Methylotrophs are a diverse group of microorganisms that can use reduced one-carbon compounds, such as methanol or methane, as the carbon source for their growth; and multi-carbon compounds that contain no carbon-carbon bonds, such as dimethyl ether and dimethylamine. This group of microorganisms also includes those capable of assimilating reduced one-carbon compounds by way of carbon dioxide using the ribulose bisphosphate pathway. [1] These organisms should not be confused with methanogens which on the contrary produce methane as a by-product from various one-carbon compounds such as carbon dioxide. Some methylotrophs can degrade the greenhouse gas methane, and in this case they are called methanotrophs. The abundance, purity, and low price of methanol compared to commonly used sugars make methylotrophs competent organisms for production of amino acids, vitamins, recombinant proteins, single-cell proteins, co-enzymes and cytochromes.
The key intermediate in methylotrophic metabolism is formaldehyde, which can be diverted to either assimilatory or dissimilatory pathways. [2] Methylotrophs produce formaldehyde through oxidation of methanol and/or methane. Methane oxidation requires the enzyme methane monooxygenase (MMO). [3] [4] Methylotrophs with this enzyme are given the name methanotrophs. The oxidation of methane (or methanol) can be assimilatory or dissimilatory in nature (see figure). If dissimilatory, the formaldehyde intermediate is oxidized completely into to produce reductant and energy. [5] [6] If assimilatory, the formaldehyde intermediate is used to synthesize a 3-Carbon () compound for the production of biomass. [2] [7] Many methylotrophs use multi-carbon compounds for anabolism, thus limiting their use of formaldehyde to dissimilatory processes, however methanotrophs are generally limited to only metabolism. [2] [5]
Single Carbon Compounds | Chemical Formula | Multi-Carbon Compounds | Chemical Formula |
---|---|---|---|
Carbon monoxide | Dimethyl ether | ||
Formaldehyde | Dimethylamine | ||
Formamide | Dimethyl sulfide | ||
Formic acid | Tetramethylammonium | ||
Methane | Trimethylamine | ||
Methanol | Trimethylamine N-oxide | ||
Methylamine | Trimethylsuphonium | ||
Methyl halide |
Methylotrophs use the electron transport chain to conserve energy produced from the oxidation of compounds. An additional activation step is required in methanotrophic metabolism to allow degradation of chemically-stable methane. This oxidation to methanol is catalyzed by MMO, which incorporates one oxygen atom from into methane and reduces the other oxygen atom to water, requiring two equivalents of reducing power. [4] [5] Methanol is then oxidized to formaldehyde through the action of methanol dehydrogenase (MDH) in bacteria, [12] or a non-specific alcohol oxidase in yeast. [13] Electrons from methanol oxidation are passed to a membrane-associated quinone of the electron transport chain to produce . [14]
In dissimilatory processes, formaldehyde is completely oxidized to and excreted. Formaldehyde is oxidized to formate via the action of Formaldehyde dehydrogenase (FALDH), which provides electrons directly to a membrane associated quinone of the electron transport chain, usually cytochrome b or c. [2] [5] In the case of associated dehydrogenases, is produced. [7]
Finally, formate is oxidized to by cytoplasmic or membrane-bound Formate dehydrogenase (FDH), producing [15] and .
The main metabolic challenge for methylotrophs is the assimilation of single carbon units into biomass. Through de novo synthesis, methylotrophs must form carbon-carbon bonds between 1-Carbon () molecules. This is an energy intensive process, which facultative methylotrophs avoid by using a range of larger organic compounds. [16] However, obligate methylotrophs must assimilate molecules. [2] [5] There are four distinct assimilation pathways with the common theme of generating one molecule. [2] Bacteria use three of these pathways [7] [11] while Fungi use one. [17] All four pathways incorporate 3 molecules into multi-carbon intermediates, then cleave one intermediate into a new molecule. The remaining intermediates are rearranged to regenerate the original multi-carbon intermediates.
Each species of methylotrophic bacteria has a single dominant assimilation pathway. [5] The three characterized pathways for carbon assimilation are the ribulose monophosphate (RuMP) and serine pathways of formaldehyde assimilation as well as the ribulose bisphosphate (RuBP) pathway of CO2 assimilation. [2] [7] [11] [18]
Unlike the other assimilatory pathways, bacteria using the RuBP pathway derive all of their organic carbon from assimilation. [5] [19] This pathway was first elucidated in photosynthetic autotrophs and is better known as the Calvin Cycle. [19] [20] Shortly thereafter, methylotrophic bacteria who could grow on reduced compounds were found using this pathway. [21]
First, 3 molecules of ribulose 5-phosphate are phosphorylated to ribulose 1,5-bisphosphate (RuBP). The enzyme ribulose bisphosphate carboxylase ( RuBisCO ) carboxylates these RuBP molecules which produces 6 molecules of 3-phosphoglycerate (PGA). The enzyme phosphoglycerate kinase phosphorylates PGA into 1,3-diphosphoglycerate (DPGA). Reduction of 6 DPGA by the enzyme glyceraldehyde phosphate dehydrogenase generates 6 molecules of the compound glyceraldehyde-3-phosphate (GAP). One GAP molecule is diverted towards biomass while the other 5 molecules regenerate the 3 molecules of ribulose 5-phosphate. [7] [20]
A new pathway was suspected when RuBisCO was not found in the methanotroph Methylmonas methanica. [22] Through radio-labelling experiments, it was shown that M. methanica used the ribulose monophosphate (RuMP) pathway. This has led researchers to propose that the RuMP cycle may have preceded the RuBP cycle. [5]
Like the RuBP cycle, this cycle begins with 3 molecules of ribulose-5-phosphate. However, instead of phosphorylating ribulose-5-phosphate, 3 molecules of formaldehyde form a C-C bond through an aldol condensation, producing 3 molecules of 3-hexulose 6-phosphate (hexulose phosphate). One of these molecules of hexulose phosphate is converted into GAP and either pyruvate or dihydroxyacetone phosphate (DHAP). The pyruvate or DHAP is used towards biomass while the other 2 hexulose phosphate molecules and the molecule of GAP are used to regenerate the 3 molecules of ribulose-5-phosphate. [6] [22]
Unlike the other assimilatory pathways, the serine cycle uses carboxylic acids and amino acids as intermediates instead of carbohydrates. [5] [23] First, 2 molecules of formaldehyde are added to 2 molecules of the amino acid glycine. This produces two molecules of the amino acid serine, the key intermediate of this pathway. These serine molecules eventually produce 2 molecules of 2-phosphoglycerate, with one molecule going towards biomass and the other being used to regenerate glycine. Notably, the regeneration of glycine requires a molecule of as well, therefore the Serine pathway also differs from the other 3 pathways by its requirement of both formaldehyde and . [22] [23]
Methylotrophic yeast metabolism differs from bacteria primarily on the basis of the enzymes used and the carbon assimilation pathway. Unlike bacteria which use bacterial MDH, methylotrophic yeasts oxidize methanol in their peroxisomes with a non-specific alcohol oxidase. This produces formaldehyde as well as hydrogen peroxide. [24] [25] Compartmentalization of this reaction in peroxisomes likely sequesters the hydrogen peroxide produced. Catalase is produced in the peroxisomes to deal with this harmful by-product. [17] [24]
The dihydroxyacetone (DHA) pathway, also known as the xylulose monophosphate (XuMP) pathway, is found exclusively in yeast. [24] [26] This pathway assimilates three molecules of formaldehyde into 1 molecule of DHAP using 3 molecules of xylulose 5-phosphate as the key intermediate.
DHA synthase acts as a transferase (transketolase) to transfer part of xylulose 5-phosphate to DHA. Then these 3 molecules of DHA are phosphorylated to DHAP by triokinase. Like the other cycles, 3 molecules are produced with 1 molecule being directed for use as cell material. The other 2 molecules are used to regenerate xylulose 5-phosphate. [27]
As key players in the carbon cycle, methylotrophs work to reduce global warming primarily through the uptake of methane and other greenhouse gases. In aqueous environments, methanogenic archaea produce 40-50% of the world's methane. Symbiosis between methanogens and methanotrophic bacteria greatly decreases the amount of methane released into the atmosphere. [28]
This symbiosis is also important in the marine environment. Marine bacteria are very important to food webs and biogeochemical cycles, particularly in coastal surface waters but also in other key ecosystems such as hydrothermal vents. There is evidence of widespread and diverse groups of methylotrophs in the ocean that have potential to significantly impact marine and estuarine ecosystems. [29] One-carbon compounds used as a carbon and energy source by methylotrophs are found throughout the ocean. These compounds include methane, methanol, methylated amines, methyl halides, and methylated sulfur compounds, such as dimethylsulfide (DMS) and dimethylsulfoxide (DMSO). [30] Some of these compounds are produced by phytoplankton and some come from the atmosphere. Studies incorporating a wider range of one-carbon substrates have found increasing diversity of methylotrophs, suggesting that the diversity of this bacterial group has not yet fully been explored. [30]
Because these compounds are volatile and impact the climate and atmosphere, research on the interaction of these bacteria with these one-carbon compounds can also help understanding of air-sea fluxes of these compounds, which impact climate predictions. [31] [29] For example, it is uncertain whether the ocean acts as a net source or sink of atmospheric methanol, but a diverse set of methylotrophs use methanol as their main energy source. In some regions, methylotrophs have been found to be a net sink of methanol, [32] while in others a product of methylotroph activity, methylamine, has been found to be emitted from the ocean and form aerosols. [29] The net direction of these fluxes depends on the utilization by methylotrophs.
Studies have found that methylotrophic capacity varies with the productivity of a system, so the impacts of methylotrophy are likely seasonal. Because some of the one-carbon compounds used by methylotrophs, such as methanol and TMAO, are produced by phytoplankton, their availability will vary temporally and seasonally depending on phytoplankton blooms, weather events, and other ecosystem inputs. [33] This means that methylotrophic metabolism is expected to follow similar dynamics, which will then impact biogeochemical cycles and carbon fluxes. [29]
Impacts of methylotrophs were also found in deep-sea hydrothermal vents. Methylotrophs, along with sulfur oxidizers and iron oxidizers, expressed key proteins associated with carbon fixation. [34] These types of studies will contribute to further understanding of deep sea carbon cycling and the connectivity between deep ocean and surface carbon cycling. The expansion of omics technologies has accelerated research on the diversity of methylotrophs, their abundance and activity in a variety of environmental niches, and their interspecies interactions. [35] Further research must be done on these bacteria and the overall effect of bacterial drawdown and transformation of one-carbon compounds in the ocean. Current evidence points to a potentially substantial role for methylotrophs in the ocean in the cycling of carbon but also potentially in the global nitrogen, sulfur and phosphorus cycles as well as the air-sea flux of carbon compounds, which could have global climate impacts. [31]
The use of methylotrophs in the agricultural sector is another way in which they can potentially impact the environment. Traditional chemical fertilizers supply nutrients not readily available from soil but can have some negative environmental impacts and are costly to produce. [36] Methylotrophs have high potential as alternative biofertilizers and bioinoculants due to their ability to form mutualistic relationships with several plant species. [37] Methylotrophs provide plants with nutrients such as soluble phosphorus and fixed nitrogen and also play a role in the uptake of said nutrients. [36] [37] Additionally, they can help plants respond to environmental stressors through the production of phytohormones. [36] Methylotrophic growth also inhibits the growth of harmful plant pathogens and induces systemic resistance. [37] Methylotrophic biofertilizers used either alone or together with chemical fertilizers have been shown to increase both crop yield and quality without loss of nutrients. [36]
Primary nutritional groups are groups of organisms, divided in relation to the nutrition mode according to the sources of energy and carbon, needed for living, growth and reproduction. The sources of energy can be light or chemical compounds; the sources of carbon can be of organic or inorganic origin.
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.
Biological carbon fixation, or сarbon assimilation, is the process by which living organisms convert inorganic carbon to organic compounds. These organic compounds are then used to store energy and as structures for other biomolecules. Carbon is primarily fixed through photosynthesis, but some organisms use chemosynthesis in the absence of sunlight. Chemosynthesis is carbon fixation driven by chemical energy rather than from sunlight.
Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NADPH as a reducing agent ('hydrogen source'). NADPH is the reduced form, whereas NADP+ is the oxidized form. NADP+ is used by all forms of cellular life.
The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in the stroma, the fluid-filled region of a chloroplast outside the thylakoid membranes. These reactions take the products of light-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and reducing power of NADPH from the light dependent reactions to produce sugars for the plant to use. These substrates are used in a series of reduction-oxidation (redox) reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of CO2 to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.
Methanotrophs 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 survive.
Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.
Pink-Pigmented Facultative Methylotrophs, commonly abbreviated to PPFMs, are bacteria that are members of the genus Methylobacterium and are commonly found in soil, dust, various fresh water supplies and on plant surfaces. Although Gram negative, Methylobacteria often stain gram variable and are easily isolated using methanol-based mineral medium. Their pigmentation, which is frequently pink but may also be yellow or orange, is thought to provide protection from solar UV radiation which damages the DNA of bacteria at low doses because of their small cell size. This color is present due to the carotenoid pigments within the cell.
Methylorubrum extorquens is a Gram-negative bacterium. Methylorubrum species often appear pink, and are classified as pink-pigmented facultative methylotrophs, or PPFMs. The wild type has been known to use both methane and multiple carbon compounds as energy sources. Specifically, M. extorquens has been observed to use primarily methanol and C1 compounds as substrates in their energy cycles. It has been also observed that use lanthanides as a cofactor to increase its methanol dehydrogenase activity
Methylobacillus flagellatus is a species of aerobic bacteria.
In enzymology, a methanol dehydrogenase (MDH) is an enzyme that catalyzes the chemical reaction:
3-hexulose-6-phosphate synthase is an enzyme with systematic name D-arabino-hex-3-ulose-6-phosphate formaldehyde-lyase (D-ribulose-5-phosphate-forming). This enzyme catalyses the following chemical reaction
6-phospho-3-hexuloisomerase is an enzyme with systematic name D-arabino-hex-3-ulose-6-phosphate isomerase. This enzyme catalyses the following chemical reaction
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.
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.
Methylophaga thiooxydans is a methylotrophic bacterium that requires high salt concentrations for growth. It was originally isolated from a culture of the algae Emiliania huxleyi, where it grows by breaking down dimethylsulfoniopropionate from E. hexleyi into dimethylsulfide and acrylate. M. thiooxydans has been implicated as a dominant organism in phytoplankton blooms, where it consumes dimethylsulfide, methanol and methyl bromide released by dying phytoplankton. It was also identified as one of the dominant organisms present in the plume following the Deepwater Horizon oil spill, and was identified as a major player in the breakdown of methanol in coastal surface water in the English Channel.
Bacillus methanolicus is a gram positive, thermophilic, methylotrophic member of the genus Bacillus. The most well characterized strain of the species, Bacillus methanolicus MGA3, was isolated from freshwater marsh soils, and grows rapidly in cultures heated to up to 60 °C using only methanol as a carbon source. The genome of B. methanolicus MGA3 was fully sequenced in 2014, revealing a 3,337,035 bp linear chromosome and two natural plasmids, pBM19 and pBM69.
John Rodney (Rod) Quayle FRS (1926–2006) was a microbial biochemist, West Riding Professor of Microbiology and Head of Department at University of Sheffield (1965–1983) and then Vice-Chancellor of Bath University (1983–1992). He adopted techniques for dissecting enzymic reactions using radioactive carbon-14. He focused on microbes that used compounds containing one atom of carbon as their sources of energy and biomass.
Formatotrophs are organisms that can assimilate formate or formic acid to use as a carbon source or for reducing power. Some authors classify formatotrophs as one of the five trophic groups of methanogens, which also include hydrogenotrophs, acetotrophs, methylotrophs, and alcoholotrophs. Formatotrophs have garnered attention for applications in biotechnology as part of a "formate bioeconomy" in which synthesized formate could be used as a nutrient for microoganisms. Formate can be electrochemically synthesized from CO2 and renewable energy, and formatotrophs may be genetically modified to enhance production of biochemical products to be used as biofuels. Technical limitations in culturing formatotrophs have limited the discovery of natural formatotrophs and impeded research on their formate-metabolizing enzymes, which are of interest for applications in carbon sequestration and astrobiology.
Hydrocarbonoclastic bacteria are a heterogeneous group of prokaryotes which can degrade and utilize hydrocarbon compounds as source of carbon and energy. Despite being present in most of environments around the world, several of these specialized bacteria live in the sea and have been isolated from polluted seawater.