Nitrogen fixation

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Nitrogen fixation is a chemical process by which molecular dinitrogen (Template:Chem) is converted into ammonia (Template:Chem).<ref name=Rees>Template:Cite journal</ref> It occurs both biologically and abiologically in chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases.<ref>Template:Cite journal</ref> These enzyme complexes are encoded by the Nif genes (or Nif homologs) and contain iron, often with a second metal (usually molybdenum, but sometimes vanadium).<ref name="Wagner">Template:Cite journal</ref>

Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses and aquatic ferns such as Azolla.<ref name="Zahran">Template:Cite journal</ref> Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi.<ref name="Sapountzis">Template:Cite journal</ref> It occurs naturally in the air by means of NO_x production by lightning.<ref>Template:Cite book</ref><ref>Template:Cite journal</ref>

Fixed nitrogen is essential to life on Earth. All nitrogen-containing organic compounds such as DNA and proteins contain nitrogen. Industrial nitrogen fixation underpins the manufacture of all nitrogenous industrial products, which include fertilizers, pharmaceuticals, textiles, dyes and explosives.

HistoryEdit

File:Nitrogen Cycle.svg

Schematic representation of the nitrogen cycle. Abiotic nitrogen fixation has been omitted.

Biological nitrogen fixation was discovered by Jean-Baptiste Boussingault in 1838.<ref>Template:Cite journal and 69: 353–367.</ref><ref>Template:Cite book</ref> Later, in 1880, the process by which it happens was discovered by German agronomist Hermann Hellriegel and Template:Interlanguage link<ref name=Hellriegel>Template:Cite book</ref> and was fully described by Dutch microbiologist Martinus Beijerinck.<ref name="Beijerinck">Template:Cite journal</ref>

"The protracted investigations of the relation of plants to the acquisition of nitrogen begun by de Saussure, Ville, Lawes, Gilbert and others, and culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887."<ref>Howard S. Reed (1942) A Short History of Plant Science, page 230, Chronic Publishing</ref>

"Experiments by Bossingault in 1855 and Pugh, Gilbert & Lawes in 1887 had shown that nitrogen did not enter the plant directly. The discovery of the role of nitrogen fixing bacteria by Herman Hellriegel and Herman Wilfarth in 1886–1888 would open a new era of soil science."<ref>Margaret Rossiter (1975) The Emergence of Agricultural Science, page 146, Yale University Press</ref>

In 1901, Beijerinck showed that Azotobacter chroococcum was able to fix atmospheric nitrogen. This was the first species of the azotobacter genus, so-named by him. It is also the first known diazotroph, species that use diatomic nitrogen as a step in the complete nitrogen cycle.<ref>Template:Cite journal</ref>

BiologicalEdit

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a nitrogenase enzyme.<ref name=Rees/> The overall reaction for BNF is:

Template:Chem2 → Template:Chem2

The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of Template:Chem. The conversion of Template:Chem into ammonia occurs at a metal cluster called FeMoco, an abbreviation for the iron-molybdenum cofactor. The mechanism proceeds via a series of protonation and reduction steps wherein the FeMoco active site hydrogenates the Template:Chem substrate.<ref name=Rees/> In free-living diazotrophs, nitrogenase-generated ammonia is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway. The microbial nif genes required for nitrogen fixation are widely distributed in diverse environments.<ref>Template:Cite journal</ref>

Nitrogenases are rapidly degraded by oxygen. For this reason, many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as leghemoglobin.<ref name="postgate">Template:Cite book</ref><ref>Template:Cite journal</ref>

Importance of nitrogenEdit

Template:Biogeochemical cycle sidebar Atmospheric nitrogen cannot be metabolized by most organisms,<ref>Template:Cite book</ref> because its triple covalent bond is very strong. Most take up fixed nitrogen from various sources. For every 100 atoms of carbon, roughly 2 to 20 atoms of nitrogen are assimilated. The atomic ratio of carbon (C) : nitrogen (N) : phosphorus (P) observed on average in planktonic biomass was originally described by Alfred Redfield,<ref name="REDFIELD 1958 230A–221">Template:Cite journal</ref> who determined the stoichiometric relationship between C:N:P atoms, The Redfield Ratio, to be 106:16:1.<ref name="REDFIELD 1958 230A–221"/>

NitrogenaseEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The protein complex nitrogenase is responsible for catalyzing the reduction of nitrogen gas (N₂) to ammonia (NH₃).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In cyanobacteria, this enzyme system is housed in a specialized cell called the heterocyst.<ref>Template:Cite journal</ref> The production of the nitrogenase complex is genetically regulated, and the activity of the protein complex is dependent on ambient oxygen concentrations, and intra- and extracellular concentrations of ammonia and oxidized nitrogen species (nitrate and nitrite).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit N_Fix, specifically when intracellular concentrations of 2-oxoglutarate (2-OG) exceed a critical threshold.<ref>Template:Cite journal</ref> The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.<ref>Template:Cite book</ref>

Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). Three iron-dependent proteins are known: molybdenum-dependent, vanadium-dependent, and iron-only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase is most common.<ref name=Rees/> The different types of nitrogenase can be determined by the specific iron protein component.<ref>Template:Cite book</ref> Nitrogenase is highly conserved. Gene expression through DNA sequencing can distinguish which protein complex is present in the microorganism and potentially being expressed. Most frequently, the nifH gene is used to identify the presence of molybdenum-dependent nitrogenase, followed by closely related nitrogenase reductases (component II) vnfH and anfH representing vanadium-dependent and iron-only nitrogenase, respectively.<ref>Template:Cite journal</ref> In studying the ecology and evolution of nitrogen-fixing bacteria, the nifH gene is the biomarker most widely used.<ref>Template:Cite journal</ref> nifH has two similar genes anfH and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.<ref>Template:Cite journal</ref>

Evolution of nitrogenaseEdit

Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga),<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> although some isotopic support showing nitrogenase evolution as early as around 3.2 Ga.<ref>Template:Cite journal</ref> Nitrogenase appears to have evolved from maturase-like proteins, although the function of the preceding protein is currently unknown.<ref>Template:Cite journal</ref>

Nitrogenase has three different forms (Nif, Anf, and Vnf) that correspond with the metal found in the active site of the protein (molybdenum, iron, and vanadium respectively).<ref>Template:Cite journal</ref> Marine metal abundances over Earth's geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common.<ref>Template:Cite journal</ref> Currently, there is no conclusive agreement on which form of nitrogenase arose first.

MicroorganismsEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Diazotrophs are widespread within domain Bacteria including cyanobacteria (e.g. the highly significant Trichodesmium and Cyanothece), green sulfur bacteria, purple sulfur bacteria, Azotobacteraceae, rhizobia and Frankia.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Mus-2016">Template:Cite journal</ref> Several obligately anaerobic bacteria fix nitrogen including many (but not all) Clostridium spp. Some archaea such as Methanosarcina acetivorans also fix nitrogen,<ref>Template:Cite journal</ref> and several other methanogenic taxa, are significant contributors to nitrogen fixation in oxygen-deficient soils.<ref>Template:Cite journal</ref>

Cyanobacteria, commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as nitrate, nitrite, ammonium, urea, or some amino acids. Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the Archean eon.<ref>Template:Cite journal</ref> Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine.<ref name="Pierella Karlusich-2021">Template:Cite journal</ref><ref>Template:Cite journal</ref> Indeed, the amount of nitrogen fixed in the ocean is at least as much as that on land.<ref>Template:Cite journal</ref> The colonial marine cyanobacterium Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally.<ref>Template:Cite journal</ref> Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Species of nitrogen fixing cyanobacteria in fresh waters include: Aphanizomenon and Dolichospermum (previously Anabaena).<ref>Template:Cite journal</ref> Such species have specialized cells called heterocytes, in which nitrogen fixation occurs via the nitrogenase enzyme.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

AlgaeEdit

One type of organelle, originating from cyanobacterial endosymbionts called UCYN-A2,<ref name="Thompson_2012" /><ref>Template:Cite journal</ref> can turn nitrogen gas into a biologically available form. This nitroplast was discovered in algae, particularly in the marine algae Braarudosphaera bigelowii.<ref>Template:Cite journal</ref>

Diatoms in the family Rhopalodiaceae also possess cyanobacterial endosymbionts called spheroid bodies or diazoplasts.<ref>Template:Cite journal</ref> These endosymbionts have lost photosynthetic properties, but have kept the ability to perform nitrogen fixation, allowing these diatoms to fix atmospheric nitrogen.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera Hemiaulus, Rhizosolenia and Chaetoceros.<ref>Template:Cite journal</ref>

Root nodule symbiosesEdit

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Legume familyEdit

File:Root nodules on fava bean plant.jpg

Nodules are visible on this broad bean root

Plants that contribute to nitrogen fixation include those of the legume family—Fabaceae— with taxa such as kudzu, clover, soybean, alfalfa, lupin, peanut and rooibos.<ref name="Mus-2016" /> They contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants.<ref>Template:Cite journal</ref> When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the soil.<ref name=postgate/><ref>Template:Cite book</ref> The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional farming practices, fields are rotated through various types of crops, which usually include one consisting mainly or entirely of clover.Template:Citation needed

Fixation efficiency in soil is dependent on many factors, including the legume and air and soil conditions. For example, nitrogen fixation by red clover can range from Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Non-leguminousEdit

File:A sectioned alder root nodule gall.JPG

A sectioned alder tree root nodule

The ability to fix nitrogen in nodules is present in actinorhizal plants such as alder and bayberry, with the help of Frankia bacteria. They are found in 25 genera in the orders Cucurbitales, Fagales and Rosales, which together with the Fabales form a nitrogen-fixing clade of eurosids. The ability to fix nitrogen is not universally present in these families. For example, of 122 Rosaceae genera, only four fix nitrogen. Fabales were the first lineage to branch off this nitrogen-fixing clade; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the most recent common ancestors of all these plants, but only evolved to full function in some of them.<ref>Template:Cite book</ref>

In addition, Trema (Parasponia), a tropical genus in the family Cannabaceae, is unusually able to interact with rhizobia and form nitrogen-fixing nodules.<ref>Template:Cite journal</ref>

Non-legumious nodulating plants
Family	Genera	Species
Betulaceae	Template:Plainlist	Most or all species
Boraginaceae	Template:Plainlist	Template:Plainlist
Cannabaceae	Template:Plainlist	Template:Plainlist
Casuarinaceae	Template:Plainlist
Coriariaceae	Template:Plainlist	Template:Plainlist
Datiscaceae	Template:Plainlist
Elaeagnaceae	Template:Plainlist
Myricaceae	Template:Plainlist
Posidoniaceae	Template:Plainlist
Rhamnaceae	Template:Plainlist
Rosaceae	Template:Plainlist

Other plant symbiontsEdit

Some other plants live in association with a cyanobiont (cyanobacteria such as Nostoc) which fix nitrogen for them:

Some lichens such as Lobaria and Peltigera
Mosquito fern (Azolla species)
Cycads<ref>{{#invoke:citation/CS1|citation

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Gunnera
Blasia (liverwort)
Hornworts<ref>Template:Cite journal</ref>

Some symbiotic relationships involving agriculturally-important plants are:<ref>Template:Cite journal</ref>

Sugarcane and unclear endophytes
Foxtail millet and Azospirillum brasilense
Kallar grass and Azoarcus sp. strain BH72
Rice and Herbaspirillum seropedicae
Wheat and Klebsiella pneumoniae
Maize landrace 'Sierra Mixe' / 'olotón'<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> and various Bacteroidota and Pseudomonadota

Industrial processesEdit

HistoricalEdit

A method for nitrogen fixation was first described by Henry Cavendish in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the Birkeland–Eyde process of 1903.<ref>Template:Cite journal</ref> The fixation of nitrogen by lightning is a very similar natural occurring process.

The possibility that atmospheric nitrogen reacts with certain chemicals was first observed by Desfosses in 1828. He observed that mixtures of alkali metal oxides and carbon react with nitrogen at high temperatures. With the use of barium carbonate as starting material, the first commercial process became available in the 1860s, developed by Margueritte and Sourdeval. The resulting barium cyanide reacts with steam, yielding ammonia. In 1898 Frank and Caro developed what is known as the Frank–Caro process to fix nitrogen in the form of calcium cyanamide. The process was eclipsed by the Haber process, which was discovered in 1909.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref>

Haber processEdit

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File:THC 2003.902.022 D. C. Bardwell Study of Nitrogen Fixation.tif

Equipment for a study of nitrogen fixation by alpha rays (Fixed Nitrogen Research Laboratory, 1926)

The dominant industrial method for producing ammonia is the Haber process also known as the Haber-Bosch process.<ref>Smil, V. 2004. Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production, MIT Press.</ref> Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial ecosystem. Ammonia is a required precursor to fertilizers, explosives, and other products. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), which are routine conditions for industrial catalysis. This process uses natural gas as a hydrogen source and air as a nitrogen source. The ammonia product has resulted in an intensification of nitrogen fertilizer globally<ref>Template:Cite journal</ref> and is credited with supporting the expansion of the human population from around 2 billion in the early 20th century to roughly 8 billion people now.<ref>Template:Cite journal</ref>

Homogeneous catalysisEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of lowering energy requirements. However, such research has thus far failed to approach the efficiency and ease of the Haber process. Many compounds react with atmospheric nitrogen to give dinitrogen complexes. The first dinitrogen complex to be reported was [[pentaamine(dinitrogen)ruthenium(II) chloride|Template:Chem(Template:Chem)²⁺]].<ref>Template:Cite journal</ref> Some soluble complexes do catalyze nitrogen fixation.<ref name=Peters>Template:Cite journal</ref>

LightningEdit

File:Lightning Pritzerbe 01 (MK).jpg

Lightning heats the air around it in a high-temperature plasma, breaking the bonds of Template:Chem, starting the formation of nitrous acid (Template:Chem).

Nitrogen can be fixed by lightning converting nitrogen gas (Template:Chem) and oxygen gas (Template:Chem) in the atmosphere into Template:NOx (nitrogen oxides). The Template:Chem molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms.<ref name="Tuck-1976">Template:Cite journal</ref> Lightning produces enough energy and heat to break this bond<ref name="Tuck-1976" /> allowing nitrogen atoms to react with oxygen, forming Template:Chem. These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form Template:Chem,<ref>Template:Cite journal</ref> which in turn reacts with water to produce Template:Chem (nitrous acid) or Template:Chem (nitric acid). When these acids seep into the soil, they make NO₃⁻ (nitrate), which is of use to plants.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Tuck-1976" />

ReferencesEdit

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External linksEdit

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