Methanotrophs (sometimes called methanophiles) are
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
s that
metabolize
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
as their source of
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
chemical energy
Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (2018). "How ...
. They are
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
or
archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
, can grow
aerobically or
anaerobically, and require single-carbon
compounds to survive.
Methanotrophs are especially common in or near environments where methane is produced, although some methanotrophs can oxidize
atmospheric methane
Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising.
The 20-year globa ...
. Their habitats include wetlands, soils, marshes, rice paddies, landfills, aquatic systems (lakes, oceans, streams) and more. They are of special interest to researchers studying
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, as they play a significant role in the global methane budget, by reducing the amount of methane emitted to the atmosphere.
Methanotrophy is a special case of
methylotrophy
Methylotrophs are a diverse group of microorganisms that can use reduced one-carbon compounds, such as methanol or methane, as the carbon source for their growth; and multi-carbon compounds that contain no carbon-carbon bonds, such as dimethyl e ...
, using single-carbon compounds that are more reduced than carbon dioxide. Some methylotrophs, however, can also make use of multi-carbon compounds; this differentiates them from methanotrophs, which are usually
fastidious
A fastidious organism is any organism that has complex or particular nutritional requirements. In other words, a fastidious organism will only grow when specific nutrients are included in its medium. The more restrictive term fastidious microorga ...
methane and methanol oxidizers. The only facultative methanotrophs isolated to date are members of the genus ''
Methylocella silvestris
''Methylocella silvestris'' is a bacterium from the genus ''Methylocella spp'' which are found in many acidic soils and wetlands. Historically, ''Methylocella silvestris'' was originally isolated from acidic forest soils in Germany, and it is de ...
'', ''
Methylocapsa aurea
''Methylocapsa aurea'' is a Gram-negative, aerobic, non-motile bacteria from the genus of Methylocapsa which was isolated from forest soil in Germany. It is a facultative methanotroph
Methanotrophs (sometimes called methanophiles) are prokaryo ...
'' and several ''
Methylocystis
''Methylocystis'' is a genus of bacteria from the family ''Methylocystaceae
The Methylocystaceae are a family of bacteria that are capable of obtaining carbon and energy from methane. Such bacteria are called methanotrophs, and in particular ...
'' strains.
In functional terms, methanotrophs are referred to as methane-oxidizing bacteria. However, methane-oxidizing bacteria encompass other organisms that are not regarded as sole methanotrophs. For this reason, methane-oxidizing bacteria have been separated into subgroups: methane-assimilating bacteria (MAB) groups, the methanotrophs, and autotrophic
ammonia-oxidizing bacteria Nitrifying bacteria are chemolithotrophic organisms that include species of genera such as ''Nitrosomonas'', '' Nitrosococcus'', ''Nitrobacter'', '' Nitrospina'', ''Nitrospira'' and '' Nitrococcus''. These bacteria get their energy from the oxidati ...
(AAOB), which cooxidize methane.
[
]
Classification
Methanotrophs can be either bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
or archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
. Which methanotroph species is present is mainly determined by the availability of 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. Electron acceptors are sometimes mista ...
s. Many types of methane oxidizing bacteria (MOB) are known. Differences in the method of formaldehyde fixation and membrane structure divide these bacterial methanotrophs into several groups. There are several subgroups among the methanotrophic archaea.
Aerobic
Under aerobic conditions, methanotrophs combine oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
to form formaldehyde
Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
, which is then incorporated into organic compounds via the serine pathway or the ribulose monophosphate (RuMP) pathway, and carbon dioxide, which is released. Type I and type X methanotrophs are part of the Gammaproteobacteria
Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically imp ...
and they use the RuMP pathway to assimilate carbon. Type II methanotrophs are part of the Alphaproteobacteria
Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...
and use the serine pathway of carbon assimilation. They also characteristically have a system of internal membranes within which methane oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
occurs. Methanotrophs in Gammaproteobacteria
Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically imp ...
are known from the family ''Methylococcaceae
The Methylococcaceae are a family of bacteria that obtain their carbon and energy from methane, called methanotrophs..
They comprise the type I methanotrophs, in contrast to the Methylocystaceae or type II methanotrophs. They belong to Gammapr ...
''. Methanotrophs from Alphaproteobacteria
Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...
are found in families ''Methylocystaceae
The Methylocystaceae are a family of bacteria that are capable of obtaining carbon and energy from methane. Such bacteria are called methanotrophs, and in particular the Methylocystaceae comprise the type II methanotrophs, which are structurally ...
'' and ''Beijerinckiaceae
The Beijerinckiaceae are a family of Hyphomicrobiales named after the Dutch microbiologist Martinus Willem Beijerinck. '' Beijerinckia'' is a genus of free-living aerobic nitrogen-fixing bacteria. Acidotolerant ''Beijerinckiaceae'' has been sho ...
''.
Aerobic methanotrophs are also known from the '' Methylacidiphilaceae'' (phylum Verrucomicrobiota
Verrucomicrobiota is a phylum of Gram-negative bacteria that contains only a few described species. The species identified have been isolated from fresh water, marine and soil environments and human faeces. A number of as-yet uncultivated species ...
). In contrast to Gammaproteobacteria
Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically imp ...
and Alphaproteobacteria
Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...
, methanotrophs in the phylum Verrucomicrobiota
Verrucomicrobiota is a phylum of Gram-negative bacteria that contains only a few described species. The species identified have been isolated from fresh water, marine and soil environments and human faeces. A number of as-yet uncultivated species ...
are mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...
s. In 2021 a bacterial bin from the phylum Gemmatimonadota
The Gemmatimonadota are a phylum of bacteria established in 2003. The phylum contains two classes Gemmatimonadetes and Longimicrobia.
Species
The type species '' Gemmatimonas aurantiaca'' strain T-27T was isolated from activated sludge in a sewa ...
called "''Candidatus'' Methylotropicum kingii" showing aerobic methanotrophy was discovered thus suggesting methanotrophy to be present in the four bacterial phyla.
In some cases, aerobic methane oxidation can take place in anoxic environments. "''Candidatus'' Methylomirabilis oxyfera" belongs to the phylum NC10 bacteria, and can catalyze nitrite reduction through an "intra-aerobic" pathway, in which internally produced oxygen is used to oxidise methane. In clear water lakes, methanotrophs can live in the anoxic water column, but receive oxygen from photosynthetic
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
organisms, which they then directly consume to oxidize methane.
No aerobic methanotrophic archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
are known.
Anaerobic
Under anoxic conditions, methanotrophs use different 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. Electron acceptors are sometimes mista ...
s for methane oxidation. This can happen in anoxic
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
habitats such as marine or lake sediments
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand a ...
, oxygen minimum zone
The oxygen minimum zone (OMZ), sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about , depending on local circumstances. OMZs are found worl ...
s, anoxic water columns, rice paddies and soils. Some specific methanotrophs can reduce nitrate, nitrite, iron, sulfate, or manganese ions and couple that to methane oxidation without syntrophic partner. Investigations in marine environments revealed that methane can be oxidized anaerobically by consortia of methane oxidizing archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
and sulfate-reducing bacteria
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...
. This type of anaerobic oxidation of methane
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
(AOM) mainly occurs in anoxic marine sediments. The exact mechanism is still a topic of debate but the most widely accepted theory is that the archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
use the reversed methanogenesis
Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...
pathway to produce carbon dioxide and another, unknown intermediate, which is then used by the sulfate-reducing bacteria to gain energy from the reduction of sulfate to hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
and water.
The anaerobic methanotrophs are not related to the known aerobic methanotrophs; the closest cultured relatives to the anaerobic methanotrophs are the methanogen
Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are com ...
s in the order Methanosarcinales
In taxonomy, the Methanosarcinales are an order of the Methanomicrobia.
Large amounts of methane are produced in marine sediments but are then consumed before contacting aerobic waters or the atmosphere. Although no organism that can consume ...
.
Special species
''Methylococcus capsulatus
''Methylococcus capsulatus'' is an obligately methanotrophic gram-negative, non-motile coccoid bacterium. ''M. capsulatus'' are thermotolerant; their cells are encapsulated and tend to have a diplococcoid shape. In addition to methane, ''M. ...
'' is used to produce animal feed from natural gas.
In 2010 a new bacterium ''Candidatus'' Methylomirabilis oxyfera from the phylum NC10 was identified that can couple the anaerobic oxidation of methane
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
to nitrite reduction without the need for a syntrophic partner. Based on studies of Ettwig et al.,[ it is believed that ''M. oxyfera'' oxidizes methane anaerobically by utilizing oxygen produced internally from the ]dismutation
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...
of nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
into nitrogen and oxygen gas.
Taxonomy
Many methanotrophic cultures have been isolated and formally characterized over the past 5 decades, starting with the classical study of Whittenbury (Whittenbury et al., 1970). Currently, 18 genera of cultivated aerobic methanotrophic ''Gammaproteobacteria'' and 5 genera of ''Alphaproteobacteria'' are known, represented by approx. 60 different species.
Methane oxidation
Methanotrophs oxidize methane by first initiating reduction of an oxygen atom to H2O2 and transformation of methane to CH3OH using methane monooxygenases (MMOs). Furthermore, two types of MMO have been isolated from methanotrophs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO).
Cells containing pMMO have demonstrated higher growth capabilities and higher affinity for methane than sMMO containing cells. It is suspected that copper ions may play a key role in both pMMO regulation and the enzyme catalysis, thus limiting pMMO cells to more copper-rich environments than sMMO producing cells.
See also
* Borg (microbiology)
References
{{Reflist, 2
External links
*Anaerobic oxidation of methane
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
Methane-Eating Bug Holds Promise For Cutting Greenhouse Gas
Media Release, GNS Science, New Zealand
Pseudomonadota