(Redirected from Nitrogen fixing)'Nitrogen fixation' is the process by which
nitrogen is taken from its relatively inert molecular form (N
2) in the
atmosphere and converted into nitrogen compounds (such as, notably,
ammonia,
nitrate and
nitrogen dioxide)
[ Nitrogen Fixation, 3rd Edition, Postgate, J, , , Cambridge University Press, Cambridge UK, 1998, ] useful for other chemical processes.
Nitrogen fixation is performed naturally by a number of different
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 with diazotrophs.
Nitrogen fixation also occurs as a result of non-biological processes. These include
lightning, industrially through the
Haber-Bosch Process, and combustion.
[1]
Biological nitrogen fixation was discovered by the Dutch microbiologist
Martinus Beijerinck.
Biological nitrogen fixation
Biological Nitrogen Fixation ('BNF') occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called
nitrogenase.
[ 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 protonated into 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).[ 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).][.]
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
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''
Microorganisms that fix nitrogen
★ Diazotrophs
★ Cyanobacteria
★ Azotobacteraceae
★ Rhizobia
★ Frankia
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. Many compounds can react with atmospheric nitrogen under ambient conditions (eg lithium makes lithium nitride if left exposed), but the products of such reactions are not easily converted into biologically accessible nitrogen sources. After the first dinitrogen complex was discovered in 1965 based on ammonia coordinated to ruthenium ([Ru(NH3)5(N2)]2+), research in chemical fixation focused on transition metal complexes. However, progress has been slow; dinitrogen is a poor ligand and the N-N triple bond is very strong.
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[3]. 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]
Unfortunately, the catalytic reduction only undergoes a few turnovers before the catalyst dies.
Synthetic nitrogen reduction Yandulov 2006
References
1. http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm
2.
3. C. E. Laplaza and C. C. Cummins, Science, '1995', 268, pp 861
4. ''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)
5. ''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
6. 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 the chromium metallocene CrCp2
★ where Cp
★ stands for the pentamethylcyclopentadiene ligand.
See also
★ Denitrification
★ George Washington Carver
★ Nitrification
★ Nitrogen cycle
★ Nitrogen deficiency
★ Nitrogenase
★ Birkeland-Eyde process
External links
★ Nitrogen Fixation
★ NITROGEN FIXATION
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