196 



Nitrogen Fixation— Nonsymbiotic 



although there is found in the hterature a statement by Bach that 

 such enzyme complexes can be obtained. Apparently the production 

 and activity of this enzyme are inseparably linked with the synthesis 

 of cell substance. Burk designated the enzyme as "growthbound," 

 for which the name "phyo-enzyme" was suggested. The enzyme 

 nitrogenase requires a certain concentration of calcium (or stron- 

 tium) for its activity. It is incapable of exerting any effect at reac- 



FiG. 82. Azotohactcr agilis, showing Hagella stain (from Beijerincki 



tions below pH 6. It is strongly activated by minute concentrations 

 of molybdenum and to a lesser degree by vanadium. 



The primary product of nitrogen fixation is not fully known, as 

 shown later. Winogradsky reported it to be ammonia. Virtanen 

 and others have said that an oxime compound seems involved. Most 

 of the fixed nitrogen is present in the cultures as cell material. Small 

 quantities of combined nitrogen may be secreted into the medium as 

 long as growth takes place. When the medium is exhausted of 

 nutrients and growth has ceased, a rapid production of ammonia 

 from the cell material sets in, because of the lytic processes that 

 take place. 



Azotohactcr can also assimilate various combined forms of nitro- 

 gen, such as nitrate, ammonia, and simple amino compounds. The 

 presence of these compounds in the medium represses the fixation 

 of free nitrogen. This appears to be due not so much to preferential 

 assimilation, as to the inactivation of the nitrogenase. Burk and 



