226 PLANT BIOCHEMISTRY 



tion, mainly to nitrogen, puts this critical element into an unavailable 

 form in the atmosphere. The evolutionary solution of this problem 

 has taken the form of species fixing molecular nitrogen. Again energy 

 is needed for the reduction, which is carried out aerobically by species 

 of Azotobacter, anaerobically by Clostridia, and photosynthetically by 

 the various photosynthetic bacteria, the blue-green algae, and members 

 of the bacterial genus Rhizobium in collaboration with leguminous 

 plants. The first two groups are bacterial genera and use organic 

 energy sources. The last groups employ solar energy in reducing the 

 nitrogen. 



Knowledge of the mechanism of biological nitrogen fixation is of 

 great potential importance to industry, biology, and agriculture. Yet 

 in spite of a great expenditure of effort this mechanism is almost com- 

 pletely unknown. Many schemes have been postulated on the basis of 

 indirect evidence of diverse kinds. Most of these schemes list inorganic 

 compounds as intermediates between molecular nitrogen and am- 

 monia. However, there is room for doubt that even the ammonia 

 observed is a primary biological product. The present authors feel 

 that the ammonia may be a by-product derived from one or more 

 reactions of the types mentioned above in connection with the forma- 

 tion of ammonia from amino acids. It seems more probable that molec- 

 ular nitrogen is fixed almost immediately in an organic form which 

 is then reduced stepwise to amino nitrogen. Proof of this or any other 

 theory appears to lie in the future. 



These transformations may be summarized in the following nitro- 

 gen cycle: 



Organic nitrogen 



A I 



NH 



+ 



^nitrification 



denitrification 



The combination of Rhizobium and legume is agriculturally im- 

 portant in nitrogen fixation. This sort of association is designated 

 as symbiotic, meaning that both biological components are essential 

 for the process. In fact one of the major frustrations in the study of 

 nitrogen fixation has been the lack of an experimental technique for 

 obtaining fixation in cultures of these bacteria grown without plants. 

 Under natural conditions the bacteria live in nodules on the roots 



