THE DARK REACTIONS 



137 



of one mole of hydrogen per liter of cell volume, and shows that the 

 substrate of hydrogenation is a major cell component, whose concentra- 

 tion is considerably larger than that of chlorophyll (the latter being of the 

 order of 0.01 mole per liter, c/. page 411). 



The liberation of hydrogen can be increased by the addition of an 

 external fermentation substrate, e. g., glucose (Fig. 10). In the presence 

 of 0.07% glucose, (in phosphate buffer of pH 6.2) 1 ml. of Scenedesmus 

 cells produced hydrogen steadily at the rate of 0.2 ml. per hour. 



2 4 



Tim*, hours 



8 



Fig. 9. — Liberation of molecular hydrogen by fermentation in Scenedesmus (after 

 Gaffron 19420. 



0.027 ml. of cells in culture medium with 0.01 M phosphate buffer of pH 6, con- 

 taining 0.2% glucose. 25° C. Curve b: KOH solution in side arm of vessel, absorbing 

 carbon dioxide. 



Among several carbohydrates investigated by Gaffron and Rubin, 

 glucose alone was found to stimulate fermentation (in all its forms) 

 immediately. All others acted with a lag, indicative of a need for 

 preliminary enzymatic transformation. In contrast to respiration and 

 photosynthesis, the hydrogen fermentation was dependent on pH, with 

 an optimum between pH 6 and 7. 



The "acid fermentation" continues independently of the hydrogen 

 fermentation. Therefore, the ratio ACO2/AH2 can vary in the widest 

 limits. Because of the simultaneous appearance of hydrogen fermenta- 

 tion and photoreduction in Scenedesmus, it is safe to assume that the 



