374 DANIEL I. ARNON 



in Chroniatiiim a light-dependent transfer of electrons from thiosulphate 

 or succinate not only to pyridine nucleotides but also to H + and N.,. The 

 transfer of electrons to H +, a reaction that is catalyzed by hydrogenase, 

 results in photoproduction of hydrogen gas. The transfer of electrons to 

 N2 constitutes photofixation of N.,. These light reactions will now be 

 discussed in more detail. 



PHOTOPRODUCTION OF HYDROGEN GAS 



P'igure 16 illustrates a vigorous photoproduction of molecular hydrogen 

 from thiosulphate [136] and Fig. 17 shows photoproduction of molecular 



10 20 30 40 50 

 minutes 



Fig. 16. Light-dependent evolution of hydrogen gas from thiosulphate by 

 Chromotiinn cells. The reaction mixture included o- i g. of washed cells, suspended 

 in 2-6 ml. of a modified nutrient solution from which nitrogen compounds were 

 omitted, 0-3 ml. of 0-5 M tris buffer, pH 7-2, and o-i ml. of 0-2 M sodium thio- 

 sulphate. o-i ml. of 2o"o KOH was placed in the centre wells of the Warburg 

 nianometer flasks. The reaction was run at 30 . Gas phase argon. Illumination 

 50 000 Lux (Losda, Nozaki, and Arnon [136]). 



hydrogen from succinate [i]. In both cases the evolution of hydrogen 

 occurred in the presence of KOH and seemed to be independent of COg 

 assimilation. Gas evolution ceased when light was turned off and resumed 

 when light was turned on again. The evolved gas was identified as hydrogen 

 (Table VII) by adsorption on palladium asbestos [137]. Photoproduction 

 of H2 was inhibited by carbon monoxide (Table VII). 



The evolution of hydrogen by illuminated Chromatium cells showed a 

 marked pH dependence (Fig. 18). The reaction was most vigorous at the 

 more acid pH. Little hydrogen was evolved at pH S-o. 



Similar to the inhibition of hydrogen evolution in the presence of 

 organic hydrogen donors [138, 139], photoproduction of hydrogen from 



