SOURCES OF ENERGY FOR THE AZOTOBACTER 



265 



are rapidly multiplying, and it decreases as their metabolic products 

 accumulate. Hoffmann and Hammer claim this to be due in impure 

 cultures to a loss of nitrogen or free ammonia occasioned by the 

 decomposition of the cells of Azotobactcr. This explanation would 

 hardly hold in the presence of pure cultures, unless we ascribe the 

 breaking down to an autolytic ferment secreted by the Azotobacter 

 cell. According to Koch and Seydel this indicates that in the latter 

 stages of fixation, when there occurs an accumulation of nitrogenous 

 material in the medium, the organisms employ the carbohydrates 



35 



3.0 



2.5 



2.0 



J.5 



10 



0.5 



5 fO J5 20 



FIG. 33. Graph showing the fixation of nitrogen and decomposition of sugar in 

 mixed cultures of Azotobacter chroococcum and Clostridium pasteurianum. 



for other purposes than for nitrogen-fixation. Under natural con- 

 ditions in the soil this accumulation and concentration of nitrogenous 

 material by the Azotobacter is not likely to occur; hence, they assume 

 that the organism will continue fixing nitrogen at the high ratio 

 noted in the early part of laboratory experiments. 



The quantity of nitrogen fixed, however, is dependent upon factors 

 other than the source of energy; e. g., Krzemieniewski found with 

 A. chroococcum that the addition of humates to the cultural solutions 

 increased the nitrogen fixed from a maximum of 2.4 mgm. to 14.9 



