382 DANIEL I. ARNON 



they absorbed more gas than in the control treatment in which the gas 

 phase consisted solely of hydrogen (Table X). This seems to indicate that 

 cell-free Chromatium preparations were fixing Ng, with the aid of hydrogen 

 as the electron donor — an interpretation that w^as strengthened by experi- 

 ments with ^^N isotope (Table XI). These findings support the view 

 that the role of light in photofixation of Ng is to generate electrons with a 

 reducing potential that is at least equal to that of Ho. When H., was supplied 

 in the gas phase, light was no longer necessary for the fixation of N2. It 

 seems that pyridine nucleotides mediate the reduction of N.2 (Tables X 

 and XI). However, the experiments on N2 fixation by cell-free Chromatium 

 preparations are at an early stage and the drawing of final conclusions 

 would be premature. 



TABLE XI 



^'"No Fixation in the Dark by Cell- Free Extracts of C/inmKitiiini 

 (Losada, Nozaki, Tagawa, and Arnon [155, 154]) 



The reaction mixture contained in a final volume of 3 ml. : cell-free extract, 

 containing 0-3 mg. bacteriochlorophyll, and the following in micromoles : tris 

 buffer, pH 7-8, 100; MgCl.,, 5; benzyl viologen, 0-2. Treatments i and 3 received 

 0-5 /^tm DPN, and Treatment 2, i /^tm DPNHo. All vessels received 0-5 atmos- 

 phere nitrogen containing 96 atom-",, excess ^^N. Treatment 3 received in addition 

 0-5 atmosphere hydrogen gas. The experiment was run at 25" for 2 hr. in the 

 dark. 



To recapitulate, the photofixation of No and the photoproduction of 

 Ho, from electron donors such as thiosulphate or succinate, are taken as 

 evidence for a non-cyclic electron flow mechanism, that supplements the 

 cyclic mechanism for ATP production. Preliminary experiments indicate 

 that the non-cyclic electron transport in Chromatium that results in 

 pyridine nucleotide reduction is coupled with the formation of ATP [155]. 



A diagrammatic representation of the proposed non-cyclic electron 

 flow mechanism in photosynthetic bacteria is shown in Fig. 24. Three of 

 the external electron acceptors have now been identified : pyridine nucleo- 

 tides, nitrogen gas, and protons. It seems likely that protons serve as 

 electron acceptors and hydrogen gas is evolved when electrons activated 

 by light become surplus, i.e. when they are not consumed in metabolic 



