Jan. 2S, 1918 



Nitrogen- A ssimilaiing Bacteria 



195 



Table IX. — Influence of nitrates on the fixation of nitrogen by Azotobacter {strain B) 



on agar films 



Cul- 

 ture 

 No. 



Treatment (nitrate in looc.c. of medium). 



Nitrogen contained in 100 c. c. of medium. 



Inoculated. 



Found. Average 



Uninoculated. 



Found. Average. 



Nitrogen 

 fixed. 



75 mgm. of NO3 as potassixxm ni- 

 trate 



...do 



150 mgm. of NO3 as potassium ni- 

 trate 



....do 



75 mgm. of NO3 as sodium nitrate 



do 



150 mgm. of NO3 as sodium nitrate 



do 



75 mgm. of NO3 as calcium nitrate 



do 



150 mgm. of NO3 as calcium nitrate 

 do 



15-50 

 15-70 

 15.60 



25. 20 

 25.40 



36.40 

 36.90 

 25. 60 

 25. 70 

 37.60 

 37. 20 

 20. 10 

 19. 60 

 32.80 

 33-30 



Mgm. 



15.60 



25-30 



Mgm,. 



}36 



I 3i 



Mgm,. 



6. 40 



} 13-85 



I 23.60 

 I 13-00 

 } 25. 80 

 I 12.3s 

 } 24. 8s 



Mgm. 

 9. 20 



11-45 



13-05 



12.65 



II. 60 



7-50 



8. 20 



A glance at the results (Tables VIII and IX) shows that an increase in 

 nitrogen fixation occurred where potassium and sodium nitrates were 

 present, whereas a marked decrease in the total nitrogen content was 

 observed where calcium nitrate was used. Whether the calcium itself 

 is detrimental to an increase in organic nitrogen or whether it is the com- 

 bination of calcium with nitrate can not be stated. It is significant, 

 however, that this decrease in fixation of nitrogen was noted throughout 

 all the experiments where calcium nitrate was employed. It is very- 

 evident that calcium nitrate exerts some detrimental effect on the nitro- 

 gen assimilating properties of the organism. 



There seems to be but a slight difference in the nitrogen-fixing ability 

 of the two strains studied. In the absence of nitrates the amount fixed 

 varies but little. Also in the presence of potassium and sodium nitrates 

 the relative increase in amount of nitrogen fixed remains about the 

 same. Calcium nitrate offers an exception where it is employed. The 

 detrimental effect seems to be more marked in the case of strain A than 

 with strain B. Strain A under normal conditions fixed slightly less nitro- 

 gen than strain B, so it may be possible that this strain is weaker. 



The formation of pigment by the Azotobacter in the presence of the 

 nitrates is of interest. Strain A normally produced no pigment by the 

 end of three weeks' incubation. But when grown on the agar films in 

 the presence of nitrate a most marked pigment production appeared. 

 This pigment was especially noticeable in the presence of the calcium 



