THE DARK REACTIONS 141 



5. VI). In other words, in the presence of carbon dioxide, for every two 

 molecules of hydrogen transferred to oxygen, up to one molecule finds 

 its way to carbon dioxide. 



In place of molecular hydrogen, hydrogen from organic donors can 

 be used by adapted algae in reactions with oxygen, as shown by the 

 diminution of hydrogen consumption caused by the addition of 0.05-1% 

 of glucose, or yeast autolysate (c/. curves a and h in Fig. 12). The occur- 

 rence of coupled carbon dioxide reduction remains to be demonstrated 

 in this case. 



The explanation of the oxyhydrogen reaction in adapted algae requires 

 one new assumption (in addition to the presence of a hydrogenase and 

 of an oxidase, which already have been postulated in the interpretation 

 of the adaptation phenomena). This assumption is that the oxidant 

 {O2}' can react not only with the hydrogenase, Eh (thus causing de- 

 adaptation) but also with the intermediate reductant, H2EH, thus com- 

 pleting the transfer of hydrogen to oxygen. The fact that the ratio 

 AH2/AO2 in the absence of carbon dioxide often is closer to 1 than to 2 

 indicates that this reaction takes place in two steps: 



(6.11a) {OsT + HoAh >{H202!+Ah 



(6.11b) IH2O2! + HoAh > 2 H2O + Ah 



and that the second step can be replaced by reaction with an internal 

 hydrogen donor: 



(6.11c) {H2O2I+H2R >2H20 + R 



thus reducing the hydrogen consumption to one molecule of hydrogen 

 per molecule of oxygen. 



This mechanism of the oxyhydrogen reaction is represented in scheme 

 6.IIA, which is merely a partial elaboration of scheme 6.1. 



In the presence of carbon dioxide, reaction (6.11) runs to completion, 

 and one molecule of carbon dioxide can be reduced simultaneously with 

 the reduction of two molecules of oxygen. This indicates that reaction 

 (6.11c) is replaced by a "coupled" reaction: 



(6.12) 4 H2AH 1^ j(.Q^j ^ j(^jj,Oi + H2O + 2 Ah 



represented by scheme 6.IIB. A combination of (6.11) with (6.12) leads 

 to the ratios AH2/AO2 = 3 and AC02/(AH2 - 2AO2) = h, in agreement 

 with the experimental values. 



The nature of the coupling between the oxyhydrogen reaction and 

 the reduction of carbon dioxide, symbolized by bracketing in (6.12) will 

 be discussed in chapter 9 (page 235). 



Equation (6.12) shows that the " chemosynthetic " reduction of 

 carbon dioxide in adapted algae requires the existence of an enzymatic 



