76 THE METABOLISM OF ALGAE 



ammonia in the medium or some carbon reserve within the 

 cells becomes exhausted. Ammonia uptake by cells not 

 initially deficient in nitrogen is much less marked but is 

 increased, although to a level lower than that of nitrogen- 

 deficient cells, if glucose is supplied in the medium. The 

 supply of glucose to nitrogen-deficient cells does not in- 

 crease their rate of ammonia assimilation. These observa- 

 tions were made with cell suspensions kept in the dark; 

 light has little effect on the rate of ammonia assimilation by 

 nitrogen-deficient cells but doubles the assimilation rate of 

 normal cells. 



TABLE 6 



RATES OF OXYGEN ABSORPTION (mM.^/mG. DRY WT./hR.) BY 



Chlorella vulgaris^' ^ 



Nitrogen-starved fno glucose 

 cells l + i?o glucose 



no glucose 



Normal cells 



+ 1 



glucose 



Before addition 

 of (NH4)oSO« 



2-74 



7-75 



3-05 



12-48 



After addition 

 of (NH4)2SO« 



i6-o8 



i6-66 



8-52* 2-32 



17-53 



* This rate was maintained for 10 minutes only. 



The assimilation of ammonia by nitrogen-starved Chlor- 

 ella is accompanied by a marked increase in respiration rate. 

 When the addition of an ammonium salt to a cell suspension 

 is not followed by very rapid ammonia assimilation respira- 

 tion is not stimulated in this manner (Table 6). The assimi- 

 lation of ammonia and the respiration accompanying it are 

 both cyanide-sensitive. Fermentation, in cell suspensions 

 in an atmosphere of nitrogen, is not aflPected by addition of 

 ammonia. There is thus a close correlation between the 

 rates of ammonia assimilation and of aerobic respiration. ^^^ 



The ammonia assimilated by nitrogen-starved Chlorella 

 is converted largely into soluble organic compounds. At 

 first, most of the ammonia which disappears from the 

 medium can be accounted for as free or combined amino- 

 nitrogen and amide nitrogen within the algal cells (Fig. 13). 



