206 



S. MIYACHI, T. HIUOKAW A, H. TAMIYA 



gated the process of background CO2 fixation as it is influenced by- 

 oxygen and light. 



The experimental organism was Chlorella ellipsoidea and the ap- 

 paratus used was virtually the same as that employed by Benson 

 et at. (2, see also 4). Algal cells were suspended in 0.02 M phosphate 

 buffer of pH 7, and the final concentration of labeled NaHCOs given 

 to the algae was 6.9 to 9.6 X 10-W. The experimental temperature 

 was 25°C. 



EFFECT OF OXYGEN 



It was found, in conformity with the earlier findings of Brown, 

 Fager, and Gaffron (5,6), that the level of D in the dark was mark- 



cpm 



20 40 



TIME IN MINUTES 



Fig. 1. Dark Ci^02-fixation by algal cells that have been kept dark (without pro- 

 vision of CO2), first in N2 and later in O2 atmosphere. Ordinate: cpm of C'^ fixed 

 in 20 minutes' contact with Ci^02 in the dark; abscissa: time of preincubation 

 in the dark with flushing of N2 and O2 as indicated. 



edly increased when the cells were exposed to oxygen. In the ex- 

 periment shown by Fig. 1, the algal suspension was first aerated with 

 nitrogen, and at the expiration of 20 minutes the nitrogen was 

 promptly replaced with oxygen, all procedures being carried out in the 

 dark without provision of CO2. At various intervals during this 

 process, aliquots of algal suspension were brought into contact with 

 C"02 in the dark, and the radioactivity fixed in 20 minutes was 

 measured. It was found that at the transition from N? to O2 atmos- 

 phere, there occurred an abrupt increase in the relative D level, 

 which, however, gradually tapered off and eventually attained a 



