230 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



TIME IN DAYS 



Figure 1. — Growth curve of Chlorella in standard culture 

 medium. Dots represent growth curve of Chlorella; 

 circles, phosphorus concentration ; triangles, pH. 



in the cells is reduced to one-half the normal con- 

 tent. Thus the cells in this experiment probably 

 were able to continue dividing after all phosphorus 

 disappeared from the medium by using intra- 

 cellular phosphorus absorbed in excess of that 

 needed during the first 4 days of the experiment. 

 The division rate for Chlorella (fig. 2) was greater 

 on the second than on the first day. The division 

 rate on the third and fourth days was about the 

 same as on the first day. From the fourth day 

 through the seventh there was a steady decrease 

 in the division rate. 



Determinations of pH made at the end of each 

 illumination period, when the culture medium was 

 found to be most alkaline, are shown in figure 1. 

 The pH rose from 7.2 on the first day to 8.3 on the 

 second day, and continued to increase daily dur- 

 ing the illumination period for the duration of 

 the experiment. Determinations made at the end 

 of each period of darkness showed that the pH 

 had dropped each day to about 7.0 or 7.2. 



GROWTH CURVE AND DIVISION RATE OF 

 NITZSCHIA 



The growth curve of Nitzschia under similar 

 conditions of adequate nutrients was obtained by 

 following the procedure described for Chlorella 

 except that cultures were prepared with 10 million 

 Nitzschia cells per liter. As shown in figure 3, the 

 Nitzschia population reached a peak on the fifth 



day and was relatively constant for the remainder 

 of the experiment. The division rate as shown 

 in figure 4 varied from 1.5 to 1.2 on the first 3 

 days of the experiment and decreased rapidly on 

 the fourth and fifth days. All measurable phos- 

 phate in the culture medium had been used by the 

 end of the fifth day. Very little division occurred 

 on the sixth day, and there was none on the sev- 

 enth day. It appears that Nitzschia cells cannot 

 continue to divide for as long a time as Chlorella 

 cells after phosphorus has disappeared from the 

 medium. Tt was found that Chlorella cells di- 

 vide for 3 days after phosphorus disappears from 

 the medium while Nitzschia cells divide for only 

 1 day. This may be due to Chlorella cells when 

 grown in the presence of phosphorus absorbing 

 much more than needed while Nitzschia cells 

 grown in the presence of phosphorus may ab- 

 sorb only that needed for immediate use. The 

 pH concentration, measured at the end of the light 

 period, increased from day to day and reached a 

 peak at the end of the third day. Beginning on 

 the fifth day and continuing through the seventh 

 day, the pH did not increase as much at the end 

 of the light period as on previous days. Since 

 the culture medium was not buffered, the pH in- 

 creased as the size of the population increased. 

 Thus the smaller increase in pH from the fifth day 

 on was probably due to a reduced metabolic rate. 



3 4 



TIME IN DAYS 



Figure 2. — Division rate of Chlorella in standard culture 

 medium. 



