VOL. 12 (1953) GROWTH OF Chlorella 29 



During the course of transformation in normal medium, the packed cell volume of the suspension 

 increased (in terms of kg) at first rapidly and later more slowly. The initial velocity of increase of 

 packed cell volume corresponds to that of photosynthesis; for example, a suspension which contained 

 2.55 ml per liter of active dark cells (J = 0.90), increased its packed cell volume up to 6.16 ml per liter 

 within the first 6 hours of illumination with 20,000 lux light at 25° C, during which time the cell 

 number remained unchanged. This increase corresponds to the relative growth rate of kg = 1.53, 

 which is almost equal to the rate of photosynthesis {kp = 1.8 1) shown by active dark cells under the 

 same condition. 



The data presented above show clearly that photosynthesis forms the key event 

 occurring in the transformation of dark cells into light cells. It must, however, be noted 

 that the process — especially at its later stages — involves also other metabolic processes, 

 which is evidenced by the effect of N- and P-deprivation* as well as the formation of 

 chlorophyll during the process in normal medium. 



Transformation of light cells into dark cells 



While the dark cells remain unchanged when they are not illuminated, the light 

 cells are rather labile and under ordinary conditions, regardless of whether they are in 

 the light or in the dark, they change sooner or later into dark cells. Using a suspension 

 of light cells it was found that the light (at least up to the intensity of 20,000 lux) 

 does not exert any influence upon the process of their transformation into dark cells. 

 In Figs. 5 and 6 is shown the process of transformation taking place in the dark. The 

 light cells which formed 85% of the total population at the beginning gradually de- 

 creased to give rise to dark cells, and within about 50 hours more than 90% of the whole 

 population had turned into dark cells. Noteworthy is the fact that the transformation 

 occurred only under aerobic conditions** and was completely halted when the cell 

 suspension was deprived of oxygen***. (See Fig. 6.) 



It is apparent from the figure that, while the cell number and the dark cell ratio 

 had increased during the process, the packed cell volume remained almost unchanged or, 

 actually, slightly decreased, which may be attributed to the consumption of cell materials 

 by respiration. From the data given in the figure, and other data not presented here, 

 it was found that the number of dark cells produced by the division of one light cell was 

 between 6 and 7. This fact indicates that the actual number of n in the process L — > wD 

 is 4 or 8 in most cases'*". By comparing the frequency polygons given in Fig. 5 with those 

 in Fig. 3 it will be noticed that the dark cells derived from light cells in the dark were 

 smaller in size than those taken from continuously illuminated cultures. The former are 

 what we call nascent dark cells and the latter the active dark cells. 



In the experiment given in Figs. 5 and 6, the cells were suspended in normal culture medium. 

 It was found in other experiments that the transformation (under aerobic conditions) was not 

 markedly affected by deprival of phosphate from the medium, but was considerably retarded, 

 although eventually proceeded to completion, on deprival of nitrate, and was most strongly suppressed 

 in distilled water. The dark cells produced in distilled water or in N-deficient medium showed a 



* Photosynthesis, in the ordinary sense, can proceed for an appreciable length of time without 

 provision of any nutrient substances. It was ascertained that the dark cells can photosynthesize in 

 distilled water at a rate of 90-95% of normal rate (observed in ordinary phosphate buffer), and this 

 activity lasted more than two hours without showing any sign of decrease. 



** It was confirmed by other experiments that the provision of COg is not necessary for provoking 

 the transformation. 



*** The light cells which had been subjected to anaerobiosis for 67 hours showed quite normal 

 growth when supplied with COg-air and light, indicating that they had remained intact during the 

 anaerobiosis. 



t This was borne out by microscopic observation of a large number of cells in the dividing state. 



References p. 40. 



