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.
Describe various types of electron acceptors and donors including: nitrate, sulfate, hydrgoen, carbon dioxide and ferric iron
- 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.
- 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.
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.
Outline the processes of nitrate reduction and denitrification and the organisms that utilize it
- 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.
- 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.
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.
Outline the process of sulfate and sulfur reduction including its various purposes
- 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.
- 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.
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 is a form of anaerobic respiration that uses carbon as a electron acceptor and results in the production of methane.
Recognize the characteristics associated with methanogenesis
- 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.
- 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:
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.
Anaerobic respiration utilizes highly reduced species – such as a proton gradient – to establish electrochemical membrane gradients.
Outline the role of the proton motive force in metabolism
- 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.
- 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.
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.
Describe the process of anoxic hydrocarbon oxidation in regards to marine environments
- 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%.
- 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.
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.