How Do Microbes "Fix" Nitrogen 

 From the Air? ' 



By D. J. D. Nicholas 



Long Ashton Research Station, University of Bristol 



All living things contain nitrogen compounds and all agriculture 

 depends upon the presence of combined nitrogen in the soil. A supply 

 of nitrogen compounds on Earth has been built up over a period of 

 millions of years by physical forces, notably lightning, whereby nitric 

 acid is formed in thunderstorms. The bulk of the combined nitrogen 

 comes, however, from the remarkable ability to convert the nitrogen 

 gas of the air into protein, possessed by a few species of free-living 

 microorganisms that inhabit soils and seas, and by others that live in 

 interdependence (symbiosis) with the roots or leaves of plants. These 

 microorganisms "fix" about 100 million tons of nitrogen a year and 

 play a vital part in the nitrogen cycle in nature (fig. 2) , without which 

 all plant, animal, and human life would come to a halt. 



The relatively few microorganisms so far known to fix atmospheric 

 nitrogen gas are listed in table 1. Of the free-living bacteria, only 

 Azotobacter requires oxygen for growth (fig. 1). The majority, how- 

 ever, are "anaerobes," inhibited by oxygen and thriving only when it 

 is absent. Other types of nitrogen-fixing bacteria also ultilize carbon 

 dioxide from the air, however, in the process of photosynthesis, and 

 some blue-green algae perform this dual function of carbon dioxide 

 and nitrogen fixation. 



What of the microorganisms that fix nitrogen while living in sym- 

 liiotic association with higher plants? The best known plants taking 

 part in this process are probably the leguminous species, including the 

 economic crops, clover, alfalfa, peas and beans, where various strains 

 of rhizobia bacteria invade the roots, forming nodules. It is not 

 widely known that about nine kinds of ordinary flowering plants (non- 

 legumes) also have root nodules in which nitrogen fixation takes place, 

 such as alder in temperate zones and Casiatrina and Mi/t'/ra in warmer 



^Reprinted by permission fnnn Nck Scicnlint ('So. 1()9), London, Deconilicr 19r)r>. 

 766-746 — 65^^35 449 



