378 DANIEL I. ARNON 



The results with thiosulphate provided the first experimental evidence 

 for a light-dependent hydrogen evolution from an inorganic electron donor 

 by a photosynthetic organism [136, cf. 143]. Photoproduction of hydrogen 

 was first observed in algae by Gaffron and Rubin [140] and in photo- 

 synthetic bacteria by Gest and Kamen [138]. In algae the photoproduction 

 of hydrogen seemed to depend on internal electron donors of metabolic 

 origin [140] whereas photoevolution of hydrogen by photosynthetic 

 bacteria appeared to depend on exogenous organic acids and CO2 [138, 

 141, 142, 143, 139, 144, 145]. 



The evolved hydrogen has previously been ascribed to photodecom- 

 position of water [140, 145] or to decomposition of a-ketoglutaric acid 

 [139]. We regard the photoproduction of molecular hydrogen from thio- 

 sulphate (or succinate) as evidence for a "non-cyclic" electron flow 

 mechanism in bacterial photosynthesis as depicted below: 



H, 



11 + 



hydrogenase 

 thiosulphate — -=^ cytochromes — "- — > chlorophyll 

 sulphate 



light 



PHOTOFIXATION OF NITROGEN GAS 



Nitrogen fixation by photosynthetic organisms [149, 141, 139, 150-153] 

 may also be viewed as resulting from a non-cyclic electron flow in which 

 electrons pass from an external electron donor, via cytochromes, to 

 chlorophyll excited by light, and thence to molecular nitrogen. 



This interpretation was substantiated by using thiosulphate and 

 succinate as electron donors for fixation of nitrogen gas by illuminated 

 Chromatiiun cells [154, 155]- Figure 21 shows photofixation of nitrogen 

 gas with thiosulphate as the electron donor and Fig. 22 shows photo- 

 fixation of nitrogen with succinate as the electron donor. In both cases, 

 Chromatium cells fixed N., only in light. Fixation ceased when the light was 

 turned off and resumed when the light was turned on again. The depend- 

 ence of N2 fixation on light and an external electron donor, was confirmed 

 with the use of ^^N isotope* (Table VIII). 



In the case of thiosulphate, photofixation of Ng was greatly increased 

 by the addition of oxaloacetate which probably acted as an amino group 



* We are indebted to Dr. C. C. Delwiche for the determinations of ^^N. 



