Jan. 2S, i9i8 Nitrogen- Assimilating Bacteria 193 



The results of the study of nitrates and their influence on Azotobacter 

 in sterilized soil show very clearly that small amounts of nitrate cause 

 a great increase in the number of Azotobacter cells. Higher concentra- 

 tions are not so favorable to the growth of the organisms, and the highest 

 concentrations studied prevented the development of the Azotobacter 

 in sterilized soil. 



From a study of the results of these experiments, it seems that the 

 increase in number of Azotobacter in the presence of small amounts 

 of nitrate is a direct result of nuclear stimulation. Later studies to 

 be cited (pp. 205-208) show that nitrates exerted considerable influence 

 on the internal structure of the Azotobacter cell. It appears reasonable 

 to expect that the nitrate affected the nuclear structure in such a manner 

 that an increase in cell multiplication resulted. It seems probable that 

 the action of nitrate as a simple nutrient would be shown by a slower 

 increase in cell multiplication. 



INFLUENCE OF NITRATES ON THE FIXATION OF NITROGEN BY .\ZOTOBACTER 



It has been shown in the preceding paragraphs that the presence of 

 small quantities of nitrate in sterilized soil bring about a large increase 

 in the number of Azotobacter. This increase was noted in the case 

 of both strains of Azotobacter. It would be of interest to know whether 

 the increase in bacterial numbers was accompanied by a corresponding 

 increase in the amount of nitrogen assimilated. 



The results secured by a few investigators indicate that in the presence 

 of combined nitrogen as nitrates the nonsymbiotic nitrogen-fixing 

 organisms will not fix atmospheric nitrogen. Stoklasa {44, p. 492-50J) 

 studied the influence of Azotobacter on sodium nitrate in aerobic and 

 anaerobic liquid cultures. He found only a small gain in organic nitro- 

 gen and from these results he concluded that in the presence of nitrates 

 Azotobacter could not assimilate atmospheric nitrogen. It has been 

 shown by Hanzawa {20) that in a hquid culture containing 12 mgm, 

 of nitrate (from potassium nitrate) in 100 c. c. of medium, a mixed 

 culture of Azotobacter fixed 5.25 mgm. of nitrogen. Under the same 

 conditions with 60 mgm. of nitrate present in 100 c. c. of medium he 

 found but 5.35 mgm. of nitrogen fixed. He concluded that nitrates 

 remained, as far as small quantities were concerned, almost without 

 influence on the amount of atmospheric nitrogen fixed by Azotobacter. 



Some studies have been carried on with respect to the influence of ni- 

 trates on the nonsymbiotic anaerobic nitrogen-assimilating organism, 

 Clostridium spp. Bredemann (9) showed that ammonium nitrate in 

 solution caused a decrease in the amount of nitrogen fixed by species 

 of Clostridium. Pringsheim (40) grew cultures of C. atnericamim in 

 solutions containing potassium nitrate. He found that in the presence 

 of available energy the organism fixed some nitrogen when nitrate was 



