AZOTOBACTER RELATION TO OTHER ORGANISMS 275 



may grow and thus be benefited. Beijerinck and van Delden early 

 recognized that an apparent symbiosis exists between Azotobacter 

 and other bacteria, and that the nitrogen fixed is considerably 

 greater in the mixed than in the pure cultures. This symbiosis, 

 though in many cases beneficial to Azotobacter, is not essential for 

 nitrogen-fixation. Radiobacter, with which the Azotobacter are 

 usually associated, have only slight nitrogen-fixing powers, yet they 

 increase the nitrogen-fixing powers of Azotobacter. The carbo- 

 hydrates disappear more rapidly from mixed than from pure cultures 

 and with a greater fixation per gram of carbohydrate utilized. 

 There is also a greater fixation when two strains of Azotobacter are 

 grown in conjunction with each other. This is especially marked in 

 an aqueous solution of mannite. Results have been reported where 

 Azotobacter fixed twice as much in the presence of Pseudomonas as 

 when grown alone. 



The manner in which this mutual benefit is exerted is not clear. 

 In some cases it may be due to the associated organism rendering 

 more available the carbonaceous material. 



Omelianski and Salunskov offer the following explanation con- 

 cerning the association of aerobic and anaerobic nitrogen-fixers: 



"The synergetic activity of nitrogen-fixing and accompanying 

 microbes, is both in laboratory experiments and under natural 

 conditions (cultivable stratum of the soil) of a different character 

 according to the properties of the species taking part in the process 

 and their environment; in both cases the function of the satellite 

 organism seems to consist in fixing the oxygen of the air and creating 

 the anaerobic environment for Clostridium pasteurianum. The 

 species added to the cultures of nitrogen-fixing microbes sometimes 

 supply the compounds of carbon needed for the process of fixing 

 nitrogen as energetic substance. In the case of the combination: 

 Azotobacter and Clostridium pasteurianum, the function of the former 

 is not confined to fixing the oxygen of the air only, and consequently 

 to creating an anaerobic environment for the Clostridium. But this 

 combination is also useful inasmuch as it destroys the injurious 

 products of disassimilation created by the second (chiefly butyric 

 acid) and maintains the action of the environment. (Azotobacter 

 is alkaligenic and the Clostridium acidogenic.) 



"The satellite species may also unfavorably affect the nitrogen- 

 fixing microbe, either through products of assimilation or by con- 

 sumption of the carbon compounds needed by this microbe for 

 nitrogen-fixing. The energetic fixation of oxygen by the satellite 

 aerobic species creates conditions favorable to the development of 

 Clostridium pasteurianum, but at the same time hinders the growth 

 of the Azotobacter, which is necessarily aerobic. 



" The form endowed with the maximum vitality and at the same 

 time the most common form in which combination of the nitrogen- 



