Methanotrophs are especially common in or near environments where methane is produced, although some methanotrophs can oxidize
atmospheric methane. 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, as they play a significant role in the global methane budget, by reducing the amount of methane emitted to the atmosphere.[1][2]
Methanotrophy is a special case of
methylotrophy, 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 methane and methanol oxidizers. The only facultative methanotrophs isolated to date are members of the genus Methylocella silvestris,[3][4]Methylocapsa aurea[5] and several Methylocystis strains.[6]
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 (AAOB), which cooxidize methane.[2]
Classification
Methanotrophs can be either
bacteria or
archaea. Which methanotroph species is present is mainly determined by the availability of
electron acceptors. 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 and
methane to form
formaldehyde, 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 and they use the RuMP pathway to assimilate carbon. Type II methanotrophs are part of the
Alphaproteobacteria and use the serine pathway of carbon assimilation. They also characteristically have a system of internal membranes within which methane
oxidation occurs. Methanotrophs in
Gammaproteobacteria are known from the family Methylococcaceae.[7] Methanotrophs from
Alphaproteobacteria are found in families Methylocystaceae and Beijerinckiaceae.
In some cases, aerobic methane oxidation can take place in anoxic environments. "CandidatusMethylomirabilis 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.[12][13] In clear water lakes, methanotrophs can live in the anoxic water column, but receive oxygen from
photosynthetic organisms, which they then directly consume to oxidize methane.[14]
Under anoxic conditions, methanotrophs use different
electron acceptors for methane oxidation. This can happen in
anoxic habitats such as marine or lake
sediments,
oxygen minimum zones, anoxic water columns, rice paddies and soils. Some specific methanotrophs can reduce nitrate,[15] nitrite,[16] iron,[17] sulfate,[18] 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 and
sulfate-reducing bacteria.[19][20] This type of
anaerobic oxidation of methane (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 use the reversed
methanogenesis 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 and water.
The anaerobic methanotrophs are not related to the known aerobic methanotrophs; the closest cultured relatives to the anaerobic methanotrophs are the
methanogens in the
orderMethanosarcinales.[21]
In 2010 a new bacterium Candidatus Methylomirabilis oxyfera from the
phylum NC10 was identified that can couple the
anaerobic oxidation of methane to nitrite reduction without the need for a
syntrophic partner.[12] Based on studies of Ettwig et al.,[12] it is believed that M. oxyfera oxidizes methane anaerobically by utilizing oxygen produced internally from the
dismutation of
nitric oxide 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.[23]
Methane oxidation
RuMP pathway in type I methanotrophs
Serine pathway in type II methanotrophs
Methanotrophs oxidize methane by first initiating reduction of dioxygen to H2O2 and transformation of methane to CH3OH using
methane monooxygenases (MMOs).[24] 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.[24] 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.[25]
^Thauer RK (June 2011). "Anaerobic oxidation of methane with sulfate: on the reversibility of the reactions that are catalyzed by enzymes also involved in methanogenesis from CO2". Current Opinion in Microbiology. 14 (3): 292–9.
doi:
10.1016/j.mib.2011.03.003.
PMID21489863.
^Lieberman RL, Rosenzweig AC (2004). "Biological methane oxidation: regulation, biochemistry, and active site structure of particulate methane monooxygenase". Critical Reviews in Biochemistry and Molecular Biology. 39 (3): 147–64.
doi:
10.1080/10409230490475507.
PMID15596549.
S2CID21628195.