OXYGEN EVOLUTION AND PHOTOREDUCTION 235 



light and only a relatively thin depth of very dilute suspension. The 

 intensities were varied by interposing neutral screens in the beam, 

 and were monitored with a Weston photocell and suitable galvanom- 

 eter. 



As is shown in Fig. 1, Scenedesmus evolves oxygen upon weak 

 illumination, even after several hours of dark incubation in an atmos- 

 phere of pure hjdrogen plus 2% carbon dioxide. However, at equal 

 Ught intensities, the rate is always greater if the carrier gas is nitrogen 

 rather than hydrogen. In obtaining these curves, the light intensity 

 was increased stepwise from zero to the highest value shown on the 

 graph. At this intensity the rate of oxygen evolution (in hydrogen 

 plus carbon dioxide) corresponds approximately to one volume of 

 oxygen per volume of algae per hour. With such low levels of 

 oxygen production, the possibility of deadaptation is certainly re- 

 mote. 



The data of Fig. 1 clearly indicate that an active hydrogenase does 

 not exclude the photosynthetic evolution of oxygen. However, be- 

 cause of the limitations in the apparatus used, it was not possible to 

 estimate the extent of true photoreduction. It was, therefore, nec- 

 essary to supplement these experiments with mass spectrometer and 

 manometric ones. 



THE MASS SPECTROMETER EXPERIMENTS 



The instrument used in these experiments has been described by 

 Brown et al. (3). Conventional Warburg vessels were attached to a 

 male joint containing a very fine leak through which gas from the at- 

 mosphere above the algal suspension could enter the spectrometer 

 analyzer tube at a very slow rate. In this way, there was continuous 

 sampling of the gas phase in the Warburg vessel. At intervals of 1.8 

 minutes analyses were obtained for the four gases: oxygen 32, oxygen 

 34, carbon dioxide 44, and carbon dioxide 45. 



Whereas in the Franck-Pringsheim apparatus only net oxygen pro- 

 duction could be followed, in the mass spectrometer it was possible to 

 measure oxygen and carbon dioxide consumption and production 

 separately. Another desirable feature of the mass spectrometer was 

 that it was possible to demonstrate adaptation during the course of 

 the experiments by showing that carbon dioxide production is small 

 (see column 2 of Table I) compared to the respiratory- rate (0.6 to 



