Methanogens are anaerobic archaea that produce methane as a byproduct of their energy metabolism, i.e., catabolism. Methane production, or methanogenesis, is the only biochemical pathway for ATP generation in methanogens. All known methanogens belong exclusively to the domain Archaea, although some bacteria, plants, and animal cells are also known to produce methane. However, the biochemical pathway for methane production in these organisms differs from that in methanogens and does not contribute to ATP formation. Methanogens belong to various phyla within the domain Archaea. Previous studies placed all known methanogens into the superphylum Euryarchaeota. However, recent phylogenomic data have led to their reclassification into several different phyla. Methanogens are common in various anoxic environments, such as marine and freshwater sediments, wetlands, the digestive tracts of animals, wastewater treatment plants, rice paddy soil, and landfills. While some methanogens are extremophiles, such as Methanopyrus kandleri, which grows between 84 and 110°C, or Methanonatronarchaeum thermophilum, which grows at a pH range of 8.2 to 10.2 and a Na+ concentration of 3 to 4.8 M, most of the isolates are mesophilic and grow around neutral pH.
Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. It is the fourth and final stage of anaerobic digestion. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism. In anoxic environments, it is the final step in the decomposition of biomass. Methanogenesis is responsible for significant amounts of natural gas accumulations, the remainder being thermogenic.
An acetogen is a microorganism that generates acetate (CH3COO−) as an end product of anaerobic respiration or fermentation. However, this term is usually employed in a narrower sense only to those bacteria and archaea that perform anaerobic respiration and carbon fixation simultaneously through the reductive acetyl coenzyme A (acetyl-CoA) pathway (also known as the Wood-Ljungdahl pathway). These genuine acetogens are also known as "homoacetogens" and they can produce acetyl-CoA (and from that, in most cases, acetate as the end product) from two molecules of carbon dioxide (CO2) and four molecules of molecular hydrogen (H2). This process is known as acetogenesis, and is different from acetate fermentation, although both occur in the absence of molecular oxygen (O2) and produce acetate. Although previously thought that only bacteria are acetogens, some archaea can be considered to be acetogens.
In biology, syntrophy, syntrophism, or cross-feeding is the cooperative interaction between at least two microbial species to degrade a single substrate. This type of biological interaction typically involves the transfer of one or more metabolic intermediates between two or more metabolically diverse microbial species living in close proximity to each other. Thus, syntrophy can be considered an obligatory interdependency and a mutualistic metabolism between different microbial species, wherein the growth of one partner depends on the nutrients, growth factors, or substrates provided by the other(s).
Methanosarcinales is an order of Archaea in the class Methanomicrobia, phylum Methanobacteriota. The order Methanosarcinales contains both methanogenic and methanotrophic lineages, although the latter have so far no pure culture representatives. Methanotrophic lineages of the order Methanosarcinales were initially abbreviated as ANME to distinguich from aerobic methanotrophic bacteria. Currently, those lineages receive their own names such as Ca. Methanoperedens, Ca. Methanocomedens (ANME-2a), Ca.Methanomarinus (ANME-2b), Ca. Methanogaster (ANME-2c), Ca. Methanovorans (ANME-3). The order contains archaeon with one of the largest genome, Methanosarcina acetivorans C2A, genome size 5,75 Mbp.
In taxonomy, the Methanosarcinaceae are a family of the Methanosarcinales.
The Wood–Ljungdahl pathway is a set of biochemical reactions used by some bacteria. It is also known as the reductive acetyl-coenzyme A (acetyl-CoA) pathway. This pathway enables these organisms to use hydrogen as an electron donor, and carbon dioxide as an electron acceptor and as a building block for biosynthesis.
In taxonomy, Methanococcoides is a genus of the Methanosarcinaceae.
Methanobacterium is a genus of the Methanobacteria class in the Archaea kingdom, which produce methane as a metabolic byproduct. Despite the name, this genus belongs not to the bacterial domain but the archaeal domain. Methanobacterium are nonmotile and live without oxygen, which is toxic to them, and they only inhabit anoxic environments.
Methanosphaera is a genus of microbes within the family Methanobacteriaceae. It was distinguished from other genera within Methanobacteriaceae in 1985 on the basis of the oligonucleotide sequence of its 16S RNA. Like other archaea within Methanobacteriaceae, those of Methanosphaera are methanogens, but while most use formate to reduce carbon dioxide, those of Methanosphaera use hydrogen to reduce methanol to methane.
Coenzyme F420 is a family of coenzymes involved in redox reactions in a number of bacteria and archaea. It is derived from coenzyme FO (7,8-didemethyl-8-hydroxy-5-deazariboflavin) and differs by having a oligoglutamyl tail attached via a 2-phospho-L-lactate bridge. F420 is so named because it is a flavin derivative with an absorption maximum at 420 nm.
In the taxonomy of microorganisms, the Methanothrix is a genus of methanogenic archaea within the Euryarchaeota. Methanothrix cells were first isolated from a mesophilic sewage digester but have since been found in many anaerobic and aerobic environments. Methanothrix were originally understood to be obligate anaerobes that can survive exposure to high concentrations of oxygen, but recent studies have shown at least one Candidatus operational taxonomic unit proposed to be in the Methanothrix genus not only survives but remains active in oxic soils. This proposed species, Ca. Methanothrix paradoxum, is frequently found in methane-releasing ecosystems and is the dominant methanogen in oxic soils.
Methanobrevibacter cuticularis is a species of methanogen archaeon. It was first isolated from the hindgut of the termite Reticulitermes flavipes. It is rod-shaped, ranging in size from 0.34 to 1.6 μm and possesses polar fibers. Its morphology, gram-positive staining reaction, resistance to cell lysis by chemical agents and narrow range of utilizable substrates are typical of species belonging to the family Methanobacteriaceae. It habitates on or near the hindgut epithelium and also attached to filamentous prokaryotes associated with the gut wall. It is one of the predominant gut biota.
Methanobrevibacter curvatus is a species of methanogen archaeon. It was first isolated from the hindgut of the termite Reticulitermes flavipes. It is rod-shaped, ranging in size from 0.34 to 1.6 μm and possesses polar fibers. Its morphology, gram-positive staining reaction, resistance to cell lysis by chemical agents and narrow range of utilisable substrates are typical of species belonging to the family Methanobacteriaceae.
Methanogenium frigidum is a psychrophilic, H2-using methanogen from Ace Lake, Antarctica.
Methanosphaera stadtmanae is a methanogen archaeon. It is a non-motile, Gram-positive, spherical-shaped organism that obtains energy by using hydrogen to reduce methanol to methane. It does not possess cytochromes and is part of the large intestine's biota.
Methanococcus maripaludis is a species of methanogenic archaea found in marine environments, predominantly salt marshes. M. maripaludis is a non-pathogenic, gram-negative, weakly motile, non-spore-forming, and strictly anaerobic mesophile. It is classified as a chemolithoautotroph. This archaeon has a pleomorphic coccoid-rod shape of 1.2 by 1.6 μm, in average size, and has many unique metabolic processes that aid in survival. M. maripaludis also has a sequenced genome consisting of around 1.7 Mbp with over 1,700 identified protein-coding genes. In ideal conditions, M. maripaludis grows quickly and can double every two hours.
Methanococcoides methylutens is a methylotrophic marine methanogen, the type species of its genus. It utilises trimethylamine, diethylamine, monomethylamine, and methanol as substrates for growth and methanogenesis. Cells are non-motile, non-spore-forming, irregular cocci 1 μm in diameter which stain Gram-negative and occur singly or in pairs. TMA-10 is the type strain.
Methanohalophilus mahii is an obligately anaerobic, methylotrophic, methanogenic cocci-shaped archaeon of the genus Methanohalophilus that can be found in high salinity aquatic environments. The name Methanohalophilus is said to be derived from methanum meaning "methane" in Latin; halo meaning "salt" in Greek; and mahii meaning "of Mah" in Latin, after R.A. Mah, who did substantial amounts of research on aerobic and methanogenic microbes. The proper word in ancient Greek for "salt" is however hals (ἅλς). The specific strain type was designated SLP and is currently the only identified strain of this species.
Methanosarcina barkeri is the type species of the genus Methanosarcina, characterized by its wide range of substrates used in methanogenesis. While most known methanogens produce methane from H2 and CO2, M. barkeri can also dismutate methylated compounds such as methanol or methylamines, oxidize acetate, and reduce methylated compounds with H2. This makes M. barkeri one of the few Methanosarcina species capable of utilizing all four known methanogenesis pathways. Even among other Methanosarcinales, which commonly utilize a broad range of substrates, the ability to grow on H2 and CO2 is rare due to the requirement for high H2 partial pressure. Like other Methanosarcina species, M. barkeri has a large genome (4.53 Mbp for the type strain MS, 4.9 Mbp for the Wiesmoor strain, and 4.5 Mbp for the CM2 strain), although it is significantly smaller than the largest archaeal genome of Methanosarcina acetivorans (5.75 Mbp for the type strain C2A). It is also one of the few archaea, particularly among anaerobic species, that is genetically tractable and can be used for genetic studies.
