Methanotroph

Methanotrophs (sometimes called methanophiles) are prokaryotes that metabolize methane as their only source of carbon and energy. They can be either bacteria or archaea and can grow aerobically or anaerobically, and require single-carbon compounds to survive. Methanotrophs (sometimes called methanophiles) are prokaryotes that metabolize methane as their only source of carbon and energy. They can be either bacteria or archaea and 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 also methanotrophs exist that 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. 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 which differentiates them from methanotrophs that are usually fastidious methane and methanol oxidizers. The only facultative methanotrophs isolated to date are members of the genus Methylocella and Methylocystis. 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 four subgroups: two methane-assimilating bacteria (MAB) groups, the methanotrophs, and two autotrophic ammonia-oxidizing bacteria (AAOB). Methantrophs 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. These include the Methylococcaceae and Methylocystaceae. Although both are included among the Proteobacteria, they are members of different subclasses. Other methanotroph species are found in the Verrucomicrobiae. Among the methanotrophic archaea, several subgroups are determined. 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 , 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 utilize the serine pathway of carbon assimilation. They also characteristically have a system of internal membranes within which methane oxidation occurs. No methanotrophic archaea are capable of using oxygen. 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 or nitrite, and couple that to methane oxidation. Investigations in marine environments revealed that methane can be oxidized anaerobically by consortia of methane oxidizing archaea and sulfate-reducing bacteria. This type of Anaerobic oxidation of methane (AOM) mainly occurs in anoxic marine sediments. The exact mechanism behind this 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 substance. This unknown intermediate is then used by the sulfate-reducing bacteria to gain energy from the reduction of sulfate to hydrogen sulfide. The anaerobic methanotrophs are not related to the known aerobic methanotrophs; the closest cultured relative to the anaerobic methanotrophs are the methanogens in the order Methanosarcinales.. Metal-oxides, such as manganese and iron, can also be used as terminal electron acceptors by ANME. For this, no consortium is needed. ANME shuttle electrons directly to the abiotic particles, which get reduced chemically . In some cases, aerobic methane oxidation can take place in anoxic (no oxygen) 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 organisms, that they then directly consume to oxidise methane aerobically . Methylococcus capsulatus is utilised to produce animal feed from natural gas.