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Nitrogen fixation usually refers to the biological process by which nitrogen (N2) in the atmosphere is converted into ammonia. This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g. nucleotides for DNA and amino acids for proteins. Formally, nitrogen fixation also refers to other abiological conversions of nitrogen, such as its conversion to nitrogen dioxide.
Nitrogen fixation is utilized by numerous prokaryotes, including bacteria, actinobacteria, and certain types of anaerobic bacteria. Microorganisms that fix nitrogen are called diazotrophs. Some higher plants, and some animals (termites), have formed associations (symbiosises) with diazotrophs. Nitrogen fixation also occurs as a result of non-biological processes. These include lightning, industrially through the Haber-Bosch Process, and combustion. Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.
Biological nitrogen fixation
- N2 + 6 H+ + 6 e− → 2 NH3
The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one molecule of H2. In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.
Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen. (In fact, 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.
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clover, soybeans, alfalfa, lupines and peanuts. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (family Polygonaceae), which were often referred to as "green manure."
Non-leguminous nitrogen-fixing plants
Although by far the majority of nitrogen-fixing plants are in the legume family Fabaceae, there are a few non-leguminous plants that can also fix nitrogen. These plants, referred to as "actinorhizal plants", consist of 22 genera of woody shrubs or trees scattered in 8 plant families. The ability to fix nitrogen is not universally present in these families. For instance, of 122 genera in the Rosaceae, only 4 genera are capable of fixing nitrogen.
Microorganisms that fix nitrogen
Nitrogen fixation by cyanobacteria
Cyanobacteria inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. Generally, cyanobacteria are able to utilize a variety of inorganic and organic sources of combined nitrogen, like nitrate, nitrite, ammonium, urea or some amino acids. Several cyanobacterial strains are also capable of diazotrophic growth. Genome sequencing has provided a large amount of information on the genetic basis of nitrogen metabolism and its control in different cyanobacteria. Comparative genomics, together with functional studies, has led to a significant advance in this field over the past years. 2-oxoglutarate has turned out to be the central signalling molecule reflecting the carbon/nitrogen balance of cyanobacteria. Central players of nitrogen control are the global transcriptional factor NtcA, which controls the expression of many genes involved in nitrogen metabolism, as well as the PII signalling protein, which fine-tunes cellular activities in response to changing C/N conditions. These two proteins are sensors of the cellular 2-oxoglutarate level and have been conserved in all cyanobacteria. In contrast, the adaptation to nitrogen starvation involves heterogeneous responses in different strains. Nitrogen fixation by cyanobacteria in coral reefs can fix twice the amount of nitrogen than on land – around 1.8kg of nitrogen is fixed per hectare per day.
Chemical nitrogen fixation
Nitrogen can also be artificially fixed. for use in fertilizers, explosives, or in other products. The most common method is the Haber process. Artificial fertilizer production is now the largest source of fixed nitrogen in the Earth's ecosystem.
The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), routine conditions for industrial catalysis. This highly efficient process uses natural gas as a hydrogen source and air as a nitrogen source.
Research on catalytic nitrogen-fixation
Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of reducing the energy required for this conversion. Such research has thus far failed to even approach the efficiency and ease of the Haber process, however. Many compounds react with atmospheric nitrogen under ambient conditions. For example, lithium metal converts to lithium nitride under an atmosphere of nitrogen. Treatment of the resulting nitride gives ammonia.
The first dinitrogen complex was reported in 1965 based on ammonia coordinated to ruthenium ([Ru(NH3)5(N2)]2+), research in chemical fixation focused on transition metal complexes. Since then a large number of transition metal compounds that contain dinitrogen as a ligand have been discovered. The dinitrogen ligand can be bound either to a single metal or bridge two (or more) metals. The coordination chemistry of dinitrogen is complex and currently under intense investigation. This research may lead to new ways of using dinitrogen in synthesis and on an industrial scale.
The first example of homolytic cleavage of dinitrogen under mild conditions was published in 1995. Two equivalents of a molybdenum complex reacted with one equivalent of dinitrogen, creating a triple bonded MoN complex. Since then, this triple bounded complex has been used to make nitriles .The first catalytic system converting nitrogen to ammonia at room temperature and pressure was discovered in 2003 and is based on another molybdenum compound, a proton source and a strong reducing agent.    Unfortunately, this catalytic reduction only fixates a few nitrogen molecules.
- ^ a b c d e Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK.
- ^ http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm
- ^ Smil, V (2000). Cycles of Life. Scientific American Library.
- ^ Herrero A and Flores E (editor). (2008). The Cyanobacteria: Molecular Biology, Genomics and Evolution (1st ed. ed.). Caister Academic Press. ISBN 978-1-904455-15-8 . http://www.horizonpress.com/cyan.
- ^ http://www.epa.gov/watertrain/nitroabstr.html US Enivronmental Protection Agency: Human Alteration of the Global Nitrogen Cycle: Causes and Consequences by Peter M. Vitousek, Chair, John Aber, Robert W. Howarth, Gene E. Likens, Pamela A. Matson, David W. Schindler, William H. Schlesinger, and G. David Tilman
- ^ Chem. Commun. 1965, pp. 621–622
- ^ C. E. Laplaza and C. C. Cummins, Science, 1995, 268, pp 861
- ^ Curley, Sceats, and Cummins, J. Amer. Chemical Soc., 2006, 128, pp 14036
- ^ Synthesis and Reactions of Molybdenum Triamidoamine Complexes Containing Hexaisopropylterphenyl Substituents Dmitry V. Yandulov, Richard R. Schrock, Arnold L. Rheingold, Christopher Ceccarelli, and William M. Davis Inorg. Chem.; 2003; 42(3) pp 796–813; (Article) doi:10.1021/ic020505l
- ^ Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center Dmitry V. Yandulov and Richard R. Schrock Science 4 July 2003: Vol. 301. no. 5629, pp. 76–78 doi:10.1126/science.1085326
- ^ The catalyst is based on molybdenum(V) chloride and tris(2-aminoethyl)amine substituted with three very bulky hexa-isopropylterphenyl (HIPT) groups. Nitrogen adds end-on to the molybdenum atom and the bulky HIPT substituents prevent the formation of the stable and nonreactive Mo-N=N-Mo dimer, and the nitrogen is reduced in an isolated pocket. The proton donor is a pyridinium cation which is accompanied by a tetraborate counter ion. The reducing agent is decamethylchromocene. All ammonia formed is collected as the HCl salt by trapping the distillate with a HCl solution
- ^ Note also that although the dinitrogen complex is shown in brackets this species can be isolated and characterized. Here the brackets do not indicate that the intermediate is not observed.
- George Washington Carver
- Nitrogen cycle
- Nitrogen deficiency
- Birkeland–Eyde process