Nitrogen fixation#Root nodule symbioses

File:Nitrogen Cycle.svg. Abiotic nitrogen fixation has been omitted.]]

Biological nitrogen fixation was discovered by Jean-Baptiste Boussingault in 1838.{{cite journal |last1=Boussingault |title=Recherches chimiques sur la vegetation, entreprises dans le but d'examiner si les plantes prennent de l'azote à l'atmosphere |journal=Annales de Chimie et de Physique |date=1838 |volume=67 |pages=5–54 |url=https://babel.hathitrust.org/cgi/pt?id=iau.31858046218297&view=1up&seq=9&skin=2021 |series=2nd series |trans-title=Chemical investigations into vegetation, undertaken with the goal of examining whether plants take up nitrogen in the atmosphere |language=French}} and [https://babel.hathitrust.org/cgi/pt?id=hvd.hx3dx7&view=1up&seq=357&skin=2021 69: 353–367].{{cite book | vauthors = Smil V |year=2001|title=Enriching the Earth|publisher=Massachusetts Institute of Technology}} Later, in 1880, the process by which it happens was discovered by German agronomist Hermann Hellriegel and {{interlanguage link|Hermann Wilfarth|de}}{{cite book| vauthors = Hellriegel H, Wilfarth H |year=1888|title=Untersuchungen über die Stickstoffnahrung der Gramineen und Leguminosen|trans-title=Studies on the nitrogen intake of Gramineae and Leguminosae|publisher=Buchdruckerei der "Post" Kayssler & Co.|location=Berlin, Germany|url= https://www.biodiversitylibrary.org/item/69506#page/7/mode/1up|language=German}} and was fully described by Dutch microbiologist Martinus Beijerinck.{{cite journal| vauthors = Beijerinck MW |year=1901|title=Über oligonitrophile Mikroben|trans-title=On oligonitrophilic microbes|journal=Centralblatt für Bakteriologie, Parasitenkunde, Infektionskrankheiten und Hygiene|volume=7|issue=16|pages=561–582|url=https://books.google.com/books?id=tcBFAQAAMAAJ&pg=PA561|language=German}}

"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."Howard S. Reed (1942) A Short History of Plant Science, page 230, Chronic Publishing

"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."Margaret Rossiter (1975) The Emergence of Agricultural Science, page 146, Yale University Press

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.{{Cite journal |last1=Al-Baldawy |first1=Muneer Saeed M. |last2=Matloob |first2=Ahed A. A. H. |last3=Almammory |first3=Mohammed K. N. |date=2023-11-01 |title=The Importance of Nitrogen-Fixing Bacteria Azotobacter chroococcum in Biological Control to Root Rot Pathogens (Review) |journal=IOP Conference Series: Earth and Environmental Science |volume=1259 |issue=1 |pages=012110 |doi=10.1088/1755-1315/1259/1/012110 |issn=1755-1307|doi-access=free |bibcode=2023E&ES.1259a2110A }}

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a nitrogenase enzyme. The overall reaction for BNF is:

{{chem2|N2 + 16ATP + 16H2O + 8e- + 8H+}} → {{chem2|2NH3 +H2 + 16ADP + 16P_{i}|}}

The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of {{chem|H|2}}. The conversion of {{chem|N|2}} 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 {{chem|N|2}} substrate. 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.{{cite journal | vauthors = Gaby JC, Buckley DH | title = A global census of nitrogenase diversity | journal = Environmental Microbiology | volume = 13 | issue = 7 | pages = 1790–9 | date = July 2011 | pmid = 21535343 | doi = 10.1111/j.1462-2920.2011.02488.x | bibcode = 2011EnvMi..13.1790G }}

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.{{cite book | vauthors = Postgate J |year=1998 |title= Nitrogen Fixation|edition= 3rd |publisher=Cambridge University Press|location= Cambridge}}{{cite journal | vauthors = Streicher SL, Gurney EG, Valentine RC | title = The nitrogen fixation genes | journal = Nature | volume = 239 | issue = 5374 | pages = 495–9 | date = October 1972 | pmid = 4563018 | doi = 10.1038/239495a0 | bibcode = 1972Natur.239..495S | s2cid = 4225250 }}

{{biogeochemical cycle sidebar|nutrient}}

Atmospheric nitrogen cannot be metabolized by most organisms,{{cite book | vauthors = Delwiche CC | chapter = Cycling of Elements in the Biosphere|date=1983 | title = Inorganic Plant Nutrition|pages=212–238| veditors = Läuchli A, Bieleski RL |series=Encyclopedia of Plant Physiology|place=Berlin, Heidelberg|publisher=Springer|language=en|doi=10.1007/978-3-642-68885-0_8|isbn=978-3-642-68885-0 }} 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,{{Cite journal| vauthors = Redfield AC |title=The Biological Control of Chemical Factors in the Environment|date=1958|url=https://www.jstor.org/stable/27827150|journal=American Scientist|volume=46|issue=3|pages=230A–221|jstor=27827150|issn=0003-0996}} who determined the stoichiometric relationship between C:N:P atoms, The Redfield Ratio, to be 106:16:1.

{{Main|Nitrogenase}}

The protein complex nitrogenase is responsible for catalyzing the reduction of nitrogen gas (N₂) to ammonia (NH₃).{{cite journal |doi=10.1021/acs.chemrev.9b00556 |title=Reduction of Substrates by Nitrogenases |date=2020 |last1=Seefeldt |first1=Lance C. |last2=Yang |first2=Zhi-Yong |last3=Lukoyanov |first3=Dmitriy A. |last4=Harris |first4=Derek F. |last5=Dean |first5=Dennis R. |last6=Raugei |first6=Simone |last7=Hoffman |first7=Brian M. |journal=Chemical Reviews |volume=120 |issue=12 |pages=5082–5106 |pmid=32176472 |pmc=7703680 }}{{cite journal |doi=10.1002/1873-3468.14534 |title=Biological nitrogen fixation in theory, practice, and reality: A perspective on the molybdenum nitrogenase system |date=2023 |last1=Threatt |first1=Stephanie D. |last2=Rees |first2=Douglas C. |journal=FEBS Letters |volume=597 |issue=1 |pages=45–58 |pmid=36344435 |pmc=10100503 }} In cyanobacteria, this enzyme system is housed in a specialized cell called the heterocyst.{{cite journal | vauthors = Peterson RB, Wolk CP | title = High recovery of nitrogenase activity and of Fe-labeled nitrogenase in heterocysts isolated from Anabaena variabilis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 75 | issue = 12 | pages = 6271–6275 | date = December 1978 | pmid = 16592599 | pmc = 393163 | doi = 10.1073/pnas.75.12.6271 | doi-access = free | bibcode = 1978PNAS...75.6271P }} 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).{{cite journal | vauthors = Beversdorf LJ, Miller TR, McMahon KD | title = The role of nitrogen fixation in cyanobacterial bloom toxicity in a temperate, eutrophic lake | journal = PLOS ONE | volume = 8 | issue = 2 | pages = e56103 | date = 2013-02-06 | pmid = 23405255 | pmc = 3566065 | doi = 10.1371/journal.pone.0056103 | doi-access = free | bibcode = 2013PLoSO...856103B }}{{Cite journal| vauthors = Gallon JR |date=2001-03-01|title=N2 fixation in phototrophs: adaptation to a specialized way of life |journal=Plant and Soil|language=en|volume=230|issue=1|pages=39–48 |doi=10.1023/A:1004640219659|bibcode=2001PlSoi.230...39G |s2cid=22893775|issn=1573-5036}}{{cite journal | vauthors = Paerl H | title = The cyanobacterial nitrogen fixation paradox in natural waters | journal = F1000Research | volume = 6 | pages = 244 | date = 2017-03-09 | pmid = 28357051 | pmc = 5345769 | doi = 10.12688/f1000research.10603.1 | doi-access = free }} 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.{{cite journal | vauthors = Li JH, Laurent S, Konde V, Bédu S, Zhang CC | title = An increase in the level of 2-oxoglutarate promotes heterocyst development in the cyanobacterium Anabaena sp. strain PCC 7120 | journal = Microbiology | volume = 149 | issue = Pt 11 | pages = 3257–3263 | date = November 2003 | pmid = 14600238 | doi = 10.1099/mic.0.26462-0 | doi-access = free }} The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.{{cite book | vauthors = Wolk CP, Ernst A, Elhai J | chapter = Heterocyst Metabolism and Development|date=1994 | title = The Molecular Biology of Cyanobacteria |pages=769–823| veditors = Bryant DA |series=Advances in Photosynthesis|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-011-0227-8_27|isbn=978-94-011-0227-8 }}

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. The different types of nitrogenase can be determined by the specific iron protein component.{{cite book | vauthors = Schneider K, Müller A | title = Catalysts for Nitrogen Fixation| chapter = Iron-Only Nitrogenase: Exceptional Catalytic, Structural and Spectroscopic Features|date=2004 |pages=281–307| veditors = Smith BE, Richards RL, Newton WE |series=Nitrogen Fixation: Origins, Applications, and Research Progress|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-1-4020-3611-8_11|isbn=978-1-4020-3611-8 }} 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.{{Cite journal| vauthors = Knoche KL, Aoyama E, Hasan K, Minteer SD |date=2017|title=Role of Nitrogenase and Ferredoxin in the Mechanism of Bioelectrocatalytic Nitrogen Fixation by the Cyanobacteria Anabaena variabilis SA-1 Mutant Immobilized on Indium Tin Oxide (ITO) Electrodes|url=https://www.cheric.org/research/tech/periodicals/view.php?seq=1531452|journal=Electrochimica Acta|language=ko|volume=232|pages=396–403|doi=10.1016/j.electacta.2017.02.148}} In studying the ecology and evolution of nitrogen-fixing bacteria, the nifH gene is the biomarker most widely used.{{cite journal | vauthors = Raymond J, Siefert JL, Staples CR, Blankenship RE | title = The natural history of nitrogen fixation | journal = Molecular Biology and Evolution | volume = 21 | issue = 3 | pages = 541–554 | date = March 2004 | pmid = 14694078 | doi = 10.1093/molbev/msh047 | doi-access = free | author4-link = Robert E. Blankenship }} nifH has two similar genes anfH and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.{{cite journal | vauthors = Schüddekopf K, Hennecke S, Liese U, Kutsche M, Klipp W | title = Characterization of anf genes specific for the alternative nitrogenase and identification of nif genes required for both nitrogenases in Rhodobacter capsulatus | journal = Molecular Microbiology | volume = 8 | issue = 4 | pages = 673–684 | date = May 1993 | pmid = 8332060 | doi = 10.1111/j.1365-2958.1993.tb01611.x | s2cid = 42057860 }}

Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga),{{Cite journal |last1=Garcia |first1=Amanda K. |last2=McShea |first2=Hanon |last3=Kolaczkowski |first3=Bryan |last4=Kaçar |first4=Betül |date=May 2020 |title=Reconstructing the evolutionary history of nitrogenases: Evidence for ancestral molybdenum-cofactor utilization |journal=Geobiology |language=en |volume=18 |issue=3 |pages=394–411 |doi=10.1111/gbi.12381 |issn=1472-4677 |pmc=7216921 |pmid=32065506|bibcode=2020Gbio...18..394G }}{{Cite journal |last1=Boyd |first1=E. S. |last2=Anbar |first2=A. D. |last3=Miller |first3=S. |last4=Hamilton |first4=T. L. |last5=Lavin |first5=M. |last6=Peters |first6=J. W. |date=May 2011 |title=A late methanogen origin for molybdenum-dependent nitrogenase |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1472-4669.2011.00278.x |journal=Geobiology |language=en |volume=9 |issue=3 |pages=221–232 |doi=10.1111/j.1472-4669.2011.00278.x |pmid=21504537 |bibcode=2011Gbio....9..221B |issn=1472-4677}} although some isotopic support showing nitrogenase evolution as early as around 3.2 Ga.{{Cite journal |last1=Stüeken |first1=Eva E. |last2=Buick |first2=Roger |last3=Guy |first3=Bradley M. |last4=Koehler |first4=Matthew C. |date=April 2015 |title=Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr |url=https://www.nature.com/articles/nature14180 |journal=Nature |language=en |volume=520 |issue=7549 |pages=666–669 |doi=10.1038/nature14180 |pmid=25686600 |bibcode=2015Natur.520..666S |issn=0028-0836}} Nitrogenase appears to have evolved from maturase-like proteins, although the function of the preceding protein is currently unknown.{{Cite journal |last1=Garcia |first1=Amanda K |last2=Kolaczkowski |first2=Bryan |last3=Kaçar |first3=Betül |date=2022-03-02 |editor-last=Archibald |editor-first=John |title=Reconstruction of Nitrogenase Predecessors Suggests Origin from Maturase-Like Proteins |journal=Genome Biology and Evolution |language=en |volume=14 |issue=3 |doi=10.1093/gbe/evac031 |issn=1759-6653 |pmc=8890362 |pmid=35179578}}

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).{{Cite journal |last=Eady |first=Robert R. |date=1996-01-01 |title=Structure−Function Relationships of Alternative Nitrogenases |url=https://pubs.acs.org/doi/10.1021/cr950057h |journal=Chemical Reviews |language=en |volume=96 |issue=7 |pages=3013–3030 |doi=10.1021/cr950057h |pmid=11848850 |issn=0009-2665}} Marine metal abundances over Earth's geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common.{{Cite journal |last1=Anbar |first1=A. D. |last2=Knoll |first2=A. H. |date=2002-08-16 |title=Proterozoic Ocean Chemistry and Evolution: A Bioinorganic Bridge? |url=https://www.science.org/doi/10.1126/science.1069651 |journal=Science |language=en |volume=297 |issue=5584 |pages=1137–1142 |doi=10.1126/science.1069651 |pmid=12183619 |bibcode=2002Sci...297.1137A |issn=0036-8075}} Currently, there is no conclusive agreement on which form of nitrogenase arose first.

{{Main|Diazotroph}}

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.{{Cite web|url=https://scitechdaily.com/nitrogen-inputs-in-the-ancient-ocean-underappreciated-bacteria-step-into-the-spotlight/|title=Nitrogen Inputs in the Ancient Ocean: Underappreciated Bacteria Step Into the Spotlight|first=Max Planck|last=Institute|date=6 August 2021}}{{cite journal | vauthors = Mus F, Crook MB, Garcia K, Garcia Costas A, Geddes BA, Kouri ED, Paramasivan P, Ryu MH, Oldroyd GE, Poole PS, Udvardi MK, Voigt CA, Ané JM, Peters JW | title = Symbiotic Nitrogen Fixation and the Challenges to Its Extension to Nonlegumes | journal = Applied and Environmental Microbiology | volume = 82 | issue = 13 | pages = 3698–3710 | date = July 2016 | pmid = 27084023 | pmc = 4907175 | doi = 10.1128/AEM.01055-16 | bibcode = 2016ApEnM..82.3698M | veditors = Kelly RM }} Several obligately anaerobic bacteria fix nitrogen including many (but not all) Clostridium spp. Some archaea such as Methanosarcina acetivorans also fix nitrogen,{{cite journal | vauthors = Dhamad AE, Lessner DJ | title = A CRISPRi-dCas9 System for Archaea and Its Use To Examine Gene Function during Nitrogen Fixation by Methanosarcina acetivorans | journal = Applied and Environmental Microbiology | volume = 86 | issue = 21 | pages = e01402–20 | date = October 2020 | pmid = 32826220 | pmc = 7580536 | doi = 10.1128/AEM.01402-20 | bibcode = 2020ApEnM..86E1402D | veditors = Atomi H }} and several other methanogenic taxa, are significant contributors to nitrogen fixation in oxygen-deficient soils.{{cite journal | vauthors = Bae HS, Morrison E, Chanton JP, Ogram A | title = Methanogens Are Major Contributors to Nitrogen Fixation in Soils of the Florida Everglades | journal = Applied and Environmental Microbiology | volume = 84 | issue = 7 | pages = e02222–17 | date = April 2018 | pmid = 29374038 | pmc = 5861825 | doi = 10.1128/AEM.02222-17 | bibcode = 2018ApEnM..84E2222B }}

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.{{cite journal | vauthors = Latysheva N, Junker VL, Palmer WJ, Codd GA, Barker D | title = The evolution of nitrogen fixation in cyanobacteria | journal = Bioinformatics | volume = 28 | issue = 5 | pages = 603–606 | date = March 2012 | pmid = 22238262 | doi = 10.1093/bioinformatics/bts008 | doi-access = free }} Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine.{{cite journal | vauthors = Pierella Karlusich JJ, Pelletier E, Lombard F, Carsique M, Dvorak E, Colin S, Picheral M, Cornejo-Castillo FM, Acinas SG, Pepperkok R, Karsenti E, de Vargas C, Wincker P, Bowler C, Foster RA | title = Global distribution patterns of marine nitrogen-fixers by imaging and molecular methods | journal = Nature Communications | volume = 12 | issue = 1 | pages = 4160 | date = July 2021 | pmid = 34230473 | pmc = 8260585 | doi = 10.1038/s41467-021-24299-y | bibcode = 2021NatCo..12.4160P }}{{Cite journal| vauthors = Ash C |date=2021-08-13| veditors = Ash C, Smith J |title=Some light on diazotrophs |journal=Science|language=en|volume=373|issue=6556|pages=755.7–756|doi=10.1126/science.373.6556.755-g|bibcode=2021Sci...373..755A|s2cid=238709371|issn=0036-8075}} Indeed, the amount of nitrogen fixed in the ocean is at least as much as that on land.{{cite journal | vauthors = Kuypers MM, Marchant HK, Kartal B | title = The microbial nitrogen-cycling network | journal = Nature Reviews. Microbiology | volume = 16 | issue = 5 | pages = 263–276 | date = May 2018 | pmid = 29398704 | doi = 10.1038/nrmicro.2018.9 | hdl-access = free | s2cid = 3948918 | hdl = 21.11116/0000-0003-B828-1 }} 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.{{cite journal | vauthors = Bergman B, Sandh G, Lin S, Larsson J, Carpenter EJ | title = Trichodesmium--a widespread marine cyanobacterium with unusual nitrogen fixation properties | journal = FEMS Microbiology Reviews | volume = 37 | issue = 3 | pages = 286–302 | date = May 2013 | pmid = 22928644 | pmc = 3655545 | doi = 10.1111/j.1574-6976.2012.00352.x }} Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen.{{Cite web|url=https://www.sciencedaily.com/releases/2018/06/180611133453.htm|title=Large-scale study indicates novel, abundant nitrogen-fixing microbes in surface ocean|website=ScienceDaily|access-date=8 June 2019|archive-url=https://web.archive.org/web/20190608024940/https://www.sciencedaily.com/releases/2018/06/180611133453.htm|archive-date=8 June 2019|url-status=live}} Species of nitrogen fixing cyanobacteria in fresh waters include: Aphanizomenon and Dolichospermum (previously Anabaena).{{Cite journal| vauthors = Rolff C, Almesjö L, Elmgren R |date=2007-03-05|title=Nitrogen fixation and abundance of the diazotrophic cyanobacterium Aphanizomenon sp. in the Baltic Proper|url=http://www.int-res.com/abstracts/meps/v332/p107-118/|journal=Marine Ecology Progress Series|language=en|volume=332|pages=107–118|doi=10.3354/meps332107 |bibcode=2007MEPS..332..107R|doi-access=free}} Such species have specialized cells called heterocytes, in which nitrogen fixation occurs via the nitrogenase enzyme.{{Cite journal| vauthors = Carmichael WW |date=12 Oct 2001|title=Health Effects of Toxin-Producing Cyanobacteria: "The CyanoHABs" |journal=Human and Ecological Risk Assessment|language=en|volume=7|issue=5|pages=1393–1407|doi=10.1080/20018091095087|bibcode=2001HERA....7.1393C |s2cid=83939897|issn=1080-7039}}{{cite journal | vauthors = Bothe H, Schmitz O, Yates MG, Newton WE | title = Nitrogen fixation and hydrogen metabolism in cyanobacteria | journal = Microbiology and Molecular Biology Reviews | volume = 74 | issue = 4 | pages = 529–551 | date = December 2010 | pmid = 21119016 | pmc = 3008169 | doi = 10.1128/MMBR.00033-10 }}

One type of organelle, originating from cyanobacterial endosymbionts called UCYN-A2,{{Cite journal |last1=Thompson |first1=Anne |last2=Carter |first2=Brandon J. |last3=Turk-Kubo |first3=Kendra |last4=Malfatti |first4=Francesca |last5=Azam |first5=Farooq |last6=Zehr |first6=Jonathan P. |date=October 2014 |title=Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophyte host: UCYN-A genetic diversity |url=https://cloudfront.escholarship.org/dist/prd/content/qt4687q7k8/qt4687q7k8.pdf?t=nx0365 |journal=Environmental Microbiology |language=en |volume=16 |issue=10 |pages=3238–3249 |doi=10.1111/1462-2920.12490 |pmid=24761991 |s2cid=24822220}} can turn nitrogen gas into a biologically available form. This nitroplast was discovered in algae, particularly in the marine algae Braarudosphaera bigelowii.{{Cite journal |last=Wong |first=Carissa |date=2024-04-11 |title=Scientists discover first algae that can fix nitrogen — thanks to a tiny cell structure |url=https://www.nature.com/articles/d41586-024-01046-z |journal=Nature |volume=628 |issue=8009 |page=702 |language=en |doi=10.1038/d41586-024-01046-z|pmid=38605201 |bibcode=2024Natur.628..702W }}

Diatoms in the family Rhopalodiaceae also possess cyanobacterial endosymbionts called spheroid bodies or diazoplasts.{{cite journal |last1=Moulin |first1=Solène L. Y. |last2=Frail |first2=Sarah |last3=Braukmann |first3=Thomas |last4=Doenier |first4=Jon |last5=Steele-Ogus |first5=Melissa |last6=Marks |first6=Jane C. |last7=Mills |first7=Matthew M. |last8=Yeh |first8=Ellen |date=15 April 2024 |title=The endosymbiont of Epithemia clementina is specialized for nitrogen fixation within a photosynthetic eukaryote |journal=ISME Communications |volume=4 |pages=ycae055 |doi=10.1093/ismeco/ycae055 |pmc=11070190 |pmid=38707843 |doi-access=free}} These endosymbionts have lost photosynthetic properties, but have kept the ability to perform nitrogen fixation, allowing these diatoms to fix atmospheric nitrogen.{{Cite journal |last1=Schvarcz |first1=Christopher R. |last2=Wilson |first2=Samuel T. |last3=Caffin |first3=Mathieu |last4=Stancheva |first4=Rosalina |last5=Li |first5=Qian |last6=Turk-Kubo |first6=Kendra A. |last7=White |first7=Angelicque E. |last8=Karl |first8=David M. |last9=Zehr |first9=Jonathan P. |last10=Steward |first10=Grieg F. |date=2022-02-10 |title=Overlooked and widespread pennate diatom-diazotroph symbioses in the sea |journal=Nature Communications |language=en |volume=13 |issue=1 |pages=799 |bibcode=2022NatCo..13..799S |doi=10.1038/s41467-022-28065-6 |issn=2041-1723 |pmc=8831587 |pmid=35145076}}{{Cite journal |pmc=4128115 |year=2014 |last1=Nakayama |first1=T. |title=Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=111 |issue=31 |pages=11407–11412 |last2=Kamikawa |first2=R. |last3=Tanifuji |first3=G. |last4=Kashiyama |first4=Y. |last5=Ohkouchi |first5=N. |last6=Archibald |first6=J. M. |last7=Inagaki |first7=Y. |pmid=25049384 |doi=10.1073/pnas.1405222111 |bibcode=2014PNAS..11111407N |doi-access=free}} Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera Hemiaulus, Rhizosolenia and Chaetoceros.{{Cite journal |last1=Pierella Karlusich |first1=Juan José |last2=Pelletier |first2=Eric |last3=Lombard |first3=Fabien |last4=Carsique |first4=Madeline |last5=Dvorak |first5=Etienne |last6=Colin |first6=Sébastien |last7=Picheral |first7=Marc |last8=Cornejo-Castillo |first8=Francisco M. |last9=Acinas |first9=Silvia G. |last10=Pepperkok |first10=Rainer |last11=Karsenti |first11=Eric |date=2021-07-06 |title=Global distribution patterns of marine nitrogen-fixers by imaging and molecular methods |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=4160 |doi=10.1038/s41467-021-24299-y |issn=2041-1723 |pmc=8260585 |pmid=34230473 |bibcode=2021NatCo..12.4160P}}

{{Main|Root nodule}}

Image:Root nodules on fava bean plant.jpg

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. 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.{{cite journal | vauthors = Kuypers MM, Marchant HK, Kartal B | title = The microbial nitrogen-cycling network | journal = Nature Reviews. Microbiology | volume = 16 | issue = 5 | pages = 263–276 | date = May 2018 | pmid = 29398704 | doi = 10.1038/nrmicro.2018.9 | hdl = 21.11116/0000-0003-B828-1 | s2cid = 3948918 | hdl-access = free }} When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the soil.{{cite book | vauthors = Smil V |year=2000 |title=Cycles of Life |publisher=Scientific American Library}} 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.{{citation needed|date=August 2019}}

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 {{convert|50|to|200|lb/acre|abbr=on}}.{{Cite web|url=http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|title=Nitrogen Fixation and Inoculation of Forage Legumes|archive-url=https://web.archive.org/web/20161202170130/http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|archive-date=2 December 2016|url-status=dead}}

Image:A sectioned alder root nodule gall.JPG

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.{{cite book | vauthors = Dawson JO | chapter = Ecology of Actinorhizal Plants |title=Nitrogen-fixing Actinorhizal Symbioses |volume=6 |pages=199–234 |doi=10.1007/978-1-4020-3547-0_8 |year=2008 |publisher=Springer |series=Nitrogen Fixation: Origins, Applications, and Research Progress |isbn=978-1-4020-3540-1 }}

In addition, Trema (Parasponia), a tropical genus in the family Cannabaceae, is unusually able to interact with rhizobia and form nitrogen-fixing nodules.{{cite journal | vauthors = Op den Camp R, Streng A, De Mita S, Cao Q, Polone E, Liu W, Ammiraju JS, Kudrna D, Wing R, Untergasser A, Bisseling T, Geurts R | title = LysM-type mycorrhizal receptor recruited for rhizobium symbiosis in nonlegume Parasponia | journal = Science | volume = 331 | issue = 6019 | pages = 909–12 | date = February 2011 | pmid = 21205637 | doi = 10.1126/science.1198181 | author-link11 = Ton Bisseling | s2cid = 20501765 | bibcode = 2011Sci...331..909O }}

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{{Cite web|title=Cycad biology, Article 1: Corraloid roots of cycads|url=http://www1.biologie.uni-hamburg.de/b-online/library/cycads/corraloid.htm|access-date=2021-10-14|website=www1.biologie.uni-hamburg.de}}
• Gunnera
• Blasia (liverwort)
• Hornworts{{Cite journal| vauthors = Rai AN |date=2000|title=Cyanobacterium-plant symbioses|journal=New Phytologist|volume=147|issue=3|pages=449–481|doi=10.1046/j.1469-8137.2000.00720.x|pmid=33862930|doi-access=free}}

Some symbiotic relationships involving agriculturally-important plants are:{{cite journal | vauthors = Van Deynze A, Zamora P, Delaux PM, Heitmann C, Jayaraman D, Rajasekar S, Graham D, Maeda J, Gibson D, Schwartz KD, Berry AM, Bhatnagar S, Jospin G, Darling A, Jeannotte R, Lopez J, Weimer BC, Eisen JA, Shapiro HY, Ané JM, Bennett AB | title = Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota | journal = PLOS Biology | volume = 16 | issue = 8 | pages = e2006352 | date = August 2018 | pmid = 30086128 | pmc = 6080747 | doi = 10.1371/journal.pbio.2006352 | doi-access = free }}

• 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'{{cite web | vauthors = Pskowski M |date=July 16, 2019|title=Indigenous Maize: Who Owns the Rights to Mexico's 'Wonder' Plant? |url=https://e360.yale.edu/features/indigenous-maize-who-owns-the-rights-to-mexicos-wonder-plant |website=Yale E360}} and various Bacteroidota and Pseudomonadota

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.{{cite journal| title= The Manufacture of Nitrates from the Atmosphere by the Electric Arc—Birkeland-Eyde Process | vauthors = Eyde S | journal= Journal of the Royal Society of Arts| volume= 57| issue = 2949 | year= 1909| pages= 568–576 | jstor=41338647}} 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.{{cite journal |title=Die Umwandlungsgleichung Ba(CN)₂ → BaCN₂ + C im Temperaturgebiet von 500 bis 1000 °C |trans-title=The conversion reaction Ba(CN)₂ → BaCN₂ + C in the temperature range from 500 to 1,000 °C | vauthors = Heinrich H, Nevbner R | journal = Z. Elektrochem. Angew. Phys. Chem. | volume = 40 | issue = 10 | pages = 693–698 | year = 1934 | url = http://onlinelibrary.wiley.com/doi/10.1002/bbpc.19340401005/abstract |url-access=subscription | access-date = 8 August 2016 | doi = 10.1002/bbpc.19340401005 |s2cid=179115181 | archive-url = https://web.archive.org/web/20160820203326/http://onlinelibrary.wiley.com/doi/10.1002/bbpc.19340401005/abstract | archive-date = 20 August 2016 | url-status = live }}{{cite book | url = {{google books |plainurl=y |id=87XQAAAAMAAJ}}| title = Fixed nitrogen | vauthors = Curtis HA | year = 1932}}

{{Main|Haber process}}

File:THC 2003.902.022 D. C. Bardwell Study of Nitrogen Fixation.tifs (Fixed Nitrogen Research Laboratory, 1926)]]

The dominant industrial method for producing ammonia is the Haber process also known as the Haber-Bosch process.Smil, V. 2004. Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production, MIT Press. 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{{Cite journal| vauthors = Glibert PM, Maranger R, Sobota DJ, Bouwman L |author-link1=Patricia Glibert |author-link2=Roxane Maranger |date=2014-10-01|title=The Haber Bosch–harmful algal bloom (HB–HAB) link|journal=Environmental Research Letters|volume=9|issue=10|pages=105001|doi=10.1088/1748-9326/9/10/105001|bibcode=2014ERL.....9j5001G|s2cid=154724892 |issn=1748-9326|doi-access=free}} 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.{{Cite journal| vauthors = Erisman JW, Sutton MA, Galloway J, Klimont Z, Winiwarter W |date=October 2008|title=How a century of ammonia synthesis changed the world |journal=Nature Geoscience|language=en|volume=1|issue=10|pages=636–639|doi=10.1038/ngeo325|bibcode=2008NatGe...1..636E|s2cid=94880859 |issn=1752-0908}}

{{Main|Abiological nitrogen fixation}}

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.{{cite journal | title = Nitrogenopentammineruthenium(II) complexes | vauthors = Allen AD, Senoff CV | journal = J. Chem. Soc., Chem. Commun. | issue = 24 | pages = 621–622 | year = 1965 | doi = 10.1039/C19650000621 }} Some soluble complexes do catalyze nitrogen fixation.{{cite journal | vauthors = Chalkley MJ, Drover MW, Peters JC | title = Catalytic N₂-to-NH₃ (or -N₂H₄) Conversion by Well-Defined Molecular Coordination Complexes | journal = Chemical Reviews | volume = 120 | issue = 12 | pages = 5582–5636 | date = June 2020 | pmid = 32352271 | pmc = 7493999 | doi = 10.1021/acs.chemrev.9b00638}}

File:Lightning Pritzerbe 01 (MK).jpg heats the air around it in a high-temperature plasma, breaking the bonds of {{chem|N|2}}, starting the formation of nitrous acid ({{chem|HNO|2}}).]]

Nitrogen can be fixed by lightning converting nitrogen gas ({{chem|N|2}}) and oxygen gas ({{chem|O|2}}) in the atmosphere into {{NOx}} (nitrogen oxides). The {{chem|N|2}} molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms.{{Cite journal| vauthors = Tuck AF |date=October 1976 |title=Production of nitrogen oxides by lightning discharges|journal=Quarterly Journal of the Royal Meteorological Society|volume=102|issue=434|pages=749–755|doi=10.1002/qj.49710243404|issn=0035-9009|bibcode=1976QJRMS.102..749T}} Lightning produces enough energy and heat to break this bond allowing nitrogen atoms to react with oxygen, forming {{chem|NO|x}}. These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form {{chem|NO|2}},{{cite journal| vauthors = Hill RD |date=August 1979|title=Atmospheric Nitrogen Fixation by Lightning|journal=Journal of the Atmospheric Sciences|volume=37|pages=179–192|doi=10.1175/1520-0469(1980)037<0179:ANFBL>2.0.CO;2|issn=1520-0469|bibcode=1980JAtS...37..179H|doi-access=free}} which in turn reacts with water to produce {{chem|HNO|2}} (nitrous acid) or {{chem|HNO|3}} (nitric acid). When these acids seep into the soil, they make NO₃⁻ (nitrate), which is of use to plants.{{Cite web|url=https://journals.ohiolink.edu/pg_99?126555292207822::NO::P99_ENTITY_ID,P99_ENTITY_TYPE:20567211,MAIN_FILE&cs=38gV8XNFDWVznjRjSa1erAIidpqPJBYgnOix4OM5wvpjv8vJAbG2NGhNYwWAVItkNehLIgXVYuozMCrUrENyuYA|title=Tropospheric Sources of NOx: Lightning And Biology | vauthors = Levin JS |date=1984|access-date=2018-11-29}}

• Birkeland–Eyde process: an industrial fertilizer production process
• Carbon fixation
• Denitrification: an organic process of nitrogen release
• George Washington Carver: an American botanist
• Heterocyst
• Nitrification: biological production of nitrogen
• Nitrogen cycle: the flow and transformation of nitrogen through the environment
• Nitrogen deficiency
• Nitrogen fixation package for quantitative measurement of nitrogen fixation by plants
• Nitrogenase: enzymes used by organisms to fix nitrogen
• Ostwald process: a chemical process for making nitric acid ({{chem|HNO|3}})
• Electrification of catalytic processes: electrochemical reduction of N₂

{{Reflist}}

• {{cite web|url=http://www.mcdb.ucla.edu/Research/Hirsch/imagesb/HistoryDiscoveryN2fixingOrganisms.pdf|title=A Brief History of the Discovery of Nitrogen-fixing Organisms | vauthors = Hirsch AM |date=2009|publisher=University of California, Los Angeles}}
• {{cite web|url=http://dornsife.usc.edu/labs/capone|title=Marine Nitrogen Fixation laboratory|publisher=University of Southern California}}
• {{Cite web|url=https://digital.sciencehistory.org/collections/gm80hv42t|title=Travis P. Hignett Collection of Fixed Nitrogen Research Laboratory Photographs // Science History Institute Digital Collections|website=digital.sciencehistory.org|access-date=2019-08-16}} Science History Institute Digital Collections (Photographs depicting numerous stages of the nitrogen fixation process and the various equipment and apparatus used in the production of atmospheric nitrogen, including generators, compressors, filters, thermostats, and vacuum and blast furnaces).
• "[https://books.google.com/books?id=p4o9AQAAIAAJ Proposed Process for the Fixation of Atmospheric Nitrogen]", historical perspective, Scientific American, 13 July 1878, p. 21
• [https://naturemicrobiologycommunity.nature.com/posts/a-global-ocean-snapshot-of-nitrogen-fixers-by-matching-sequences-to-cells-in-the-tara-ocean A global ocean snapshot of nitrogen fixers by matching sequences to cells in the Tara Ocean]

{{Authority control}}

Category:Nitrogen cycle

Category:Metabolism

Category:Plant physiology

Category:Soil biology