532 LIGHT AND LIFE 



gas in algae is a photodeconiposition of an endogenous hydrogen donor 

 was considered by Gaffron and Rubin but rejected by them in favor 

 of photodeconiposition of water (58) . In their view, the hychogen 

 came from water and the role of the endogenous hydrogen donors 

 was to reduce the remaining oxidized moiety (OH) of water. Like- 

 wise, in the case of photosynthetic bacteria, neither current inter- 

 pretations nor existing evidence supported the photochemical elec- 

 tron flow theory as a mechanism for evolving hydrogen from inorganic 

 and organic electron donors. 



With R. rubrum, Gest, Kamen, and Bregoff (64) found that the 

 most active substrates for photoproduction of hydrogen gas were 

 malate, oxaloacetate, and fumarate. Siegel and Kamen concluded that, 

 in general, "the presence of a certain steady-state concentration of COo 

 appears to be a major requirement for Ho evolution" (135). Thus, 

 they found succinate, which had earlier been reported as not giving 

 a photoproduction of hydrogen (64) , could evolve hydrogen gas in 

 the presence of CO2. 



The dependence of photoproduction of hydrogen gas on COo sug- 

 gested that the evolved hydrogen may be a product of interconversions 

 of certain organic hydrogen donors. However, Siegel and Kamen 

 recognized the possibility that "essentially all readily metabolized sub- 

 strates are capable of inducing Ho evolution under appropriate con- 

 ditions" (135) . A decisive test of this possibility would have been 

 evidence of hydrogen evolution from inorganic hydrogen donors 

 such as hydrogen sulfide or thiosulfate. But when this w^as experi- 

 mentally tested by Bregoff and Kamen with Chromatium, they found 

 that "using only inorganic media, no gas production could be ob- 

 served even over periods as prolonged as 2 to 3 weeks. \Vhenever 

 malate media were used, good gas production, as well as growth, 

 was observed in 2 to 3 days" (32) . 



Photoproduction of hydrogen gas in Chroinalium was also observed 

 by Newton and Wilson (113). In their experiments the photoproduc- 

 tion of hydrogen was strictly dependent on the presence of COo and 

 was inhibited by N2 and NH4+ — an inhibition that was first noted 

 by Gest and Kamen (63) . 



These experimental facts have been variously interpreted. Ne^\•ton 

 and Wilson (113) regarded the photoproduction of hydrogen by 

 Chrojnalium as resulting from a condensation of a C-4 compound 

 with COo to form a-ketoglutarate. They suggested that when a short- 

 age of nitrogen prevents the subsecpient formation of glutamate, its 

 precursor, a-ketoglutarate, is decomjiosed to give molecular hydrogen. 



