THE PHOTOCHEMICAL REACTIONS 



147 



figures (particularly those of French and Wessler) were considerably 

 above 2. Gaffron, too, at first found quotients as high as 3, but decided 

 that these high values were due: (a) to hydrogen absorption not connected 

 with photoreduction; and (6) to carbon dioxide liberation by acid 

 fermentation. 



The consumption of hydrogen increases linearly with light intensity 

 between 200 and 600 lux; but before any "light saturation" can be 

 observed "de-adaptation" sets in, as illustrated by figure 14, and photo- 

 reduction is replaced by normal photosynthesis. De-adaptation can be 

 delayed by hydroxylamine or phenantroline (c/. Chapter 12, page 319); 

 the maximum rate of photoreduction observed under these conditions 

 was three times the rate of dark respiration. 



12 16 20 24 



Time, minuses 



28 30 



Fig. 16. — Photoreduction with hydrogen in Scenedesmus (after Gaffron 1940i). 



Preceding anaerobiosis, 12 hours. 20° C. in Ho. Curve /: cells washed and sus- 

 pended in 0.01 M NaHCOs. Curve //: cells suspended in nutrient medium with 

 0.01 M NaHCOa and 0.5% glucose. 



The effect of hydrogen concentration on the rate of photoreduction in 

 nitrogen is shown in figure 15. The reaction is slowed down when the 

 hydrogen concentration is below 20% and de-adaptation starts rapidly 

 below 4%. 



It was mentioned before that hydrogen-adapted algae also may 

 function like heterotrophic purple bacteria, that is, reduce carbon dioxide 

 at the cost of hydrogen from organic donors. The delay in hydrogen 

 absorption, which was attributed above to the "cleanup" of intercellular 

 hydrogen donors, and illustrated by figure 14, can be interpreted as 

 e\'idence of this type of metabolism. This delay can be extended by a 

 supply of organic reductants. Figure 16 shows the effect of glucose on 

 the consumption of hydrogen. The hydrogen absorption in the dark is 

 inhibited entirely, and that in light is strongly reduced. An induction 



