BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



243 



fluctuation in the hydrogen-ion concentration was 

 prevented by using buffered and aerated medium. 

 The experiment with aerated medium also elimi- 

 nated lack of C0 2 as a factor responsible for 

 inhibition of growth. 



As a precautionary measure to guard against 

 the physiological condition of the cells varying 

 from one experiment to another, controls were 

 run with every experiment. A comparison of the 

 controls in different experiments revealed no 

 significant difference in the physiological condi- 

 tion of the cells used. The control and the mixed 

 cultures were always started with cells taken from 

 the same culture. 



The physical effect of one species upon the 

 other in mixed culture cannot be considered as 

 a factor in this inhibition, since experiments with 

 culture medium prepared with conditioned me- 

 dium from which all cells had been removed gave 

 an inhibitory effect on the growth of cells subse- 

 quently cultured in the medium, even though the 

 supply of nutrients and the pH were optimum. 



A criticism raised against similar work on the 

 growth of protozoa was that other organisms 

 were used as a source of food for the animals 

 under investigation and that the presence of the 

 supplementary animals in the medium altered the 

 results (Beers 1033; Johnson 1933). This criti- 

 cism does not apply to the present investigation, 

 since pure cultures of Chlorella and Nitzschia 

 were used and the liquid media contained only 

 inorganic nutrients. However, the solid medium 

 used to test the effect of chlorellin on NitzscMa 

 contained glucose and peptone in addition to the 

 agar. Thus it can be concluded that the inhibition 

 observed in these experiments was due to a sub- 

 stance originating from the algae. The mode of 

 action of these antagonistic substances seemed to 

 be only a retarding effect on the growth rate with- 

 out any change in pigmentation of the cells or 

 other observable deleterious effects. 



Chlorella and Nitzschia, through the antago- 

 nistic substances produced by them, inhibited their 

 own growth as well as the growth of the other, 

 but each species inhibited the growth of the other 

 more than its own. Rodhe (1948) working with 

 Scenedesmus concluded that similar inhibitory 

 conditions existed in his pure cultures. 



The fact that a species of alga can inhibit its 

 own further growth under any condition is of 

 extreme importance. In the present laboratory 

 experiments, population densities obtained in pure 

 cultures were much higher than those ordinarily 

 found in nature. Thus a shorter period of time 

 was presumably required for the inhibition to 

 show its effect and the degree of severity probably- 

 far exceeds that found under natural conditions. 

 Therefore the results can be applied to natural sit- 

 uations only with caution. 



The antagonistic substance produced by 

 Nitsschia inhibited the growth rate of Chlorella 

 in all experiments in which a concentration of 

 70 million Chlorella cells and 10 million Nitsschia 

 cells per liter were used as initial populations. 

 That the antagonistic substance accumulates in 

 the medium of pure cultures as the cultures age 

 was shown. Nitzschia cultures with 10 million 

 cells per liter to which Chlon Ha was added on the 

 second day gave a greater inhibition of growth 

 rate for the Chlorella cells than when they were 

 added initially with the Nitzschia. Chlorella 

 cultures started with 70 million cells per liter in- 

 hibited the growth of Nitzschia when added in a 

 concentration of 10 million cells per liter on the 

 third day. Also when Chlorella was grown on 

 agar, the antagonistic substance was absorbed by 

 the agar creating an area on which the Nitzschia 

 cells would not grow. An increase in the initial 

 concentration of Nitzschia brought about a greater 

 inhibition of the growth rate of Chlorella, and a 

 similar increase in the inhibition of small popula- 

 tions of Nitzschia Was brought about by increasing 

 the initial concentration of Chlorella. However. 

 a ratio of Chlorella to Nitzschia of 20: 1 was re- 

 quired before the antagonistic substance from 

 Chlorella would inhibit the growth rate of 

 Nitzschia. When Chlorella and Nitzschia were 

 grown together with a ratio of 7:1, the growth 

 rate of Chlorella was always inhibited. 



It is possible for antagonistic substances to ac- 

 cumulate in natural bodies of water supporting 

 relatively small phytoplankton populations pro- 

 vided decomposition of these substances is slow. 

 It will be important in the future to determine 

 how long the substances remain effective. Sim- 

 ilarly, large populations of phytoplankton may 

 not suppress subsequent growth of certain species 

 if antagonistic substances produced by them are 



