Methanogenic Bacteria
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Methanogens are microorganisms that produce
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 Ea ...
as a metabolic byproduct in
hypoxic Hypoxia means a lower than normal level of oxygen, and may refer to: Reduced or insufficient oxygen * Hypoxia (environmental), abnormally low oxygen content of the specific environment * Hypoxia (medical), abnormally low level of oxygen in the t ...
conditions. They are
prokaryotic 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 ...
and belong to the
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function * Do ...
Archaea. All known methanogens are members of the archaeal phylum
Euryarchaeota Euryarchaeota (from Ancient Greek ''εὐρύς'' eurús, "broad, wide") is a phylum of archaea. Euryarchaeota are highly diverse and include methanogens, which produce methane and are often found in intestines, halobacteria, which survive extr ...
. Methanogens are common in
wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The p ...
s, where they are responsible for
marsh gas Marsh gas, also known as swamp gas or bog gas, is a mixture primarily of methane and smaller amounts of hydrogen sulfide, carbon dioxide, and trace phosphine that is produced naturally within some geographical marshes, swamps, and bogs. The surfac ...
, and in the digestive tracts of animals such as
ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
s and many
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s, where they are responsible for the methane content of
belching Burping (also called belching and eructation) is the release of gas from the upper digestive tract (esophagus and stomach) of animals through the mouth. It is usually audible. In humans, burping can be caused by normal eating processes, or as ...
in
ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
s and flatulence in humans. In marine
sediment 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, sa ...
s, the biological production of methane, also termed methanogenesis, is generally confined to where
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
s are depleted, below the top layers. Moreover, methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments. Others are
extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
s, found in environments such as
hot spring A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by c ...
s and submarine hydrothermal vents as well as in the "solid" rock of Earth's crust, kilometers below the surface.


Physical description

Methanogens are coccoid (spherical shaped) or bacilli (rod shaped). There are over 50 described species of methanogens, which do not form a monophyletic group (since
haloarchaea Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. Halobacteria are now recognized as archaea rather than bacteria and are one of t ...
emerged from within them), although all known methanogens belong to
Euryarchaeota Euryarchaeota (from Ancient Greek ''εὐρύς'' eurús, "broad, wide") is a phylum of archaea. Euryarchaeota are highly diverse and include methanogens, which produce methane and are often found in intestines, halobacteria, which survive extr ...
. They are mostly anaerobic organisms that cannot function under aerobic conditions, but recently a species (''
Candidatus In prokaryote nomenclature, ''Candidatus'' (Latin for candidate of Roman office) is used to name prokaryotic phyla that are well characterized but yet-uncultured. Contemporary sequencing approaches, such as 16S sequencing or metagenomics, provide m ...
Methanothrix paradoxum In the alpha taxonomy, taxonomy of microorganisms, the Methanothrix are a genus of the Euryarchaeota. Phylogeny See also * List of Archaea genera References

Archaea genera {{Archaea-stub ...
'') has been identified that can function in anoxic microsites within aerobic environments. They are very sensitive to the presence of
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 ...
even at trace level. Usually, they cannot sustain oxygen stress for a prolonged time. However, ''
Methanosarcina barkeri '' Methanosarcina barkeri'' is the most fundamental species of the genus '' Methanosarcina'', and their properties apply generally to the genus ''Methanosarcina''. ''Methanosarcina barkeri'' can produce methane anaerobically through different m ...
'' is exceptional in possessing a superoxide dismutase (SOD)
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
, and may survive longer than the others in the presence of O2. Some methanogens, called
hydrogenotroph Hydrogenotrophs are organisms that are able to metabolize molecular hydrogen as a source of energy. An example of hydrogenotrophy is performed by carbon dioxide-reducing organismsStams, J.M., and Plugge, C.M. (2010) The microbiology of methanogen ...
ic, use
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
(CO2) as a source of carbon, and
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
as a
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth me ...
. The reduction of
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
into
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 Ea ...
in the presence of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
can be expressed as follows: :CO2 + 4 H2 → CH4 + 2H2O Some of the CO2 reacts with the hydrogen to produce methane, which creates an
electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and ...
across the cell membrane, used to generate ATP through
chemiosmosis Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a memb ...
. In contrast,
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s and algae use water as their
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth me ...
. Methanogens lack
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...
, a polymer that is found in the cell walls of
Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
but not in those of Archaea. Some methanogens have a cell wall that is composed of
pseudopeptidoglycan Pseudopeptidoglycan (also known as pseudomurein;White, David. (1995) ''The Physiology and Biochemistry of Prokaryotes'', pages 6, 12-21. (Oxford: Oxford University Press). . PPG hereafter) is a major cell wall component of some Archaea that differs ...
. Other methanogens do not, but have at least one
paracrystalline In materials science, paracrystalline materials are defined as having short- and medium-range ordering in their lattice (similar to the liquid crystal phases) but lacking crystal-like long-range ordering at least in one direction. Origin and ...
array (S-layer) made up of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s that fit together like a
jigsaw puzzle A jigsaw puzzle is a tiling puzzle that requires the assembly of often irregularly shaped interlocking and mosaiced pieces, each of which typically has a portion of a picture. When assembled, the puzzle pieces produce a complete picture. In t ...
.


Extreme living areas

Methanogens play a vital ecological role in anaerobic environments of removing excess hydrogen and fermentation products that have been produced by other forms of
anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing r ...
. Methanogens typically thrive in environments in which all electron acceptors other than CO2 (such as
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 ...
, nitrate, ferric
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
(Fe(III)), and
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
) have been depleted. In deep
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
ic rocks near the
mid ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverg ...
s, they can obtain their
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
from the serpentinisation reaction of
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
as observed in the hydrothermal field of Lost City. The thermal breakdown of water and water radiolysis are other possible sources of hydrogen. Methanogens are key agents of remineralization of
organic carbon Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
in
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margi ...
sediments and other aquatic sediments with high rates of sedimentation and high sediment organic matter. Under the correct conditions of pressure and temperature, biogenic methane can accumulate in massive deposits of
methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amo ...
s, which account for a significant fraction of organic carbon in continental margin sediments and represent a key reservoir of a potent greenhouse gas. Methanogens have been found in several extreme environments on Earth – buried under kilometres of ice in
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
and living in hot, dry desert soil. They are known to be the most common archaebacteria in deep subterranean habitats. Live microbes making methane were found in a glacial ice core sample retrieved from about three kilometres under Greenland by researchers from the
University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant u ...
. They also found a constant metabolism able to repair macromolecular damage, at temperatures of 145 to –40 °C. Another study has also discovered methanogens in a harsh environment on Earth. Researchers studied dozens of soil and vapour samples from five different desert environments in
Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
,
Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...
and
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
, and in
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
and
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
. Of these, five soil samples and three vapour samples from the vicinity of the
Mars Desert Research Station The Mars Desert Research Station (MDRS) is the largest and longest-running Mars surface research facility in the world and is one of two simulated Mars analog habitats owned and operated by the Mars Society. The MDRS station was built in the e ...
in Utah were found to have signs of viable methanogens. Some scientists have proposed that the presence of methane in the
Martian Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. It became the most popular celestial object in fiction in the late 1800s as the Moon was evidently lifeless. At the time, the pr ...
atmosphere may be indicative of native methanogens on that planet. In June 2019, NASA's
Curiosity Curiosity (from Latin '' cūriōsitās'', from ''cūriōsus'' "careful, diligent, curious", akin to ''cura'' "care") is a quality related to inquisitive thinking such as exploration, investigation, and learning, evident by observation in humans ...
rover detected methane, commonly generated by underground microbes such as methanogens, which signals possibility of
life on Mars The possibility of life on Mars is a subject of interest in astrobiology due to the planet's proximity and similarities to Earth. To date, no proof of past or present life has been found on Mars. Cumulative evidence suggests that during the ...
. Closely related to the methanogens are the anaerobic methane oxidizers, which utilize methane as a substrate in conjunction with the reduction of sulfate and nitrate. Most methanogens are
autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
producers, but those that
oxidize 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 ...
CH3COO are classed as
chemotroph A Chemotroph is an organism that obtains energy by the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic ( chemolithotrophs). The chemotroph designation is in contrast to phototr ...
instead.


Comparative genomics and molecular signatures

Comparative proteomic analysis has led to the identification of 31 signature proteins which are specific for methanogens (also known as ''Methanoarchaeota''). Most of these proteins are related to methanogenesis, and they could serve as potential molecular markers for methanogens. Additionally, 10 proteins found in all methanogens which are shared by ''
Archaeoglobus ''Archaeoglobus'' is a genus of the phylum Euryarchaeota. ''Archaeoglobus'' can be found in high-temperature oil fields where they may contribute to oil field souring. Metabolism ''Archaeoglobus'' grow anaerobically at extremely high temperat ...
'', suggest that these two groups are related. In phylogenetic trees, methanogens are not monophyletic and they are generally split into three clades. Hence, the unique shared presence of large numbers of proteins by all methanogens could be due to lateral gene transfers. Additionally, more recent novel proteins associated with sulfide trafficking have been linked to methanogen archaea. More proteomic analysis is needed to further differentiate specific genera within the methanogen class and reveal novel pathways for methanogenic metabolism. Modern DNA or RNA sequencing approaches has elucidated several genomic markers specific to several groups of methanogens. One such finding isolated nine methanogens from genus Methanoculleus and found that there were at least 2 trehalose synthases genes that were found in all nine genomes. Thus far, the gene has been observed only in this genus, therefore it can be used as a marker to identify the archaea Methanoculleus. As sequencing techniques progress and databases become populated with an abundance of genomic data, a greater number of strains and traits can be identified, but many genera have remained understudied. For example, halophilic methanogens are potentially important microbes for carbon cycling in coastal wetland ecosystems but seem to be greatly understudied. One recent publication isolated a novel strain from genus Methanohalophilus which resides in sulfide-rich seawater. Interestingly, they have isolated several portions of this strain's genome that are different from other isolated strains of this genus (Methanohalophilus mahii, Methanohalophilus halophilus, Methanohalophilus portucalensis, Methanohalophilus euhalbius). Some differences include a highly conserved genome, sulfur and glycogen metabolisms and viral resistance. Genomic markers consistent with the microbes environment have been observed in many other cases. One such study found that methane producing archaea found in hydraulic fracturing zones had genomes which varied with vertical depth. Subsurface and surface genomes varied along with the constraints found in individual depth zones, though fine-scale diversity was also found in this study. It is important to recognize that genomic markers pointing at environmentally relevant factors are often non-exclusive. A survey of Methanogenic Thermoplasmata has found these organisms in human and animal intestinal tracts. This novel species was also found in other methanogenic environments such as wetland soils, though the group isolated in the wetlands did tend to have a larger number of genes encoding for anti-oxidation enzymes that were not present in the same group isolated in the human and animal intestinal tract. A common issue with identifying and discovering novel species of methanogens is that sometimes the genomic differences can be quite small, yet the research group decides they are different enough to separate into individual species. One study took a group of Methanocellales and ran a comparative genomic study. The three strains were originally considered identical, but a detailed approach to genomic isolation showed differences among their previously considered identical genomes. Differences were seen in gene copy number and there was also metabolic diversity associated with the genomic information. Genomic signatures not only allow one to mark unique methanogens and genes relevant to environmental conditions; it has also led to a better understanding of the evolution of these archaea. Some methanogens must actively mitigate against oxic environments. Functional genes involved with the production of antioxidants have been found in methanogens, and some specific groups tend to have an enrichment of this genomic feature. Methanogens containing a genome with enriched antioxidant properties may provide evidence that this genomic addition may have occurred during the Great Oxygenation Event. In another study, three strains from the lineage Thermoplasmatales isolated from animal gastro-intestinal tracts revealed evolutionary differences. The eukaryotic-like histone gene which is present in most methanogen genomes was not present, eluding to evidence that an ancestral branch was lost within Thermoplasmatales and related lineages. Furthermore, the group Methanomassiliicoccus has a genome which appears to have lost many common genes coding for the first several steps of methanogenesis. These genes appear to have been replaced by genes coding for a novel methylated methogenic pathway. This pathway has been reported in several types of environments, pointing to non-environment specific evolution, and may point to an ancestral deviation.


Metabolism


Methane production

Methanogens are known to produce methane from substrates such as H2/CO2, acetate, formate, methanol and methylamines in a process called methanogenesis. Different methanogenic reactions are catalyzed by unique sets of
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s and
coenzymes A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...
. While reaction mechanism and energetics vary between one reaction and another, all of these reactions contribute to net positive energy production by creating ion
concentration gradient Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) o ...
s that are used to drive ATP synthesis. The overall reaction for H2/CO2 methanogenesis is: :CO2 + 4 H2 -> CH4 + 2 H2O (∆G˚’ = -134 kJ/mol CH4) Well-studied organisms that produce methane via H2/CO2 methanogenesis include ''Methanosarcina barkeri'', ''Methanobacterium thermoautotrophicum'', and ''Methanobacterium wolfei''. These organisms are typically found in anaerobic environments. In the earliest stage of H2/CO2 methanogenesis, CO2 binds to
methanofuran Methanofurans are a family of chemical compounds found in methanogenic archaea. These species feature a 2-aminomethylfuran linked to phenoxy group. At least three different end groups are recognized: R = tricarboxyheptanoyl (methanofuran), gluta ...
(MF) and is reduced to formyl-MF. This
endergonic In chemical thermodynamics, an endergonic reaction (; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and an additional driving fo ...
reductive process (∆G˚’= +16 kJ/mol) is dependent on the availability of H2 and is catalyzed by the enzyme formyl-MF dehydrogenase. :CO2 + H2 + MF -> HCO-MF + H2O The formyl constituent of formyl-MF is then transferred to the coenzyme
tetrahydromethanopterin Tetrahydromethanopterin (THMPT, ) is a coenzyme in methanogenesis. It is the carrier of the C1 group as it is reduced to the methyl level, before transferring to the coenzyme M. Tetrahydrosarcinapterin (THSPT, ) is a modified form of THMPT, wh ...
(H4MPT) and is catalyzed by a soluble enzyme known as formyl transferase. This results in the formation of formyl-H4MPT. :HCO-MF + H4MPT -> HCO-H4MPT + MF Formyl-H4MPT is subsequently reduced to methenyl-H4MPT. Methenyl-H4MPT then undergoes a one-step hydrolysis followed by a two-step reduction to methyl-H4MPT. The two-step reversible reduction is assisted by coenzyme F420 whose hydride acceptor spontaneously oxidizes. Once oxidized, F420’s electron supply is replenished by accepting electrons from H2. This step is catalyzed by methylene H4MPT dehydrogenase. :HCO-H4MPT + H+ -> CH-H4MPT+ + H2O (Formyl-H4MPT reduction) :CH-H4MPT+ + F420H2 -> CH2=H4MPT + F420 + H+(Methenyl-H4MPT hydrolysis) :CH2=H4MPT + H2 -> CH3-H4MPT + H+(H4MPT reduction) Next, the methyl group of methyl-M4MPT is transferred to coenzyme M via a methyltransferase-catalyzed reaction. :CH3-H4MPT + HS-CoM -> CH3-S-CoM + H4MPT The final step of H2/CO2 methanogenic involves
methyl-coenzyme M reductase In enzymology, coenzyme-B sulfoethylthiotransferase, also known as methyl-coenzyme M reductase (MCR) or most systematically as 2-(methylthio)ethanesulfonate:N-(7-thioheptanoyl)-3-O-phosphothreonine S-(2-sulfoethyl)thiotransferase is an enzyme tha ...
and two coenzymes: N-7 mercaptoheptanoylthreonine phosphate (HS-HTP) and coenzyme F430. HS-HTP donates electrons to methyl-coenzyme M allowing the formation of methane and mixed disulfide of HS-CoM. F430, on the other hand, serves as a prosthetic group to the reductase. H2 donates electrons to the mixed disulfide of HS-CoM and regenerates coenzyme M. :CH3-S-CoM + HS-HTP -> CH4 + CoM-S-S-HTP (Formation of methane) :CoM-S-S-HTP + H2 -> HS-CoM + HS-HTP (Regeneration of coenzyme M)


Wastewater treatment

Methanogens are widely used in anaerobic digestors to treat wastewater as well as aqueous organic pollutants. Industries have selected methanogens for their ability to perform
biomethanation 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 g ...
during wastewater decomposition thereby rendering the process sustainable and cost-effective. Bio-decomposition in the anaerobic digester involves a four-staged cooperative action performed by different microorganisms. The first stage is the hydrolysis of insoluble polymerized organic matter by anaerobes such as Streptococcus and Enterobacterium. In the second stage, acidogens break down dissolved organic pollutants in wastewater to
fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
. In the third stage, acetogens convert fatty acids to acetates. In the final stage, methanogens metabolize acetates to gaseous
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 Ea ...
. The byproduct methane leaves the aqueous layer and serves as an energy source to power wastewater-processing within the digestor, thus generating a self-sustaining mechanism. Methanogens also effectively decrease the concentration of organic matter in wastewater run-off. For instance,
agricultural wastewater Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or i ...
, highly rich in organic material, has been a major cause of aquatic ecosystem degradation. The chemical imbalances can lead to severe ramifications such as
eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
. Through anaerobic digestion, the purification of wastewater can prevent unexpected blooms in water systems as well as trap methanogenesis within digesters. This allocates biomethane for energy production and prevents a potent greenhouse gas, methane, from being released into the atmosphere. The organic components of wastewater vary vastly. Chemical structures of the organic matter select for specific methanogens to perform anaerobic digestion. An example is the members of
Methanosaeta In alpha taxonomy, taxonomy, ''Methanosaeta'' is a genus (biology), genus of microbes within Methanosaetaceae. Like other species in this family, those of ''Methanosaeta'' metabolize acetate as their sole source of energy. The genus contains two ...
genus dominate the digestion of palm oil mill effluent (POME) and brewery waste. Modernizing wastewater treatment systems to incorporate higher diversity of microorganisms to decrease organic content in treatment is under active research in the field of microbiological and chemical engineering. Current new generations of Staged Multi-Phase Anaerobic reactors and Upflow Sludge Bed reactor systems are designed to have innovated features to counter high loading wastewater input, extreme temperature conditions, and possible inhibitory compounds.


Strains

*'' Methanobacterium bryantii'' *'' Methanobacterium formicum'' *'' Methanobrevibacter arboriphilicus'' *''
Methanobrevibacter gottschalkii Methanobrevibacter gottschalkii is a species of methanogen archaeon, named after Gerhard Gottschalk. Description It is a coccobacillus with rounded ends, about 0.7 micrometres in width and 0.9 micrometres in length, occurring in pairs or short ...
'' *''
Methanobrevibacter ruminantium In taxonomy, ''Methanobrevibacter'' is a genus of the Methanobacteriaceae.See the NCBIbr>webpage on Methanobrevibacter Data extracted from the The species within ''Methanobrevibacter'' are strictly anaerobic archaea that produce methane, for ...
'' *''
Methanobrevibacter smithii ''Methanobrevibacter smithii'' is the predominant archaeon in the microbiota of the human gut. ''M. smithii'' has a coccobacillus shape. It plays an important role in the efficient digestion of polysaccharides (complex sugars) by consuming the ...
'' *'' Methanococcus chunghsingensis'' *'' Methanococcus burtonii'' *''
Methanococcus aeolicus ''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 ba ...
'' *'' Methanococcus deltae'' *'' Methanococcus jannaschii'' *'' Methanococcus maripaludis'' *'' Methanococcus vannielii'' *'' Methanocorpusculum labreanum'' *''
Methanoculleus bourgensis ''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 a ...
'' (''Methanogenium olentangyi'' & ''Methanogenium bourgense'') *''
Methanoculleus marisnigri ''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 a ...
'' *'' Methanoflorens stordalenmirensis'' *'' Methanofollis liminatans'' *'' Methanogenium cariaci'' *'' Methanogenium frigidum'' *''
Methanogenium organophilum ''Methanogenium'' is a genus (biology), genus of archaeans in the family Methanomicrobiaceae. The type species is ''Methanogenium cariaci''. Description and habitat The species within ''Methanogenium'' are coccus, coccoid in shape and Gram-nega ...
'' *'' Methanogenium wolfei'' *'' Methanomicrobium mobile'' *''
Methanopyrus kandleri In taxonomy, ''Methanopyrus'' is a genus of the Methanopyraceae. ''Methanopyrus'' is a genus of methanogen, with a single described species, ''M. kandleri''. It is a rod-shaped hyperthermophile, discovered on the wall of a black smoker from the ...
'' *''
Methanoregula boonei In the alpha taxonomy, taxonomy of microorganisms, ''Candidatus Methanoregula'' is a genus (biology), genus of the Methanomicrobiales. It was is isolated from an acidic peat bog. It produces methane at the lowest pH of any known organism. See a ...
'' *''
Methanosaeta concilii ''Methanosaeta concilii'' is an archaeum in the disputed genus ''Methanosaeta''. It is obligately anaerobic, gram-negative and non-motile Motility is the ability of an organism to move independently, using metabolic energy. Definitions M ...
'' *'' Methanosaeta thermophila'' *''
Methanosarcina acetivorans ''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 ...
'' *''
Methanosarcina barkeri '' Methanosarcina barkeri'' is the most fundamental species of the genus '' Methanosarcina'', and their properties apply generally to the genus ''Methanosarcina''. ''Methanosarcina barkeri'' can produce methane anaerobically through different m ...
'' *''
Methanosarcina mazei ''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 en ...
'' *'' Methanosphaera stadtmanae'' *'' Methanospirillium hungatei'' *''
Methanothermobacter defluvii In taxonomy, ''Methanothermobacter'' is a genus of the Methanobacteriaceae.See the NCBIbr>webpage on Methanothermobacter Data extracted from the The species within this genus are thermophilic and grow best at temperatures between 55°C and 6 ...
'' (''Methanobacterium defluvii'') *''
Methanothermobacter thermautotrophicus In taxonomy, ''Methanothermobacter'' is a genus of the Methanobacteriaceae.See the NCBIbr>webpage on Methanothermobacter Data extracted from the The species within this genus are thermophilic and grow best at temperatures between 55°C and 6 ...
'' (''Methanobacterium thermoautotrophicum'') *''
Methanothermobacter thermoflexus In taxonomy, ''Methanothermobacter'' is a genus of the Methanobacteriaceae.See the NCBIbr>webpage on Methanothermobacter Data extracted from the The species within this genus are thermophilic and grow best at temperatures between 55°C and 6 ...
'' (''Methanobacterium thermoflexum'') *'' Methanothermobacter wolfei'' (''Methanobacterium wolfei'') *'' Methanothrix sochngenii''


See also

*
Extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
* Hydrogen cycle *
Methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amo ...
*
Methanogens in digestive tract of ruminants 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 ...
*
Methanopyrus In taxonomy, ''Methanopyrus'' is a genus of the Methanopyraceae. ''Methanopyrus'' is a genus of methanogen, with a single described species, ''M. kandleri''. It is a rod-shaped hyperthermophile, discovered on the wall of a black smoker from the ...
*
Methanotroph Methanotrophs (sometimes called methanophiles) are prokaryotes that metabolize methane as their source of carbon and chemical energy. They are bacteria or archaea, can grow aerobically or anaerobically, and require single-carbon compounds to s ...


References

{{Extremophile Anaerobic digestion
Gen Gen may refer to: * ''Gen'' (film), 2006 Turkish horror film directed by Togan Gökbakar * Gen (Street Fighter), a video game character from the ''Street Fighter'' series * Gen Fu, a video game character from the ''Dead or Alive'' series * Gen l ...
Archaea biology Environmental microbiology