BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



233 



3 4 



TIME IN DAYS 



Figure (>. — Comparison of division rates of Chlorella in 

 Chlorella culture and In mixed culture prepared with 

 standard culture medium. Dots represent division rate 

 in Chlorella culture; circles, division rate in mixed 

 culture. 



the populations readied on the seventh day in 

 this experiment with twice the amount of nu- 

 trients was not significantly different from those 

 obtained in the previous experiments. Also the 

 division rates did not vary significantly. 



In medium enriched daily : 



Since nutrients may inhibit the growth of some 

 algae if present in too high a concentration, the 

 preceding experiment was repeated except that 

 the extra amount of nutrients was added daily 

 from the end of the second day through the sixth 

 day. This method made possible tests on the ef- 

 fect of higher concentrations of nutrients with- 

 out increasing the initial concentration. To the 

 first group of flasks, an amount equal to 10 per- 

 cent of the initial concentration of nutrients was 

 added to each flask. To the second group an 

 amount equal to 30 percent, and to the third group 

 an amount equal to 50 percent of the initial con- 

 centrations of nutrients was added daily to each 

 flask. 



At the end of these experiments, the size of 

 the populations reached in the Chlorella, the 

 Xitzschia, and the mixed cultures were not 

 significantly different from those obtained in the 

 two previous experiments. Division rates ob- 



259109—53 2 



tamed for Chlorella when grown in the presence 

 of Nitzschia in mixed cultures were lower than 

 division rates of Chlorella grown alone. As was 

 found in previous experiments, division rates for 

 Nitzschia were as high when grown in the presence 

 of Chlorella as when grown alone. From the re- 

 sults of this and the previous experiment, it can 

 be concluded that a lack of nutrients is not respon- 

 sible for the observed inhibition of growth rate of 

 Chlorella. 



In buffered medium : 



In the first experiment testing interactions be- 

 tween Chlorella and Nitzschia the hydrogen-ion 

 concentration increased considerably, especially 

 as the cultures aged. To test whether the growth 

 of either OMon lla or Nitzschia had been limited 

 by this change, experiments were run in which 

 the hydrogen-ion concentration was prevented 

 from fluctuating as widely. In this experiment 

 the culture medium was buffered at pH 7.2 with 

 .001 molar KH 2 P(), and K 2 HPG 4 . The same 



- 7 ' 



- 6 



TIME IN DAYS 



Figure 7. — Growth curve of Nitzschia in mixed culture 

 prepared with standard culture medium. Dots repre- 

 sent growth curve of Nitzschia; circles, phosphorus con- 

 centration ; triangles, pH. 



