2. Phot or eduction with Various Reductants 



Oxygen Evolution and Photoreduction by Adapted 



Scenedesmus* 



LEONARD HORWITZ and F. L. ALLEN, f Research Institutes, University 

 of Chicago (Pels Fund), Chicago, Illinois 



The reaction steps on the oxygen-hberating side of the photosyn- 

 t.lietic machinery are totally unknown. It was therefore of consider- 

 able interest when Gaffron reported that Scenedesmus ohliquus, 

 when suitably subjected to anaerobic incubation, became so adapted 

 that now irradiation with weak light in an atmosphere of hydrogen 

 and carbon dioxide resulted, not in the evolution of oxygen, but in the 

 absorption of two hydrogen molecules for each carbon dioxide mole- 

 cule consumed. In addition, he found that this adaptation allowed 

 the algae to perform the oxyhydrogen reaction, in which, at suffi- 

 ciently low oxygen tensions, the three gases' oxygen, hydrogen, and 

 carbon dioxide, are normally consumed in the dark in the ratio 2 :6: L 



He considered the possibility that the photochemical uptake of 

 hydrogen and carbon dioxide by adapted Scenedesmus consisted of no 

 more than photosynthesis and the oxyhydrogen reaction running 

 concurrently (this suggestion was also made later by Laisen et al. (1)) 

 according to the following equations: 



(A) Photosynthesis 



CO2 + H2O + light -> (CH2O) + O2 



(B) Oxyhydrogen reaction 



62 + 2H2 -* 2H2O 



(C) Dark CO2 reduction 



qCOi + 2gH2 — g(CH20) + gHjO 



(D) (1 + ?)C02 + 2(1 + 9)H2 + light -* (1 + gXCHjO) + (1 + g)H20 



* Work supported by the Atomic Energy Commission, contract nimiber 

 AT(ll-)-239, the Office of Naval Research, contract numbers NOnr-432(00)nr 

 119-272 and nr 160-030, and the Fels Fund. The isotopically enriched oxygen was 

 prepared by Dr. A. O. Nier under a grant from the American Cancer Society 

 through the Committee of Growth of the National Research Council. 



t Present address: Arthur D. Little, Inc., Cambridge, Massachusetts. 



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