BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



237 



percent of that obtained in previous experiments 

 when CMorella was added initially with Nitzschia 

 to new culture medium. The division rate of 

 CMorella added to the Nitzschia cultures was thus 

 less than the division rate of Chlorella in the con- 

 trol cultures. The decreased division rate and 

 subsequent smaller population obtained in this ex- 

 periment can be attributed to Nitzschia having 

 grown for 2 days in the medium before the ad- 

 dition of Chlorella. This gave sufficient time for 

 the medium to become conditioned through the 

 formation and accumulation of the antagonistic 

 substance. The population size and division rate 

 of Nitzschia in the cultures to which Chlorella 

 was added were not significantly different from 

 those obtained in the control cultures. 



INHIBITORY EFFECT ON NITZSCHIA OF 

 CHLOflELL<4-CONDITIONED MEDIUM 



In the previous experiments when populations 

 of CMorella and Nitzschia were grown in mixed 

 cultures, an inhibition always occurred in the 

 growth rate of CMorella, while there was no sig- 

 nificant change in the growth rate of Nitzschia. 

 Therefore, the following experiment was designed 

 to ascertain whether an inhibition of Nitzschia 

 would occur if the medium were conditioned by a 

 longer and larger growth of CMorella. The 

 same procedure used to prepare the Nitzschia- 

 conditioned medium was used to prepare the 

 CMorella-cow\.\t\on^\ medium, except that Chlo- 

 kIIii cultures were started with an initial concen- 

 tration of 70 million cells per liter. At the end 

 of the third day Nitzschia was added in sufficient 

 quantity to give 10 million cells per liter to 3 of 

 the Chlorella cultures, and 3 Chlorella cultures 

 were left as controls. Three flasks with a con- 

 centration of 10 million Nitzschia cells per liter 

 were prepared as controls. 



The size of the Nitzschia population reached at 

 the end of the experiment in the mixed cultures 

 was only 70 percent of that attained in the con- 

 trols (fig. 13). The division rate of the Nitzschia 

 cells added to the Chlorella cultures were not as 

 great as the division rate of Nitzschia in the con- 

 trol cultures. The population size and division 

 rate of CMorella in cultures to which Nitzschia 

 was added were not significantly different from 

 those obtained in the control cultures. 



time in days 



Figure 1.3. — Comparison of growth curves of Nitzschia in 

 control and in C/iJoreHo-eondltioned cultures. Dots 

 represent growth curve in control cultures; circles, 

 growth curve in CftlwcMo-Conditloned cultures. 



INHIBITORY EFFECTS OF FILTRATE FROM 

 CONDITIONED MEDIA 



This experiment was designed to test the growth 

 rates of both CMorella and Nitzschia in culture 

 media prepared from the filtrates of their own 

 conditioned media as well as from filtrates of the 

 conditioned medium of the other. NitescMor&m- 

 ditioned medium was prepared from 5-day-old 

 cultures which had been started with an initial 

 concentration of 10 million cells per liter in stand- 

 aid culture medium. At the end of the fifth day 

 the cultures were centrifuged to remove the cells. 

 The pH was adjusted to 7.0 with phosphoric 

 acid. Phosphate determinations run at this point 

 showed that approximately 5 /xg.VP/L had been 

 added in the phosphoric acid. Next, 600 cc. of 

 this conditioned medium was filtered through 

 a Herkefeld filter. From this filtered medium, 

 100-cc. portions were transferred to each of three 

 250-cc. Erlenmeyer Masks. The remaining 300 cc. 

 of the filtered, conditioned medium was washed 

 with 1-percent Norit A (carbon) by bringing the 

 medium to a boil. The medium was then paper 

 filtered to remove the Norit A, divided into 100-cc. 

 portions, placed in 250-cc. Erlenmeyer flasks, and 

 autoclaved. The same concentration of nutrients 

 used in the standard culture medium was added to 



