44 Photosynthesis* 

 Von Otto Warburg 



Ever since Chlorella has been an object of photosynthetic study, it has been known 

 that there are cells that use light efficiently and cells that use light inefficiently. In 

 recent years we have sought to discover and control the conditions that give rise 

 to efficient cells. It has been found that one of the most important conditions is the 

 light intensity at which the cells are cultured. If one employs artifkial light sources 

 without interruption, as has been the almost universal practice, the Chlorella are 

 then too far removed from their natural living conditions of the past half billion 

 years. The cells are forced to produce organic matter continuously, and more 

 material than they need for their own synthesis. As a consequence, the energy yield 

 of the cells is reduced to a small fraction of the optimum yield. 



Cells that use light efficiently result, on the other hand, when one allows the 

 intensity of the light to fluctuate so as to imitate day and night, with dimming late 

 evening and early morning 1 . We attain this by varying the operating voltage 

 automatically from 50 up to 220 volts and back to 50 volts again over a period of 

 24 hours. The relative quantum intensities of radiation were measured with the 

 chemical quantum actinometer' 2 , with results indicated by the ordinate values in 

 Fig. 1. Cells so cultured use the light best when they are placed in the manometric 

 vessels in the morning and their photosynthetic efficiency is measured thereafter 

 during the artificial day. 



Equally as important as the culturing of the cells are the conditions under which 

 the utilization of the light is measured. For example, it was found with monochro- 

 matic light that the utilization of light in the green or yellow or red was the poorer 

 the purer the spectral composition. However, good utilization was immediatly 

 restored when a relatively small amount of blue-green light was added to the main 

 beam of very pure monochromatic light. One can thus obtain good or poor yields 

 at will, simply by adding or removing the blue-green light during the measure- 

 ments of efficiency. If each such test period is made 30 minutes long, one can ob- 

 serve in an experimental day of 8 hours, with one and the same Suspension, good 

 yields eight times and poor yields eight times ! 



The different parts of the blue-green spectrum are not equally effective. The 

 action spectrum of the blue-green light shows a sharp maximum in the region of 

 460 m/>, as is shown in Fig. 2. This action spectrum is probably a Carotinoid spec- 

 trum. An inactive Carotinoid proenzyme is probably converted by the blue- 

 green light into an active lumino enzyme. As possible analogs, there may be 

 mentioned light-sensitive visual purple, and ooverdin, a Carotinoid protein dis- 

 covered by Richard Kuhn. 



Both examples — the fluctuating light during culture and the blue-green light 

 during yield measurement — suffice to make it understandable why, in the last 



Aus: Science, 128, (1958): 68—73. 



