VOL. 12 (1953) 



GROWTH OF Chlorella 



27 



Using the samples containing practically 100% of each type of cells, some of their 

 physiological and chemical characteristics were compared*. The results obtained are 

 summarized in Table L 



The following facts emerge from the data given in the table. 



1. In respect to average cell size and respiratory activity, the order is: Light cells 

 > Active dark cells > Nascent dark cells. 



2. The photosynthetic rate (both under light-saturated and -limited conditions), and 

 the contents of chlorophyll and nitrogen in cells increases in the order: Light cells < 

 Nascent dark cells < Active dark cells. 



TABLE I 



CHARACTERISTICS OF DARK AND LIGHT CELLS 



. „ Light-saturated Light-limited Respiratory 



diamUr(\ Photosynth. photosynth. activity (Qoz) 



rate at 25° C 



rate (a) 



at 25» C 



Chlorophyll 

 content ("/,,) 



Nitrogen 

 content {"/„) 



Phosphorus 

 content C/o) 



* The light-Hmited photosynthetic rate is expressed by a = (d^p/d/)i_>o (in terms of i/day- 

 kilolux), where kp is photosynthetic rate and / the light intensity. 



3. Regarding phosphorus content, no significant difference is found among different 

 kinds of cells. 



Most remarkable was the difference in light-saturated photosynthetic rate between 

 active dark cells and light cells, the activity of the former being about 6 times that of 

 the latter. Significant also was the difference in light-limited photosynthetic rate and 

 chlorophyll content, which was about 3.5 times higher in active dark cells than in 

 light cells. 



Transformation of dark cells into light cells 



The process of transformation of dark cells into light cells, which takes place only 

 in the light, was followed in detail using cell samples rich in active dark cells as starting 

 material. In Figs. 3 and 4 are shown various aspects of the transformation which occurred 

 ' when the cell suspension (in normal nutrient medium and provided with COg-enriched 

 air), was illuminated with 2,000 lux light at 9°-io° C. The starting population, consisting 

 of 90% dark cells, gradually changed its dark cell ratio and within 100 hours 90 °o of its 

 population were transformed into light cells. During this period the cell number remained 

 unchanged, while the packed cell volume and the average cell volume increased 



* The effective way of collecting light cells is to take advantage of the occurrence of periodical 

 "bursting division" which was mentioned in the footnote on p. 24. During the period of repeated 

 "bursting divisions", there are stages in which almost 90% of the whole population are hght cells. 

 In separating these cells, adequate timing of harvesting and fractionation is necessary, because they 

 will divide, sooner or later, bursting into dark cells. The division can be prevented by keeping the 

 suspension in an ice box under anaerobic conditions (see later). Nascent dark cells can easily be 

 obtained from light cells, by keeping the latter in the dark, for 2 or 3 days, under aerobic condition. 

 There are two ways to obtain active dark cells: either subject the nascent dark cells to illumination 

 for several hours, or collect the cells grown under weak light (about 150 lux or less) at relatively high 

 temperatures such as 25° C. 



References p. 40. 



