BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



241 



eight species those investigators tested, the ma- 

 jority divided poorly, and some later shrank in 

 size and died. 



Tahle 1. — Populations of Clilorella and Nitzschia obtained 

 in culture medium prepared from t?&ndoria&-conditioned 

 pond water 



[Data given in number of cells per liter] 



EFFECT OF VARYING THE INITIAL CON- 

 CENTRATIONS OF NITZSCHIA AND 

 CHLORELLA 



INHIBITORY EFFECT ON CHLORELLA OF IN- 

 CREASED CONCENTRATIONS OF NITZSCHIA 



This experiment was designed to test the effect 

 of different concentrations of Nitzschia on Chlo- 

 rella populations as listed in table 2. Two cul- 

 tures were prepared for each population in 50 cc. 

 of standard culture medium buffered with .001 

 molar K 2 HP0 4 and KH 2 PO, at pH 7.2. The cul- 

 tures were prepared in 120-cc. Erlenmeyer flasks 

 and cell counts were made for each culture at the 

 end of the first and second days. 



Chlorella populations in cultures to which Nitz- 

 schia was added did not increase as much at the 

 end of the first and second days as the Chlorella 



population in the cultures prepared as controls 

 to which no Nitzschia was added (table 2). The 

 larger the initial concentration of Nitzschia added 

 to the Chlorella cultures the greater the inhibition 

 of growth of Chlorella. " Chlorella populations of 

 14i) million cells per liter to which an initial con- 

 centration of 50 million Nitzschia cells per liter 

 were added increased approximately threefold in 

 2 days, while the same size Chlorella population 

 to which 150 million Nitzschia cells per liter had 

 been added increased only 15 percent in 2 days. 

 Chlorella populations of 560 million cells per liter 

 to which an initial concentration of 50 million 

 Nitzschia cells per liter were added reached a 

 population size 50 percent of that in the cultures 

 containing only Chlorella. Similar Chlorella 

 populations to which 150 million Nitzschia cells 

 were added were only about 50 percent of popu- 

 lations obtained in Chlorella cultures prepared 

 with 50 million Nitzschia cells, and 25 percent of 

 those obtained in cultures containing only Chlo- 

 rella. Chlorella populations of 1,680 million cells 

 per liter to which 50 million Nitzschia cells per 

 liter were added reached 75 percent of the popu- 

 lation in the Chlorella cultures. 



There was no significant difference between the 

 increased size of Nitzschia populations when 

 grown in the presence of 140 million Chlorella 

 rells per liter and when grown alone (table 2). 

 Nitzschia added to Chlorella cultures of 560 mil- 

 lion cells per liter reached population sizes slight- 

 ly smaller than those obtained in the control cul- 

 tures containing only Nitzschia, but the difference 

 was not large enough to be significant. The 

 Nitzschia added to Chlorella cultures of 1,680 mil- 



Table 2. — Effect of increased initial concentrations of Nitzschia on Chlorella 

 [Data given in number of cells per liter] 



' Control. 



25S»109^54r 



