882 LIGHT AND LIFE 



CO2 in the light with the aid of hydrogen gas (Gaffron's photore- 

 duction) . Arnon suggests that this capacity is a primitive form of 

 photosynthesis going back to a time when there was free hydrogen 

 gas in the atmosphere and when the sole function of light was the 

 formation of ATP by cyclic photophosphorylation. 



Chromatium will utilize other hydrogen donors than hydrogen gas, 

 and some of these, such as malate, will reduce pyridine nucleotides 

 in the dark. But Chromatium will also use succinate and thiosul- 

 fate as electron donors, and thiosulfate and succinate are not at a po- 

 tential sufficient to reduce pyridine nucleotides in a simple dark 

 reaction. In these cases Arnon favored the view that pyridine nucleo- 

 tides are reduced only after the electrons derived from the substrate 

 have passed first to the cytochrome-chlorophyll system and been 

 raised to the required energy level by means of light. 



In the paper by Losada, Nozaki, and Arnon, a study of this prob- 

 lem has been contributed to the Symposium. Arnon's electron flow 

 theory of photosynthesis excludes a direct photolysis of water. It 

 is proposed that the photoproduction of hydrogen might then come 

 from a reduction of protons by a hydrogenase with the aid of elec- 

 trons derived from the excited chlorophyll molecules, and fed to 

 the latter by cytochromes obtaining them from the external electron 

 donors. The primary photochemical act would suffice to raise the 

 electrons to the reducing potential of molecidar hydrogen. If this 

 were the case, then inorganic electron donors might be substituted 

 for the physiological organic electron donors. It was therefore de- 

 cided to see whether thiosulfate would produce hydrogen gas and 

 reduce cytochromes in illuminated Chromatium cells, but not in the 

 dark. The reaction was in fact found to occur, and the accompany- 

 ing scheme illustrates its agreement with the electron flow theory. 



CHLOROPHYLL 



LIGHT 



THIOSULFATE ^ > CYTOCHROMES »- EXCITED CHLOROPHYLL 



SULFATE 



H + 



HYDROGENASE 



H2 



The reaction was highly dependent on pH, being low at pW 8 



