5 The Quantum Efficiency of Photosynthesis* 



By Otto Warburg, Dean Burk, Victor Schocken 

 and Sterling B. Hendricks 



Photosynthesis is a unique endothermic photochemical reaction in which chemical 

 energy is gained from visible light energy by the combined action of several 

 quanta. Nothing similar is known in the nonliving world. It was first reported a 

 quarter of a Century ago 1 that in photosynthesis the greater part of the absorbed 

 visible light energy could be converted into chemical energy under optimum condi- 

 tions. Indeed, no more than four quanta of red light seemed to be necessary to 

 produce one molecule of oxygen gas, which is close to the thermodynamic require- 

 ment of three quanta. It is easy to understand that this result, lacking any analogy, 

 has sometimes been doubted by theoreticians, and it is a fact that certain investi- 

 gators have raised methodological objections 2 . For this reason we have reinvesti- 

 gated the question of the minimum quantum requirement of photosynthesis as 

 measured by oxygen and carbon dioxide gas exchange. The present paper is a short 

 summary of our findings by new and simplified methods. 



I. Cultivation of Cells 



A strain of Chlorella pyrenoidosa, isolated in New England and identified by Dr. 

 Florence Meier of the Smithsonian Institution, and for many years in laboratory 

 use, was cultivated in tall Drechsel gas washing bottles containing 200 ml of the 

 following salt Solution: 5 g MgS0 4 ■ 7 H 2 0, 2.5 g KNO s , 2.5 g KH 2 P0 4 , 2 g NaCI, 

 and 5 mg FeS0 4 • 7 H 2 0, in 1 liter of filtered, unsterilized well water (pH 4.5—5). 

 The cultures were maintained at a room temperature of 25 — 30° C, and were 

 aerated with 5% C0 2 in air at a rate (-500 ml per minute) rapid enough to prevent 

 cell settling, and were constantly illuminated with a 100-watt incandescent lamp 

 at a distance of about 30 cm. Cells cultivated by this method gave more uniform 

 material and more regulär manometric results than when cultivated by the older 

 method Ci> p- 427) i n which slowly aerated cells settled down in Erlenmeyer-shaped 

 flasks and became partially anaerobic until reshaken up, and in which lowered light 

 intensities were employed for the terminal cultivation phase. 



The cultures were used for the experiments in the present work after 2—10 days 

 growth, when they contained 200—1000 u\ cells, depending upon the amount of 

 initial inoculation. Usually 50—100 /d cells per 200 ml medium were employed 

 as inoculum, grown as just indicated. Bacterial growth during either cell culturing 

 or manometric experiments was found with a haemocytometer to be negligible, due 

 to the low pH, the lack of added organic matter in the synthetic medium, and 

 possible antibiotics produced by the Chlorella. 



The cells for experimental use were centrifuged in an International No. 2 Centri- 



* Aus Biochimica et Biophysica Acta 4 (1950): 335. 



