PHOTOREDUCTION IN OCHROMONAS 241 



for. At the same time 9.5 /xM of isopropyl alcohol have been oxidized 

 to acetone, which is almost the stoichiometric amount required for 

 the reduction of 4.5 ^M of CO2. These results are much too good to 

 reflect the true course of events. The total amount of CO2 fixed is 

 probably greater than the net fixation observed since some fermenta- 

 tive CO2 evolution probably also occurred. The amount of isopropyl 

 alcohol oxidized is probably greater than 9.5 mM, since acetone can 

 be further oxidized by Ochromonas and some of it undoubtedly was. 

 But the results show quite clearly that CO2 fixation by Ochromonas 

 can be accounted for if we consider it to be the sum of both photore- 

 duction and photosynthesis. These results merely extend the ob- 

 servation of photoreduction to another algal species, but Table II 

 summarizes an experiment which provides evidence for growth of 

 Ochromonas by photoreduction. Growing in the light on glucose in a 

 closed system, Ochromonas consumed 29.6 mg. of carbon and formed 

 cell material to the extent of 28.9 mg. of carbon. The ratio (mg. 



TABLE II. Growth of Ochromonas on Two Substrates 



Glucose Glycerol 



mg. C of substrate used 29. (> 22.0 



mg. C of Ochromonas formed 28 . 9 28 . 5 



Ochro7nonas was grown in closed flasks for 5 days at 30°C. Atmosphere: 

 Initially 95% N2 + 5% CO2, no net formation of O2 occurred. Illumination: ca. 

 1000 lux, fluorescent light. Glucose was determined by anthrone reagent, glycerol 

 by periodate titration, and total carbon as BaCOs after AgNOa + K2SO5 com- 

 bustion. No significant changes occurred in the dark. 



C formed) /(mg. C used) = 0.98. A similar culture grown on glycerol 

 consumed 22.0 mg. of carbon and formed cell material to the extent 

 of 28.5 mg. of carbon, which gives a ratio (mg. C formed)/(mg. C 

 used) = 1.29. In other words, when grown on glycerol, Ochromonas 

 fixed more GO2 than could be provided by the oxidation of glycerol. 

 The ratio 1.29/0.98 = 1.31 corresponds approximately to the ratio 

 between the number of hydrogens per carbon in glycerol to the num- 

 ber of hydrogens per carbon in glucose (2.66/2 = 1.33). Inasmuch as 

 the amount of organic carbon formed during growth appears to be a 

 function of the reduction level of the substrate, this experiment sug- 

 gests that Ochromonas can grow by photoreduction. It is possible that 

 this alga is limited in its ability to evolve oxygen. 



