The Maximum Efficiency of Photosynthesis 109 



sation, and is explained by Eq. 3, where z'o is the compensating light intensity for 

 thin cell suspensions. 



4. Mechanism of Compensation 



Compensation of respiration by light has been generally explained by the Oz pro- 

 duction of photosynthesis. But again and again in the history of the science of 

 photosynthesis the question has been discussed as to whether light did not inhibit 

 the very process of respiration itself, for example, by catalytically inactivating 

 respiratory enzymes or by reducing intermediates of respiration 14 . We have 

 investigated this problem by the following type of experimental procedure : 



In a rectangular vessel, bearing two side-arms, was placed 300 cu. mm. of Chlorella, suspended 

 in 6 ml. of aeid eulture medium of pH 4.9. The side-arms contained NaOH, the gas-space air. 

 The Suspension was illuminated by a beam of red light (630 — 660 m/0 which entered the cell 

 Suspension in a vertical direction through the bottom of the vessel. The total intensity of the 

 beam was about 0.25 microeinsteins min., and the cross section of the beam at the bottom of the 

 vessel was 3 cm.- The main part of the light was absorbed in the first millimeter depth of the 

 Suspension, so that the illuminated volume of the cell Suspension was only 0.3 cu. mm. or 0.05 

 of the total volume of the cell Suspension. 



When the vessel was shaken rapidly, no compensatory effect of the light was observed, that 

 is, the negative pressure changes in the light and in the dark were virtually equal. When on the 

 other hand the gas space contained 5% CO-2, everything eise being equal, quantum requirements 

 of 3 to 5 were obtained for aliquots of the same cell Suspension. Such experiments, many times 

 repeated with the same result, proved that light does not inhibit respiration, when employed at 

 intensities capable of giving maximum photosynthetic efficiencies; such light, when it compensates 

 respiration, does so by photosynthesis, the gas exchange of which happens to be the reverse of the 

 gas exchange of respiration. 



This result does not contradict earlier observations that in the near absence of 

 CO-2 in the gas space, light can compensate respiration. The light intensities then 

 employed were much higher and were not controlled by simultaneous efficiency 

 determinations. Moreover, in the present experiments we had the advantage of 

 adequate intermittent illumination. In their cycles from dark to light and vice 

 versa the cells had time to give off in the longer dark periods most of the CO 2 pro- 

 duced in their respiration, and when they entered the illuminated volume of the 

 cell Suspension they were relatively free of CO2. Thus in the competition between 

 light and alkali for the CO2 produced in respiration, our special set-up worked in 

 favor of the alkali. 



5. The Significance of Compensation 



Cultivated as described, Chlorella has shown under experimental conditions at 20° an initial 

 respiration of 0.5 to 1.2 times its own volume of O2 gas/hr. After long manometric experiments 

 values as low as 0.2 times the cell volume of O2 gas/hr. have been observed. Thus very different 

 rates of photosynthesis may be necessary to compensate respiration and the "compensating light 

 intensity" is a highly variable magnitude. 



Compensation of respiration is important for the following reasons : 

 1. When photosynthetic efficiencies were determined manometrically below the 

 compensation point by a sequence of dark and light periods, the experiments had 

 to be discontinued after some hours, because not only the respiration, but also the 

 efficiency of photosynthesis usually decreased. But when the respiration was over- 

 compensated, so that the pressure changes throughout the whole duration of the 



