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William Vidaver 



freshwater algae. CO2 concentration was 4 per cent by volume at 

 the start of the experiment. O2 was assumed to be in equilibrium 

 with the atmosphere. Light was projected in a collimated beam 

 from a lOOw tungsten incandescent lamp and focused on the algal 

 disc in the pressurized cell. 



The intensity of white light was adjusted to bring about a 

 steady oxygen evolution of approximately one-half the saturation 

 rate at the base pressure of 1000 pounds per square inch (psi) for 

 the experiments with constant illumination. Pressure of this mag- 

 nitude has an imperceptible effect on oxygen evolution and ensures 

 that gasses (CO2 and O2), in the experimental concentrations, re- 

 main dissolved in the medium. 



For the light flashes, intensity was empirically selected to 

 induce maximum transient response and was usually near saturation 

 (at 1000 psi). 



RESULTS 



Constant Light 



Time courses of the three thalloid algae show that both steady 

 oxygen evolution and the a spike were inhibited by increased hy- 

 drostatic pressure (Figure 2a, b, c). There was greater inhibi- 

 tion of steady rates than of the highest rate of the spike. 



Ankis trodesmus (Figure 2d) was affected similarly by pressure. 

 However this alga displayed a small transient peak which occurred 

 before the maximum of the a spike. The transient appeared to be 

 almost uninhibited by the highest applied pressure. 



Light Flashes 



In view of the pre-a transient observed in the induction period 

 of Ankis trodesmus . attempts were made to ascertain the possibility 

 of a pre-a transient being present in the induction response of 

 other algae. It was found that high or low temperature (4° or 

 30° C.) did induce the appearance of similar transients in both 

 Ulva and Ilea . No pre-a transient was ever observed in the res- 

 ponse of Porphyra . Figures 3 and 4 show that the pre-a transients 

 of Ulva and Ilea are also strikingly resistant to pressure inhibi- 

 tion. 



DISCUSSION 

 A pressure insensitive oxygen evolution transient was observed 



