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 useful for other chemical processes (such as, notably, ammonia, nitrate and nitrogen dioxide) <ref name=postgate>Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Pres, Cambridge UK.</ref>.

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.

Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.

1 Biological Nitrogen Fixation
2 Leguminous nitrogen-fixing plants
3 Non-leguminous nitrogen fixing plants
4 Chemical nitrogen fixation
5 References
6 See also
7 External links

[edit] Biological Nitrogen Fixation

Schematic representation of the nitrogen cycle.

Biological Nitrogen Fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called nitrogenase <ref name=postgate/>. The formula for BNF is:

N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16ADP + 16 P_i

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) <ref name=postgate/>. 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) <ref name=postgate>.

[edit] Leguminous nitrogen-fixing plants

The best-known are legumes (such as clover, beans, alfalfa and peanuts) which 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 nitrogen helps to fertilize the soil <ref name=postgate></ref> <ref>Smil, V (2000). Cycles of Life. Scientific American Library.</ref> . The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not.

[edit] Non-leguminous nitrogen fixing plants

Plants from many other families have similar associations, including: *Lobaria lichen and some other lichens

Mosquito fern (Azolla species)
Cycads
Gunnera
Alder (Alnus species)
Ceanothus (Ceanothus species)
Wax myrtle (Myrica species)
Mountain-mahogany (Cercocarpus species)
Bitterbrush (Purshia tridentata)
Buffalo berry (Shepherdia argentea)
Ironwood (Casuarina species), Sheoak (Allocasuarina species), and other genera in the Casuarinaceae

[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 <ref> 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</ref> <ref>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</ref> <ref>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. </ref>.