Physiology, Biochemistry and Genetics of Dinitrogen Fixation

Abstract

(Di)nitrogen fixation is the terminus technicus for the conversion of the dinitrogen molecule to ammonia according to the following equation: 6 H+ + 6 e- + N2=2 NH3. This reduction can proceed either chemically or biologically in a reaction catalyzed by the enzyme complex nitrogenase. The direct comparison of the data for chemical and biological nitrogen fixation reveals the importance of the latter reaction. The chemical reaction, known as the Haber–Bosch process, is used to supply nitrogen fertilizers for agriculture. Estimates of the global consumption of fertilizers in 1974 were 30 × 106 (Burns and Hardy 1975) or 40 × 106 t year- 1 (Rothamsted Subject Day 1975), and this latter compendium stated that the world–wide consumption had increased tenfold since 1950. The Haber-Bosch process is highly energy–consuming since it proceeds at a pressure of some 200 atm and temperatures between 400°–600 °C where molecular hydrogen and nitrogen are converted to ammonia in the presence of a metal catalyst and with a yield of approximately 13%. It is said (Sprent 1979) that about 1.5 kg of fuel oil is needed to deliver 1 kg of fertilizer N to the farm. In comparison, the rate of biological nitrogen fixation (in 106 t yr- 1) was calculated to be 175 (Burns and Hardy 1975), 170–270 (Soderlund and Svensson 1976) and 150 (U.S. Council of Agricultural Science and Technology, Cast, Anon, 1976).