308 BADER AND ABDEL-BASSETFigure 9 Mass spectrometric analysis of the photosynthetic oxygen evolution in Oscillatoria chalybeaas a
function of the oxygen partial pressure of the surrounding gas atmosphere. The cumulative signals were induced
by a train of 10 short (5 sec) flashes fired at a frequency of 3.3 Hz. The signal at the left was obtained at an
oxygen background signal of 26 mV equivalent to about 10.1 1015 oxygen molecules. Under completely
anaerobic conditions, no photolytic activity can be observed at all. Addition of 5 1014 oxygen molecules re-
sulted in the enhanced oxygen evolution signal shown at the right. (From Ref. 2.)
Figure 10 Hill plots of the dependence of the photosynthetic oxygen evolution of^16 O 2 (mass 32) and^18 O 2
(mass 36) on increasing oxygen partial pressure of the atmosphere. [The mixed isotope molecule^16 O^18 O (mass
34) gave a value of n3.1 and is not shown in the figure.] The value ncorresponds to the Hill coefficient cal-
culated for the respective isotope. The mV values in the graphs represent the oxygen partial pressure at half-
maximal oxygen evolution (S0.5). (From Ref. 2.)