DANIEL I. ARNON 533 



A more general hypothesis of the photoproduction of hydrogen was 

 lecently proposed by Gest (01), namely, that "the source of the H... 

 produced in these light dependent reactions is evidently the 'H' atom 

 pool generated by j)hotolysis of water" and that "when the metabolism 

 of the 'accessory donor' provides a steady stream of reducing power 

 (reduced pyridine nucleotide etc.) the 'H' atoms from water photo- 

 decomposition may be diverted through the appropriate carriers to 

 hydrogenase, thus yielding Ho gas." 



Photoproduction of hydrogen by Chromatium has recently been in- 

 vestigated in our laboratory with special reference to succinate and 

 thiosulfate as hydrogen donors. The results with thiosulfate, which 

 are presented in the companion paper by Losada, Nozaki, and Arnon 

 (95) , provided the first evidence of a light-dependent hydrogen evolu- 

 tion by photosynthetic bacteria from an inorganic electron donor. 

 There was thus an experimental basis for considering the photo- 

 production of hydrogen by photosynthetic bacteria as being primarily 

 related to a light-induced electron transport process that is funda- 

 mentally indei^endent of interconversions of organic compounds. 



This conclusion was strengthened by the results of Ogata et al. 

 with succinate (115). A vigorous production of hydrogen from suc- 

 cinate occurred only in light (Fig. 15) and was independent of COo. 

 The identity of hydrogen was established by absorbing the gas on 

 jxilladium asbestos (150) . Little hydrogen evolution occurred in 

 the dark, or in the light without the addition of succinate. The photo- 

 production of hydrogen was strongly inhibited by carbon monoxide. 

 These results are summarized in Table 8. 



Photoproduction of hydrogen gas indicates that photosynthetic 

 bacteria should be capable of reducing DPN or TPN with light, in 

 the presence of succinate or some other electron donor that is less re- 

 duced than pyridine nucleotide because they are able to form photo- 

 chemically a reductant at the level of molecular hydrogen. A photo- 

 chemical reduction of pyridine nucleotide with succinate was indeed 

 observed by Frenkel (51) and Vernon and Ash (160) and confirmed 

 by Ogata et al. (115) . 



As to the mechanism of this reaction it is not possible, on present 

 evidence, to decide whether molecular hydrogen is produced first and 

 then used to reduce pyridine nucleotide or whether the reduction is 

 accomplished by electrons generated in the photochemical act before 

 they are combined with protons to give molecular hydrogen. The 

 elucidation of the mechanisms involved in these reactions is likely 

 to come from work on cell-free systems. 



