FIXATION OF ATMOSPHERIC NITROGEN 567 



was found to vary at different stages of growth of the nitrogen-fixing 

 organism. Bonazzi 25 differentiated between the "ferment power" or 

 first stage in the growth of the organism, when nitrogen assimilation is 

 at a maximum, and the second or "maintenance" phase. During this 

 maintenance stage the carbohydrate complexes are actually reworked, 

 partially burned to liberate energy, partially utilized in the building of 

 cellular substance, and partially secreted into the medium in the form 

 of soluble by-products, as shown in table 57. In this table, S represents 

 the sugar consumed; c, the mass of cellular substance at work during the 

 period, and t, the time of incubation. During the early periods of 

 growth, a unit of cellular substance could utilize in a unit of time 5.45 

 units of sugar; after an incubation period of 30 days, a unit of cellular 

 substance utilized in a unit of time only 0.28 unit of sugar (fig. 40). Not 



TABLE 57 

 Ferment power of Azotobacter chroococcum 



the sugar itself, but the products of its decomposition form the true 

 sources of energy for Azotobacter. 



The fixation process is usually most efficient in the earlier stages of 

 growth coinciding with the period of greatest cell multiplication and 

 sugar utilization. It was found, 26 for example, that 70 to 80 mgm. of 

 nitrogen are fixed by Azotobacter per gram of glucose decomposed on 

 the second and third day of growth, and only 5 to 8 mgm. on the 

 eighth day. Although the total amount of nitrogen fixed during the 

 first five days is small, the process is most economical. In the following 

 3 to 5 days' periods, the process becomes less and less economical. In 

 the latter stages a larger part of the energy is utilized for respiration 

 only. A definite correlation is thus found between the processes of 



25 Bonazzi, A. Studies on Azotobacter chroococcum Beij. Jour. Bact., 6: 

 331-369. 1921. 



26 Koch, A., and Seydel, S. Versuche iiber den Verlauf der Stickstoffbindung 

 durch Azotobacter. Centrbl. Bakt. II, 31: 570-577. 1912. 



