PHOTOPRODUCTION OF MOLECULAR HYDROGEN 

 FROM THIOSULFATE BY CHROMATIUM CELLS 



M. LosADA, AL NozAKi, AND Daniel 1. Arnon 



Laboratory of Cell Physiology 

 University of California, Berkeley, California 



Photoproduction of molecular hydrogen by photosynthetic cells was 

 first observed by Gaffron and Rubin in the green alga Scenedesmus 



(5) and by Gest and Kamen in photosynthetic bacteria (7, 8) . These 

 findings were confirmed and extended in other laboratories (4, 9) . 



The evolution of hydrogen was attributed either to a decomposi- 

 tion of a dicarboxylic acid (9) or to a photolysis of water into (H) 

 and (OH) (5, 6) . In the latter proposal, it was suggested that (H) 

 is eventually liberated, with the aid of hydrogenase, as molecular 

 hydrogen, whereas the (OH) is reduced back to water, by reacting 

 in the case of algae with endogenous hydrogen donors (5) , and in 

 the case of photosynthetic bacteria with exogenous hydrogen donors 



(6) . Photosynthetic bacteria evolve hydrogen in light only in the 

 presence of an exogenous hydrogen donor (7, 8, 10) . 



The endogenous electron donors for algae were not chemically 

 identified (5) . The exogenous electron donors for photosynthetic 

 bacteria were organic acids, mainly malate (7, 8, 9, 10) . There was 

 no evidence that thiosulfate or any other physiologically suitable in- 

 organic electron donor was compatible with ]:)hotoproduction of 

 hydrogen (3) . 



From the standpoint of the electron flow theory of j^hotosynthesis, 

 which excludes a photolysis of water (I, 2), the photoproduction of 

 hydrogen could be viewed as a reduction of protons by a hydrogenase 

 Avith the aid of electrons, "expelled" by excited chlorophyll molecules. 

 The electrons come to the excited chloroj^hyll via cytochromes from 

 the external electron donors and are raised to the reducing poteiuial of 

 molecular hydrogen during the primary photochemical act (1, 2) . 



If this interpretation was correct, then j)hotoj)roductioti of hydro- 

 gen by photosynthetic Ixicteria should occur not only at the expense 

 of organic eledron donors but also at the expense of physiologically 



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