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 common in wetlands, where they are responsible for marsh gas, and in the digestive tracts of animals such as ruminants and many humans, where they are responsible for the methane content of belching in ruminants and flatulence in humans. In marine sediments, the biological production of methane, also termed methanogenesis, is generally confined to where sulfates are depleted, below the top layers. Moreover, methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments. Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of Earth's crust, kilometers below the surface.
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.
Methanosarcina acetivorans is a versatile methane producing microbe which is found in such diverse environments as oil wells, trash dumps, deep-sea hydrothermal vents, and oxygen-depleted sediments beneath kelp beds. Only M. acetivorans and microbes in the genus Methanosarcina use all three known metabolic pathways for methanogenesis. Methanosarcinides, including M. acetivorans, are also the only archaea capable of forming multicellular colonies, and even show cellular differentiation. The genome of M. acetivorans is one of the largest archaeal genomes ever sequenced. Furthermore, one strain of M. acetivorans, M. a. C2A, has been identified to possess an F-type ATPase along with an A-type ATPase.
Methanosarcina is a genus of euryarchaeote archaea that produce methane. These single-celled organisms are known as anaerobic methanogens that produce methane using all three metabolic pathways for methanogenesis. They live in diverse environments where they can remain safe from the effects of oxygen, whether on the earth's surface, in groundwater, in deep sea vents, and in animal digestive tracts. Methanosarcina grow in colonies.
Methanococcus is a genus of coccoid methanogens of the family Methanococcaceae. They are all mesophiles, except the thermophilic M. thermolithotrophicus and the hyperthermophilic M. jannaschii. The latter was discovered at the base of a “white smoker” chimney at 21°N on the East Pacific Rise and it was the first archaeal genome to be completely sequenced, revealing many novel and eukaryote-like elements.
In taxonomy, the Methanobacteriales are an order of the Methanobacteria. Species within this order differ from other methanogens in that they can use fewer catabolic substrates and have distinct morphological characteristics, lipid compositions, and RNA sequences. Their cell walls are composed of pseudomurein. Most species are Gram-positive with rod-shaped bodies and some can form long filaments. Most of them use formate to reduce carbon dioxide, but those of the genus Methanosphaera use hydrogen to reduce methanol to methane.
Methanospirillaceae are a family of microbes within Methanomicrobiales.
Methanogenium is a genus of archaeans in the family Methanomicrobiaceae. The type species is Methanogenium cariaci.
In taxonomy, Methanococcoides is a genus of the Methanosarcinaceae.
In taxonomy, Methanohalophilus is a genus of the Methanosarcinaceae.
In taxonomy, Methanolobus is a genus of methanogenic archaea within the Methanosarcinaceae. These organisms are strictly anaerobes and live exclusively through the production of methane, but the species within Methanolobus cannot use carbon dioxide with hydrogen, acetate or formate, only methyl compounds. The cells are irregular coccoid in form and approximately 1 μm in diameter. They do not form endospores. They are Gram negative and only some are motile, via a single flagellum. They are found in lake and ocean sediments that lack oxygen.
In taxonomy, Methanosalsum is a genus of microbes within the family Methanosarcinaceae. This genus contains two species.
In taxonomy, Methanobrevibacter is a genus of the Methanobacteriaceae. The species within Methanobrevibacter are strictly anaerobic archaea that produce methane, for the most part through the reduction of carbon dioxide via hydrogen. Most species live in the intestines of larger organisms, such as termites and are responsible for the large quantities of greenhouse gases that they produce. Mbr. smithii, found in the human intestine, may play a role in obesity.
Methanoculleus is a genus of microbes within the family Methanomicrobiaceae. The species of the genus Methanoculleus live in marine environments brackish water, and are very common in bioreactors, landfills, and wastewater. Unlike other archaea, Methanoculleus and some species of related genera can use ethanol and some secondary alcohols as electron donors as they produce methane. This has implications as the production of methane as a greenhouse gas and consequences with respect to global climate change.
In taxonomy, Methanolacinia is a genus of the Methanomicrobiaceae. The cells are bar-shaped and irregular 0.6 μm in diameter and 1.5–2.5 μm in length. They do not form endospores. Most are non-motile, but some have a single flagellum. They are strictly anaerobic. They produce methane through the reduction of carbon dioxide with hydrogen and cannot use formate, acetate or methyl compounds as substrates.
In taxonomy, Methanomicrobium is a genus of the Methanomicrobiaceae. The cells are shaped like short bars and do not form endospores. They produce methane via the reduction of carbon dioxide with hydrogen or formate. They cannot metabolize acetate, methylamines, or methanol.
In taxonomy, Methanoplanus is a genus of the Methanomicrobiaceae, comprising three species of methanogenic, or methane-producing, archaea. The cells are irregular coccoid in shape, tend to stain Gram-negative and do not form endospores.
The Methanosarcinales S-layer Tile Protein (MSTP) is a protein family found almost exclusively in Methanomicrobia members of the order Methanosarcinales. Typically a tandem repeat of two DUF1608 domains are contained in a single MSTP protein chain and these proteins self-assemble into the protective proteinaceous surface layer (S-layer) structure that encompasses the cell. The S-layer, which is found in most Archaea, and in many bacteria, serves many crucial functions including protection from deleterious extracellular substances.
Methanogens are a group of microorganisms that produce methane as a byproduct of their metabolism. They play an important role in the digestive system of ruminants. The digestive tract of ruminants contains four major parts: rumen, reticulum, omasum and abomasum. The food with saliva first passes to the rumen for breaking into smaller particles and then moves to the reticulum, where the food is broken into further smaller particles. Any indigestible particles are sent back to the rumen for rechewing. The majority of anaerobic microbes assisting the cellulose breakdown occupy the rumen and initiate the fermentation process. The animal absorbs the fatty acids, vitamins and nutrient content on passing the partially digested food from the rumen to the omasum. This decreases the pH level and initiates the release of enzymes for further breakdown of the food which later passes to the abomasum to absorb remaining nutrients before excretion. This process takes about 9–12 hours.
Ralph Stoner Wolfe was an American microbiologist, who contributed to the discovery of the single-celled archaea as the third domain of life. He was a pioneer in the biochemistry of methanogenesis.
