OXYGEN EVOLUTION ANU PHOTOKEDUCTION 233 



Reaction (A) is simply ordinary pliotosyntliesis as it is usually 

 written. Reaction (B) is the oxyhydrogen reaction which supplies 

 energy to accomplish the coupled carbon dioxide reduction indi- 

 cated by reaction (C). The efficiency of the coupling between (B) 

 and (C) is measured by q, which is normally V'2. The overall result 

 of (A) + (B) + (C) is reaction (D) which differs from (E) below only 

 by the additional gas consumption {q) due to the coupling of (B) to 

 (C). 



However, on the basis of the evidence available to him, Gaffron 

 concluded that the light process he observed in adapted Scenedesrmis 

 was a true photoreduction, essentially like the process occurring in 

 various photosynthetic bacteria in which molecular oxygen ap- 

 parently does not play any role at all. 



(E) CO2 + 2H2 + light — (CH2O) + H2O 



There were indications, however, in the work of Franck, Prings- 

 heim, and Lad (2) that the situation was more complicated. At the 

 suggestion of Gaffron, we have repeated and extended their ex- 

 periments. The results indicate that although adapted algae can 

 perform a true photoreduction they do not necessarily lose the ca- 

 pacity to evolve oxygen. True photoreduction with hydrogen and 

 photosynthesis can exist side by side in adapted algae, and either 

 may predominate depending upon conditions. 



EXPERIMENTS WITH THE FRANCK-PRINGSHEIM OXYGEN 



APPARATUS 



A careful comparison was made, using Scenedesnms ohliquus, strain 

 D3, between the curve of oxygen evolution versus light intensity ob- 

 tained in an atmosphere of hydrogen and that obtained in an at- 

 mosphere of nitrogen, both with 2% carbon dioxide. The rate of 

 oxygen evolution was measured, as in the work of Franck et al. (2), 

 by the method of phosphorescence quenching. Oxygen-free carrier 

 gas passing over or through a cell suspension flushes away any oxygen 

 that it may produce. After passing through a liquid nitrogen trap to 

 remove moisture, it flows over a dye adsorbed on silica gel, whose 

 phosphorescence is a measure of the amount of oxygen the carrier gas 

 contains. 



The algae were used at concentrations ranging from 0.0002% to 

 0.02%, and were flushed with oxygen-free carrier gas for a minimum 



