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关于固氮作用的简单而明了的介绍——转载

已有 5517 次阅读 2007-11-22 21:55 |个人分类:微生物生理学专栏|文章来源:转载

以下内容来自http://www.answers.com/topic/nitrogen-fixation

nitrogen fixation
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Nitrogen fixation

The chemical or biological conversion of atmospheric nitrogen (N2) into compounds which can be used by plants, and thus become available to animals and humans. In the 1990s, chemical and biological processes together contributed about 260 million tons (230 million metric tons) of fixed nitrogen per year globally. Industrial production of nitrogen fertilizer accounted for about 85 million tons (80 million metric tons) of nitrogen per year, while spontaneous chemical processes, such as lightning, ultraviolet irradiation, and combustion, leading to the synthesis of nitrogen oxides from O2 and N2, may have accounted for 44 million tons (40 million metric tons) per year. The remainder, roughly half of the global input of newly fixed nitrogen, arose from biological processes. World agriculture, which is very dependent on nitrogen fixation, is increasingly reliant on chemical nitrogen sources.

Chemical fixation

Three chemical processes for fixing atmospheric nitrogen have been developed. All require considerable thermal or electrical energy and yield different products. In arc processes, which are now rarely used, air is passed through an electric arc and about 1% nitric oxide is formed, which can be chemically converted to nitrates. In the cyanamide process, which is now obsolete, heating calcium carbide in nitrogen generates calcium cyanamide, which when moistened hydrolyzes to urea and ammonia. In the widely used Haber process, hydrogen (generated by heating natural gas) is mixed with nitrogen (from air), and burned to yield a nitrogen-hydrogen mixture. The nitrogen-hydrogen mixture is compressed (10–80 megapascals) and heated (200–700°C or 390–1300°F) in the presence of a metal oxide catalyst to give ammonia. The Haber process is the major source of ammonia used for fertilizer. See also Ammonia; Cyanamide; Electrochemical process; Fertilizer; High-pressure processes.

Biological fixation

Only prokaryotes—bacteria, archaea, and cyanobacteria (earlier called blue-green algae)—fix nitrogen. Nitrogen-fixing microbes, called diazotrophs, fall into two main groups, free-living and symbiotic. See also Archaebacteria; Bacteria; Cyanobacteria; Prokaryotae.

The free-living diazotrophs are subclassified. Aerobic diazotrophs, of which there are over 50 genera, including Azotobacter, methane-oxidizing bacteria, and cyanobacteria, require oxygen for growth and fix nitrogen when oxygen is present. Azotobacter, some related bacteria, and some cyanobacteria fix nitrogen in ordinary air, but most members of this group fix nitrogen only when the oxygen concentration is low. Free-living diazotrophs, which fix nitrogen only when oxygen is absent or vanishingly low, are widespread. The genera Bacillus and Klebsiella include many strains of this type, and representatives of symbiotic diazotrophs behave in this way as well. See also Algae; Bacterial physiology and metabolism.

The best-known symbiotic bacteria belong to the genus Rhizobium. Species of Rhizobium, or related genera, such as Bradyrhizobium and Sinorhizobium, colonize the roots of leguminous plants and stimulate the formation of nodules within which they fix nitrogen microaerobically. Both plants and bacteria show specificity; for example, certain types of plants require special strains of rhizobia. Some types of rhizobium, such as Bradyrhizobium, can fix nitrogen in the absence of plant tissue, but require low oxygen, though most rhizobia fix nitrogen only within the nodules. See also Soil microbiology.

The enzymes responsible for nitrogen fixation are called nitrogenases. The most common nitrogenase consists of two proteins, one large containing molybdenum, iron, and inorganic sulfur (the MoFe-protein or dinitrogenase), the other smaller containing iron and inorganic sulfur (the Fe-protein or dinitrogenase reductase). Nitrogenase reduces one molecule of N2 to two of ammonia (NH3), a reaction which is accompanied by the conversion of 16 molecules of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and the release of one molecule of H2 as a by-product. Nitrogenase is irreversibly destroyed by air, so all aerobic diazotrophs have developed means of restricting access of oxygen to the active enzyme.

 

  1. The conversion of atmospheric nitrogen into compounds, such as ammonia, by natural agencies or various industrial processes.
  2. The conversion by certain soil microorganisms, such as rhizobia, of atmospheric nitrogen into compounds that plants and other organisms can assimilate.

以下内容来自:http://www.grainlegumes.com/aep/layout/set/print/layout/set/print/content/view/full/22693

Importance of nitrogen for life
The element 'Nitrogen' is an essential nutrient for all organisms' growth. It enters in large amounts in the form of many basic chemical compounds, such as proteins, nucleic acids and other cellular constituents.
The earth's atmosphere is very abundant in nitrogen in the form of N2 gas (nearly 79% N2, 21% O2). However this free gaseous nitrogen cannot be used directly by animals or by higher plants to build the chemicals necessary for growth and reproduction. In fact, the N2 molecule is composed of two atoms of nitrogen linked by a very strong triple bond that requires a large amount of energy to be broken. Therefore the N2 molecule is quite chemically unreactive.
To be incorporated by living organisms the atmospheric N2 must first be combined with oxygen or hydrogen into biological compounds such as ammonia (NH3, NH4+) or nitrates (NO3-). This conversion process is commonly referred to as nitrogen fixation that may be accomplished chemically or biologically. In this latter case, it is known as Biological Nitrogen Fixation (BNF).



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