Nitrogen fixation
From Wikipedia, the free encyclopedia
Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds (such as, notably, ammonia, nitrate and nitrogen dioxide)[1] useful for other chemical processes.
Nitrogen fixation is performed naturally by a number of different prokaryotes, including bacteria, and actinobacteria certain types of anaerobic bacteria. Microorganisms that fix nitrogen are called diazotrophs. Some higher plants, and some animals (termites), have formed associations with diazotrophs.
Nitrogen fixation also occurs as a result of non-biological processes. These include lightning, industrially through the Haber-Bosch Process, and combustion.[2]
Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.
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[edit] Biological Nitrogen Fixation
Biological Nitrogen Fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called nitrogenase.[1] The formula for BNF is:
- N2 + 8H+ + 8e− + 16 ATP → 2NH3 + H2 + 16ADP + 16 Pi
Although ammonia (NH3) is the direct product of this reaction, it is quickly ionized to ammonium (NH4+). In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.
In most bacteria, the nitrogenase enzymes are very susceptible to destruction by oxygen (and many bacteria cease production of the enzyme in the presence of oxygen).[1] Low oxygen tension is achieved by different bacteria by: living in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein (e.g. leghaemoglobin).[1][3] The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not.
[edit] 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.
Family Genera
Betulaceae: Alnus
Casuarinaceae: Allocasuarina
- Casuarina
- Gymnostoma
Coriariaceae: Coriaria
Datiscaceae: Datisca
Elaeagnaceae: Elaeagnus
- Hippophae
- Shepherdia
Myricaceae: Morella
- Myrica
- Comptonia
Rhamnaceae: Ceanothus
- Colletia
- Discaria
- Kentrothamnus
- Retanilla
- Trevoa
Rosaceae: Cercocarpus
- Chamaebatia
- Purshia
- Dryas
There are also several nitrogen-fixing symbiotic associations that involve cyanobacteria (such as Nostoc). These include some lichens such as Lobaria and Peltigera:
- Mosquito fern (Azolla species)
- Cycads
- Gunnera
[edit] Microrganisms that fix nitrogen
[edit] Chemical nitrogen fixation
Nitrogen can also be artificially fixed for use in fertilizer, explosives, or in other products. The most popular method is by the Haber process. This artificial fertilizer production has achieved such scale that it is now the largest source of fixed nitrogen in the Earth's ecosystem.
The Haber process requires high pressures and very high temperatures and active research is committed to the development of catalyst systems that convert nitrogen to ammonia at ambient temperatures. The first dinitrogen complex was discovered in 1965 based on ammonia coordinated to ruthenium ([Ru(NH3)5(N2)]2+) This discovery was followed by the first example of homolytic cleavage of nitrogen by a molybdenum complex to two equivalents of a triple bonded MoN complex (1995). The first catalytic system converting nitrogen to ammonia at room temperature and 1 atmosphere was discovered in 2003 and is based on another molybdenum catalyst, a proton source and a strong reducing agent.[4][5][6]
[edit] References
- ^ a b c d Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Pres, Cambridge UK.
- ^ http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm
- ^ Smil, V (2000). Cycles of Life. Scientific American Library.
- ^ 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 purpose of the bulky HIPS ligands is to prevent the formation of the stable and nonreactive Mo-N=N-Mo dimer, the actual reduction takes place in a cavity created by these ligands. The proton donor is a pyridinium cation which is accompanied by a tetraborate counter ion. The reducing agent is the chromium metallocene CrCp2* where Cp* stands for the pentamethylcyclopentadiene ligand.
[edit] See also
- Denitrification
- George Washington Carver
- Nitrification
- Nitrogen cycle
- Nitrogen deficiency
- Nitrogenase
