INFLUENCE OF ARTIFICIAL CHANGES OF CHLOROPHYLL CONTENT 1271 



ing deficiency of nitrogen ; it could be remedied by repeated addition of 

 nitrate to the suspension medium. (This influence of nitrogen concentra- 

 tion, too, was discussed in chapter 13; cj. page 339). 



To sum up, experiments with nitrogen-deficient and magnesium-de- 

 ficient cell cultures gave less consistent results as to the correlation be- 

 tween chlorophyll concentration and the capacity for photosynthesis than 

 did experiments in iron-deficient media. However, in the latter case, too, 

 we probably are dealing not with direct effects of changes in chlorophyll 

 concentration, but with variations in the general development of the photo- 

 catalytic apparatus, which includes, in addition to chlorophyll, also the 

 several catalysts participating in the "dark" stages of photosynthesis. 



Myers (1946) noted that, when Chlorella pijrenoidosa was grown in a 

 continuous growth apparatus, the chlorophyll content per unit cell volume 

 increased by a factor of 5 when illumination was decreased from 360 to 

 6 foot-candles. At the same time, however, the number of cells in one 

 cubic centimeter of packed cell material increased from 6 X 10^ to 29 X 10^ 

 so that the average number of chlorophyll molecules in a single cell was 

 ahnost unchanged, the small "shade cells" containing about as much 

 chlorophyll as the large "light cells." Among these different cultures, the 

 highest saturation rates of photosynthesis per unit cell volume were shown 

 —in alkahne buffer as well as in acid Knop's medium— by those grown in 

 fight of 25-50 foot-candles; the saturation rate per unit cell volume de- 

 cfined sharply for cultures grown < 25 foot-candles and gradually for cul- 

 tures grown >50 foot-candles. The saturation rate per cell (and thus, be- 

 cause of the above-mentioned constancy of the chlorophyll amount per 

 cell, also the saturation rate per chlorophyll molecule, i. e., the assimilation 

 number), increased continuously with increasing intensity of the culture 

 light. (It should be noted that the "saturation rate" was measured at 

 600 foot-candles— a light intensity that may be insufficient to completely 

 saturate fight-adapted Chlorella cells.) 



Myers noted that the maximum rate of growth of Chlorella was reached at about 100 

 foot-candles, and suggested that in stronger light the cells are incapable of utilizing all 

 the immediate products of photosynthesis, and develop a special mechanism of dissimila- 

 tion ("hght respiration," or photoxidation?) which disposes of surplus photosynthates. 

 In other words, below 100 foot-candles growth is limited by the rate of photosynthesis, 

 while above 100 foot-candles it is limited by a dark process (such as nitrogen assimila- 

 tion), which the primary products of photosynthesis (carbohydrates?) must undergo to 

 be converted into balanced cell material. 



In Chapter 31, (p. 1228) we described Sorokin and Myers' discovery of 

 a "thermophilic" Chlorella pyrenoidosa strain, with a peak capacity for 

 growth at 39° C. (instead of the usual 26° C). At its optimum tempera- 

 ture, this strain required a higher fight intensity for saturation, and pro- 



