THE BACKGROUND CO2 FIXATION 21 1 



indication at all for the occurrence of photochemical decarboxylation 

 when algae were given C'*02 in O2 atmosphere. 



DISCUSSION AND SUMMARY 



We have seen that the C^^02 fixation by green cells in the dark 

 occurs much faster in O2 atmosphere than in N2 atmosphere. By 

 applying flashing light lasting 1 to 3 seconds it w^as demonstrated 

 that a part of C* fixed in N2 was photochemically decarboxylated, 

 whereas no such hght effect was detectable when C^'*02 fixation 

 occurred in O2 atmosphere. These facts indicate that the processes of 

 dark CO2 fixation are different under aerobic and anaerobic condi- 

 tions. Indeed, it was observed by Brown, Fager, and Gaffron (6) 

 that the products of background C^*02 fixation were different accord- 

 ing to the presence or absence of oxygen; namely, whereas the C" 

 fixed under anaerobic condition was all found in the water-soluble 

 fraction, those fixed under aerobic condition were both in water- 

 soluble and water-insoluble fractions, being incorporated even m 

 fats, starch, and proteins. In view of these observations our experi- 

 mental results may be explamed as follows. Under anaerobic condi- 

 tions the products of background CO2 fixation may remain mostly in 

 the state of rather simple carboxylated compounds, at least part of 

 which are sensitive to photodecarboxylation. In the presence of 

 oxygen, these compounds will be subject to further transformations, 

 the primary step of which may be a reduction or exchange of car- 

 boxyl groups, so that the decarboxylating effect of light, if any, will 

 become undetectable by the tracer technique we applied. 



By the "subtracting method" we used, it was shown (1) that when 

 algal cells w^ere illuminated after prolonged darkness, the background 

 C02-fixing capacity decreased temporarily, and then increased 

 gradually to attain a steady level which was considerably higher than 

 the steady level observed in the darkness, and (2) that when the 

 light was turned off after continued illumination, the background 

 C02-fixing capacity increased temporarily, and, after attaining a 

 certain maximum value, it gradually decreased to a final level which 

 was lower than the steady level observed during the illumination. 

 The temporary decrease of C02-fixing capacity occurring immedi- 

 ately after transition from dark to light, as well as the photochemical 

 decarboxylation of some product of background CO2 fixation, may 



