M. LOSADA, M. NOZAKl, AND DANIEL I. ARNON 571 



suitable inorj^aiiic electron donors, aiul the latter should be capable 

 of reducing the bacterial cytochromes ihai are oxidized by light in 

 the course ol i)h()tosynthesis (2) . An investigation of photoproduc- 

 tion of hydrogen from thiosulfate by Clnonuitiuni was undertaken to 

 test this prediction. 



As shown in Fig. 1, in the light but not in the dark, gas was 

 evolved by Clirouuiliinn cells in the presence of thiosulfate. No COy 

 source was supplied and KOH was present in the reaction vessel. 

 Gas evolution ceased when light was turned off and resumed when 

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

 (Table 1) by adsorption on palladium asbestos (11). 



The evolution of hydrogen by illuminated Chromatium cells 

 showed a marked pH dependence (Fig. 2) . The reaction was most 

 vigorous at the more acid pH. Little hydrogen was evolved at pH 

 8.0. 



Similar to the inhibition of hydrogen evolution in the presence of 

 organic hydrogen donors (7, 9) , photoproduction of hydrogen from 



PHOTOPRODUCTION OF H2 FROM THIOSULFATE 



m/nufes 

 Fig. 1. Light-dependent evolution of hydrogen from thiosulfate by Chromatium 

 cells. The reaction mixture included O.l g of washed cells, suspended in 2.6 ml 

 of a modified nutrient solution from which nitrogen was omitted, 0.3 ml of 0.5 

 M Tris buffer, pH 7.2, and 0.1 ml of 0.2 M sodium thiosulfate.. 0.1 ml of 20% 

 KOH was placed in the center well of the \\'arburg manometer flasks. The reaction 

 was run at SO^C. Gas phase, argon. Illumination, .'jO.OOO Lux. 



