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Some Conditions Influencing Nitrogen Fixation by Aerobic 

 Organisms. 



By W. B. Bottomley, M.A., Professor of Botany, King's College, London. 



(Communicated by Dr. J. Eeynolds Green, F.R.S. Received May 30, — 

 Eead June 27, 1912.) 



The fixation of nitrogen by Azotobacter and Pseudomonas depends upon the 

 presence of a fermentable carbohydrate as a source of energy. Although a 

 large number of carbohydrates can be utilised by these organisms there is a 

 considerable variation in their efficiency, and for effective fixation it is 

 necessary to work with a sugar specially suitable for the organism under 

 investigation. It is generally recognised that mannite is the most efficient 

 kind for Azotobacter and maltose for Pseudomonas. Unfortunately mannite 

 1 is practically useless for Pseudomonas and maltose for Azotobacter. Further 

 it was found when working with a mixed culture of Azotobacter and 

 Pseudomonas that the mixture of carbohydrates (mannite and maltose) was 

 not satisfactory. It was desirable, therefore, to obtain a carbohydrate 

 which would be equally efficient as a source of energy for both organisms. 

 This has been obtained in dextrin. 



In order to compare the influence of dextrin with mannite and maltose 

 upon Azotobacter and Pseudomonas respectively, six series of Erlenmeyer 

 flasks, four flasks in each series, were arranged. All the flasks contained 

 100 c.c. of a nutrient solution consisting of di-potassium phosphate - 2 grm., 

 magnesium sulphate 0*02 grm., basic slag - 4 grm., in 100 c.c. distilled water. 



The series were treated as follows : — A received 1 grm. dextrin ; B, 1 grm. 

 maltose ; C, 1 grm. dextrin ; D, 1 grm. mannite ; E, 2 grm. dextrin ; F, 1 grm. 

 maltose and 1 grm. dextrin. 



A and B were each inoculated with 1 c.c. of a pure culture of Pseudomonas, 

 C and D with 1 c.c. of a pure culture of Azotobacter, and E and F with 2 c.c. 

 of a mixed culture (1 c.c. of Pseudomonas and 1 c.c. of Azotobacter). In each 

 series flask 1 was autoclaved immediately after inoculation and served as a 

 control ; flask 2 was used for testing the carbohydrate ; flasks 3 and 4 were 

 analysed for amount of nitrogen fixation. All the flasks were incubated at 

 26° C. until the carbohydrate was completely used up as shown by flasks 2. 

 Mtrogen determinations were then made and the following results 

 obtained : — 



