microbiol.137:33-41. [Google Scholar]

15. Schink, B. 1989. Mikrobielle Lebensgemeinschaften in Gewässersedimenten. Naturwissenschaften76:364-372. [Google Scholar]

16. Schulz, H. N., T. Brinkhoff, T. G. Ferdelman, M. H. Marine, A. Teske, and B. B. Jørgensen. 1999. Dense populations of a giant sulfur bacterium in Namibian shelf sediments. Science284:493-495. [PubMed] [Google Scholar]

17. Schulz, H. N., and B. B. Jørgensen. 2001. Big bacteria. Annu. Rev. Microbiol.55:105-137. [PubMed] [Google Scholar]

18. Seelert, H., A. Poetsch, N. A. Dencher, A. Engel, H. Stahlberg, and D. J. Müller. 2000. Proton-powered turbine of a plant motor. Nature405:418-419. [PubMed] [Google Scholar]

19. Seitz, H.-J., and H. Cypionka. 1986. Chemolithotrophic growth of Desulfovibrio desulfuricans with hydrogen coupled to ammonification of nitrate and nitrite. Arch. Microbiol.146:63-67. [Google Scholar]

20. Simon, J. 2002. Enzymology and bioenergetics of respiratory nitrite ammonification. FEMS Microbiol. Rev.26:285-309. [PubMed] [Google Scholar]

21. Stouthamer, A. H. 1979. The search for correlation between theoretical and experimental growth yields. Int. Rev. Biochem. Microb. Biochem.21:1-47. [Google Scholar]

22. Stouthamer, A. H. 1988. Dissimilatory reduction of oxidized nitrogen compounds, p. 245-303. In A. J. B. Zehnder (ed.), Biology of anaerobic microorganisms. John Wiley and Sons, New York, NY.

23. Stouthamer, A. H. 1988. Bioenergetics and yields with electron acceptors other than oxygen, p. 345-437. In L. E. Erickson and D. Y.-C. Fung (ed.), Handbook on anaerobic fermentations. Marcel Dekker Inc., New York, NY.

24. Stouthamer, A. H., A. P. de Boer, J. van der Oost, and R. J. van Spanning. 1997. Emerging principles of inorganic nitrogen metabolism in Paracoccus denitrificans and related bacteria. Antonie Leeuwenhoek71:33-41. [PubMed] [Google Scholar]

25. Thauer, R. K., K. Jungermann, and K. Decker. 1977. Energy conservation in chemotrophic anaerobic bacteria. Bacteriol. Rev.41:100-180. [PMC free article] [PubMed] [Google Scholar]

26. Tiedje, J. M. 1988. Ecology of denitrification and dissimilatory nitrate reduction to ammonium, p. 179-244. In A. J. B. Zehnder (ed.), Biology of anaerobic microorganisms. John Wiley and Sons, New York, NY.

27. Tiedje, J. M., A. J. Sexstone, D. D. Myrold, and J. A. Robinson. 1982. Denitrification: ecological niches, competition and survival. Antonie Leeuwenhoek48:569-583. [PubMed] [Google Scholar]

28. Tran, Q. H., and G. Unden. 1998. Changes in the proton potential and the cellular energetics of Escherichia coli during growth by aerobic and anaerobic respiration or by fermentation. Eur. J. Biochem.251:538-543. [PubMed] [Google Scholar]

29. Widdel, F., and N. Pfennig. 1981. Studies on dissimilatory sulfate-reducing bacteria that decompose fatty acids. I. Isolation of new sulfate-reducing bacteria enriched with acetate from saline environments. Description of Desulfobacter postgatei gen. nov., sp. nov. Arch. Microbiol.129:395-400. [PubMed] [Google Scholar]

30. Zehnder, A. J. B., and W. Stumm. 1988. Geochemistry and biogeochemistry of anaerobic habitats, p. 1-38. In A. J. B. Zehnder (ed.), Biology of anaerobic microorganisms. John Wiley and Sons, New York, NY.

31. Ziegler, K., R. Buder, J. Winter, and G. Fuchs. 1989. Activation of aromatic acids and aerobic 2-aminobenzoate metabolism in a denitrifying Pseudomonas strain. Arch. Microbiol.151:171-176. [Google Scholar]

32. Zumft, W. G. 1997. Cell biology and molecular basis of denitrification. Microbiol. Mol. Biol. Rev.61:533-616. [PMC free article] [PubMed] [Google Scholar]

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