The Maximum Efficiency of Photosynthesis 107 



comes constant. Thus, with increasing light intensities the efficiency of photo- 

 synthesis decreases, owing, as is generally assumed, to the limiting influence of a 

 dark reaction. It has long been considered as a major difficulty of efficiency deter- 

 minations, that the maximum efficiency can only be obtained at very low light 

 intensities ; this maximum has to be computed from small pressure changes that 

 involve important "corrections for respiration". It will be shown in the following 

 that this difficulty may be overcome by taking advantage of intermittent Illumi- 

 nation with high light intensities. 



In the steady State at light Saturation, our Chlorella produces about 30 times its own cell volume 

 of oxygen gas/hr. at 20 C. ; and in the region of proportional intensities it produces several times 

 its cell volume of 02/hr. As earlier indicated 11 - 12 , Chlorella and most plants may commonly show 

 a rectangularly hyperbolic function of rate of photosynthesis against light intensity, which means 

 Virtual proportionality up to at least 20 — 30",, of maximum rate under favorable conditions 1 



(pp. 289 3 329). 



The Chlorella suspensions used for our efficiency determinations were not optically thin, but 

 so dense that the red light was virtually completely absorbed in the suspensions. For example, 

 300 cu. mm. of cells suspended in 7 ml. of culture medium, were placed in shaken rectangular 

 vessels of 8 cm.- bottom area. Imagine now a device whereby the 300-cu. mm. cells could be 

 homogeneously illuminated as in optically thin suspensions, then the 300-cu. mm. cells at Sa- 

 turation intensity would produce 30 300 9000 cu. mm. of O2 gas/hr., and hence at pro- 

 portional intensities, at the very least, 1000 cu. mm. O2 gas/hr. This means that maximum efficien- 

 cies could be determined at light intensities that produced as much as 1000 cu. mm. of O2 /hr. 



This is not the case, when the cell Suspension is at rest during the illumination. Then the 

 cells in the outer layers of the Suspension are more strongly illuminated than in the inner layers 

 and the oxygen production is unequally distributed over the 300-cu. mm. cells. The cells in the 

 inner layers might produce no O2 at all, and some cells in the outer layers would produce much 

 more O2 than three times their cell volume per hour. Thus in the outer layers the rate of O2 pro- 

 duction may be less than proportional to the light intensity, and the efficiency, determined for 

 the total cell Suspension, could not be the maximum efhciency. 



But when the cell Suspension is shaken and the cells move in cycles from higher to lower inten- 

 sities and vice versa, then in a time that is large compared with the duration of a 1-cell cycle, 

 all of the 300-cu. mm. cells may absorb the same amount of light, and produce, proportional to 

 the light intensity, the same amount of oxygen. In this State the maximum efficency can be ob- 

 tained. 



To reach this State the motion of the cells must be so rapid that the light energy absorbed in 

 any fraction of time in any part of the cell Suspension does not diminish the concentrations of the 

 limiting dark reactions. The more a given total light intensity is concentrated in space and the 

 stronger the spectral region of the light is absorbed by the cells, the smaller will be the illuminated 

 space inside the cell Suspension, and the more rapid therefore must be the shaking to distribute 

 the light evenly over the cells of the Suspension. 



For example, when a narrow beam of red light was sent into a Suspension of 300-cu. mm. 

 cells, maximum efficiency was obtained in one instance up to an O2 production of 70 cu. mm./hr., 

 but less than maximum efficiency was obtained when the light intensity of the narrow beam was 

 doubled, the motion of the cells being insufficient for the higher light intensity. When on the 

 other hand the greater part of the cell Suspension was illuminated with diffuse white light, maximum 

 efficiency was obtained from the initial beam of red light up to an over-all O2 production of 500 cu. 

 mm./hr. as effected by both light sources. This approached an oxygen production of two times 

 the cell volume per hour, a value up to which proportionality to the light intensity may be readily 

 observed in the steady State in experiments with optically thin suspensions. 



These facts suggest that the efficieny of photosynthesis should be determined by 

 illuminating cell suspensions from all sides in diffuse light of a spectral region that 

 is only weakly absorbed by the cells. But even with the help of the new actino- 

 metric technique 13 , this procedure would not allow a correct measurement of the 

 absorbed light. Only when a collimated narrow light beam enters the cell suspen- 



