946 



CONCENTRATION FACTORS 



CHAP. 27 



nated in chapter 6 by Ah (i. e., the compound assumed there to be hydro- 

 genated to AhH2 by the mediation of the hydrogenase). 



However, in interpreting the hydrogen saturation of photoreduction, 

 one has to keep in mmd that hydrogenation equilibrium is only one of the 

 two possible explanations. As in the case of the carbon dioxide curves, 

 the hydrogen curves may also be affected — or completely determined — by 

 kinetic influences, such as slow rate of hydrogen supply and slow hydrogena- 

 tion (in the linearly ascending part), and limitations of light supply, of 

 oxidant supply, or of the availability of a catalyst, in the horizontal part of 

 the curves, which follows saturation. 



ro . 



e 



E 



O 



o 



u. 

 o 



UJ 



a. 



3 



0.2 0.4 0.6 0.8 1.0 1.2 



THIOSULFATE CONCENTRATION, % 



P^ig. 27.12. Thiosulfate concentration and rate of photo- 

 reduction of carbon dioxide in Chromatium (after Wassink, 

 Katz and Dorrestein 1942): 5%C02; pH6.3; 29° C; strong 

 light. 



Wassink (1942) made reaction rate studies on the same organism, Chro- 

 matium, also with varying concentrations of gaseous hydrogen sulfide as re- 

 ductant. As shown in figure 27.1 IB, the (initial) rate was found to be 

 proportional to [H2S] up to about 2% (in nitrogen) ; no signs of saturation 

 were noticeable in the investigated range of concentrations. Higher doses 

 of hydrogen sulfide could not be used because of rapid poisoning. 



The influence of the concentration of thiosulfate as reductant was also 

 studied on Chromatium. Eymers and Wassink (1938) first found the rate 

 to be constant between 0.16 and 2% thiosulfate, indicating that the satu- 

 rating concentration was <0.16%. More detailed measurements were 

 made by Wassink, Katz and Dorrestein (1942), who detennined a complete 

 "thiosulfate curve," P = f [thiosulfate], reproduced in figure 27.12. It 

 shows full saturation near 0.5% and half saturation a little below 0.1% 

 thiosulfate (corresponding to 0.06 mole Na2S203/l.). 



