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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



lion cells per liter reached smaller populations 

 than those in the control cultures. While this 

 decrease in the Nitzschia population was not large 

 enough to be important it occurred in all the mixed 

 cultures. It is believed that the Chlorella popula- 

 tion was approaching a concentration large 

 enough to bring about an inhibition in the growth 

 of these Nitzschia populations. Also the antago- 

 nistic substance produced by Nitzschia was divided 

 among an increased number of Chlorella cells at 

 the beginning of the experiment, thus producing 

 less inhibition. 



INHIBITORY EFFECT ON NITZSCHIA OF IN- 

 CREASED CONCENTRATIONS OF CHLORELLA 



In the previous experiment the effect of vary- 

 ing the initial population of Nitzschia on a popu- 

 lation of Chlorella was tested. Since it has been 

 shown that the concentrations of Nitzschia used 

 in the mixed cultures in previous experiments in- 

 hibited the growth of Chlorella, it was now desir- 

 able to test the effect of large concentrations of 

 Chlorella on a small concentration of Nitzschia. 

 For this experiment, cultures were prepared as 

 in previous experiments, except that different 

 populations of Chlorella, as listed in table 3, were 

 used. 



The size of the Nitzschia population was smaller 



at the end of the second clay in all cultures to 

 which Chlorella had been added than in the con- 

 trol cultures containing only Nitzschia. The 

 greatest inhibition in the division rate of Nitzschia 

 occurred in cultures to which 5,000 million 

 Chlorella cells per liter had been added. In these 

 cultures, a 70-percent inhibition occurred in the 

 division rate of Nitzschia. In the cultures which 

 contained initially 20 million Nitzschia cells per 

 liter and 400 million Chlorella cells per liter, a 

 greater inhibition occurred in the division rate of 

 Chlorella than of Nitzschia. However, in the cul- 

 tures containing initially 20 million Nitzschia 

 cells and 1,200 million Chlorella cells per liter 

 considerable inhibition occurred in the division 

 rate of both Nitzschia and Chlorella. 



Since neither nutrients nor pH were limiting 

 factors in these experiments, it can be concluded 

 that the division rate of both Chlorella and 

 Nitzschia are inhibited by varying the initial con- 

 centration of one or the other. The division rate 

 of large concentrations of Chlorella is inhibited 

 by relatively small concentrations of Nitzschia, 

 while extremely large initial concentrations of 

 Chlorella- are required to inhibit the division rate 

 of Nitzschia. It appears that Chlorella is more 

 sensitive to the antagonistic substance produced 

 by Nitzschia than Nitzschia is to chlorellin. 



Table 3. — Effect of increased initial concentrations of Chlorella on Nitzschia 



[Data given in number of cells per liter] 



' Control. 



EFFECT OF ANTAGONISTIC SUBSTANCES 

 ON PHYTOPLANKTON GROWTH 



Up to the present time, our best understanding 

 of the influence one plant exerts on another grow- 

 ing in association with it has been based on knowl- 

 edge of the competition for some factor essential 

 to growth, such as nutrients. Thus a thorough 

 study was required to demonstrate conclusively 

 the more complicated interactions between 

 Chlorella and Nitzschia and the existence of 

 antagonistic substances produced by them. 



In the experiments reported in this paper, those 

 chemical and physical factors which are known 

 to have an effect upon the growth of algae were 

 eliminated as responsible for the observed in- 

 hibition of growth by either maintaining a con- 

 stant and adequate value for them or having them 

 present in excess. Physical factors such as light 

 and temperature were kept constant within small 

 ranges. Nutrients were added in excess initially 

 to the culture medium in some experiments, and 

 daily to cultures in other experiments. Extreme 



