Nitrogen fixation

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) (Postgate, 1998).

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.

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 (Postgate 1998). The formula for BNF is:

N₂ + 8H⁺ + 8e^âˆ' + 16 ATP â†' 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 baceria cease production of the enzyme in the presence of oxygen) (Postgate 1998). 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) (Postgate 1998).

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 (Postgate, 1998; Smil, 2000). The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not.

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

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. Artificial fertilizer production has achieved such scale that it is now the largest source of fixed nitrogen in the Earth's ecosystem.

References

See also

*Denitrification
*George Washington Carver
*Nitrification
*Nitrogen cycle
*Nitrogen deficiency

External links

*Nitrogen Fixation