DANIEL I. ARNON 531 



which Gaftion named photorcduction (55) , appears to be the same 

 type ol photosynthesis as that in Chruniatiiun when it is supplied 

 with hythogcn <;as. It seems likely that photoreduc tion by algae is 

 a reversion to a primiti\e photosynthesis in which the COo reductant 

 was hychogen gas present in the enviromnent, and the sole contribu- 

 tion of light was the lormation ot ATP by cyclic photophosphorylation. 



14. Photoproduction of Hydrogen Gas and Its Significance to 



Photosynthesis 



Chromatium, like other photosynthetic bacteria, grows photosyn- 

 thetically with hydrogen donors more oxidized than molecular hydro- 

 gen (158, 109) . Is the role of light in these cases also limited to ATP 

 formation? Or is light energy used for the reduction of pyridine 

 nucleotide, as is the case in green plants? 



Among the hydrogen donors used by Chromatium for growth are 

 some w^hich are known to reduce pyridine nucleotides enzymatically 

 in the dark. In these cases, assuming the requisite enzymes are present, 

 there is no need to invoke the participation of light in that step. A 

 case in point is malate, an excellent hydrogen donor for Chromatium 

 cells, which are known to contain the malic enzyme that catalyzes 

 the reduction of TPN. But Chromatium can also grow photosyn- 

 thetically with an inorganic electron donor, such as thiosulfate, or 

 with succinate (158, 109), a substrate that donates electrons at the 

 level of flavin and is therefore unable to reduce pyridine nucleotides. 

 How, in terms of the electron flow theory (9) , can pyridine nucleotides 

 be reduced in bacterial photosynthesis with thiosulfate or succinate as 

 hydrogen donors? 



An attractive hypothesis would be to consider photoproduction of 

 hydrogen gas as resulting from an electron flow mechanism that gen- 

 erates, with the aid of light energy, a photosynthetic reductant at the 

 level of molecular hydrogen, from various electron donors having a 

 potential less reducing than hydrogen. Photoproduction of hydrogen 

 would be limited to those photosynthetic cells that are either known 

 to have or to be able to form hydrogenase adaptively, namely, certain 

 species of algae (58, 48a) and photosynthetic bacteria (63, 64, 135, 

 32, 113, 62, 61). The source of the evolved hydrogen gas would be 

 either an endogenous electron donor, as seems to be the case in 

 adapted algae (58) , or an exogenous electron donor, organic or inor- 

 ganic, that is always required for the growth of photosynthetic bacteria 

 (158, 109) . 



A similar hypothesis that the origin of the photoproduced hydrogen 



