442 



Norman I. Bishop and H. Gaffron 



GENETIC BLOCK OF OXYGEN AND HYDROGEN PRODUCTION 



Recently, a mutant of Scenedesmus (mutant 11) has been de- 

 scribed which, while unable to evolve oxygen either in photosyn- 

 thesis or in the quinone-Hill reaction, is fyJJy equipped to 

 assimilate carbon dioxide via photoreducti on ^ ' . Despite the 

 presence of an active hydrogenase and the ability to utilize ex- 

 ternal hydrogen, adapted suspensions of this mutant do not evolve 

 hydrogen photochemi cal 1 y. The capacity of the parent strain of 

 Scenedesmus and mutant 11 to evolve hydrogen in the dark and In 

 the light are compared in Fig. 1. The genetic alteration produces 

 a two-fold effect: hydrogen is not evolved in the light and the 

 small rate of hydrogen production in the dark (which is identical 

 to the reaction of the parent strain) is abolished in the light. 



Three other mutant strains resemble mutant /^l 1 . In the light 

 they are neither able to evolve oxygen aerobically (these will be 

 termed "oxygen mutants"), nor produce hydrogen anaerobi cal 1 y. 

 Their capacities for hydrogen formation are summarized in Table 1. 



Table I. Gas Metabolism of Adapted "Oxygen Mutants" of 



Scenedesmus D'j 



1000 ul cells in 3 cc of 0.05 M phosphate buffer, pH = 6.5. Tem- 

 perature = 25°C. Gas Phase =-- N2. 0.2 cc 19% KOH in center well 

 of each vessel, CCP (final concentration = lO'^M) added after 

 adaptation. Period of adaptation - ]h hours. Light Intensity = 

 300 lux. 



