240 W. VISHNIAC AND G. H. REAZIN, JR. 



tions of the activities of resting cell suspensions established the 

 occurrence of photoreduction, and measurements of growth in a 

 closed system showed that growth by photoreduction can occur. 



Several attempts at inducing suspensions of Ochromonas to evolve 

 or consume hydrogen failed but we do not consider these results con- 

 clusive. Isopropyl alcohol was found to be a suitable hydrogen donor 

 for CO2 fixation by Ochromonas. Its use was suggested by the work of 

 Foster (9), who observed CO2 fixation by Rhodopseudomonas with 

 the concomitant light-dependent oxidation of isopropyl alcohol to 

 acetone. The results summarized in Table I can be interpreted both in 



TABLE I.* CO2 Fixation by Resting Cell Suspension of Ochromonas 



Light, 

 Li!?ht, 200 mM 



Dark no addition isopropyl alcohol 



COiCjuM) +10.5 +9.2 -15.2 

 O2UM) +1.2 +10.7 

 Acetone found (^M) 0.3 0.2 9^ 



* 15-m1- cells per vessel, suspended in 2.0 ml. 0.05 M K-PO4 buffer, pH 5.5. 

 Atmosphere: Initially 95% N2 + 5% COo. Illumination: 12,000 lux, incandescent 

 light. Dark control wrapped in tin foil. Temperature: 30°C. Isopropyl alcohol 

 added from sidearm at zero time. Time: 2 hours. Gas changes were determined 

 as described in reference (10). Acetone was determined after distillation by con- 

 densation with salicyl aldehyde. 



a qualitative and in a quantitative fashion. Qualitatively, they show 

 that a net fixation of CO2 occurs in the light only in the presence of 

 isopropyl alcohol. This alone suggests the occurrence of photoreduc- 

 tion. A quantitative interpretation of the experiment is complicated 

 by our inability to decide what corrections should be applied for dark 

 metabolism. Taking the figures at face value, we find that in the 

 light without isopropyl alcohol 1.2 /xM of oxygen is formed, pre- 

 sumably by photosynthesis, which therefore should correspond to the 

 fixation of 1 juM of carbon dioxide. Yet we find a net evolution of 9.2 

 uM of carbon dioxide. Allowing for the carbon dioxide presumably 

 fixed by photosynthesis, the total CO2 evolution is 10.4 yM, which 

 agrees well with the 10.5 pM of CO2 given off by suspensions in the 

 dark. In the light in the presence of isopropyl alcohol 10.7 juM of 

 oxygen were given off, which should correspond to 10.7 mM of CO2 

 fixed by photosynthesis, yet the net uptake of carbon dioxide ob- 

 served is 15.2 /xM, which leaves 4.5 ^M of CO2 fixed to be accounted 



