70 P- M. COOK 



completeness of this cycle is a function of the length of exposure 

 and the intensity of the light. It also has been shown (Myers, 

 1946; Myers and Burr, 1940) that at a certain critical intensity 

 of light and above, a cell is light-saturated and has reached its 

 maximum rate of photosynthesis. At a lower critical intensity, 

 the maximum growth rate is reached (Myers, 1946; 1946a). 

 Therefore, we can expect to gain no further benefit from in- 

 creased light intensity, widi the exception of having more volume 

 growing at maximum rate due to greater penetration of light 

 through the culture. We have demonstrated that at various wave 

 lengths and intensities of light a Chlorella culture closely follows 

 Beer-Lambert's Law on the absorption of light. This will show 

 that at high light intensities Chlorella absorbs considerably more 

 light than can be used, due to the light-saturation point of each 

 cell. In a practical system, then, under high light intensities, a 

 considerable portion of the light will be wasted. We can visualize 

 no practical system to overcome this inherent difficulty. More 

 data still must be obtained on the light-saturation point of Chlo- 

 rella under various conditions, but it is now clear that as much 

 as 85% of the light is wasted under certain conditions of popu- 

 lation density and culture, even though the culture is deep 

 enough to permit light extinction. This seriously limits the pos- 

 sible amount of conversion of light in the synthesis of organic mat- 

 ter. In spite of this we have succeeded in converting approximately 

 2/4% of the incident solar radiation in the production of organic 

 matter. Some improvement may be expected by maintaining a 

 greater depth of culture in the new pilot plant, inasmuch as the 

 culture in the column is not absorbing all the light at times of 

 maximum intensity. 



Population Density 



For maximum production in sunlight under the best culture 



conditions now known, population density should be maintained 



at 0.36 grams dry weight per liter. For all practical purposes, 



this can vary by ± 10% without an important reduction in yield. 



;If a system allows a wider variation of population density, an 



