BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



239 



X 5000- 



Figube 14. — Comparison of typical growth curves of 

 Chlorella in culture medium prepared with Norit-washed 

 and autoclaved Chlorella- and AWzscftMi-conditioned 

 medium with CTiZoreJJa-condltioned medium and with 

 NitZSChia-conditioned medium. Dots represent growth 

 curve in culture medium prepared with Norit-washed 

 and autoclaved Chlorella- and A T !tesc/!ia~conditioned me- 

 dium ; circles, growth curve in culture medium prepared 

 with (7i?orc/ta-eonditioned medium ; X's, growth curve 

 in culture medium prepared with Xitzseh in-conditioned 

 medium. 



the periods and the numbers of cells used in these 

 experiments. 



The size of the Nitzschia population reached at 

 the end of the fifth day in Berkefeld-filtered and 

 Norit-washed medium prepared from Chlorella- 

 and A/fesc/ua-conditioned medium is not signifi- 

 cantly different from that obtained in culture 

 medium prepared with distilled water and the 

 same concentration of nutrients. The Nitzschia 

 grown in culture medium prepared from 

 Nitzschia- and CMore/fa-conditioned medium did 

 not reach a population size as large as when grown 

 in similar medium which in addition had been 

 washed in Norit A (fig. 15). Nitzschia grown in 

 culture medium prepared from Nitzschia-con- 

 ditioned medium reached a population size larger 

 than when grown in Chlorella-conditioned medi- 

 um. The division rates of Nitzschia and of 

 Chlorella in culture medium prepared from Berke- 

 feld-filtered, Nitzschia- and 6Wore//a-conditioned 



medium is less than in similar medium which had 

 been washed with Norit A. Thus it has been 

 shown that both Nitzschia and Chlorella produce 

 substances that remain in the medium after the 

 cells have been filtered off which inhibit not only 

 their own growth but also the growth of the other. 

 Also, the antagonistic substances under the con- 

 ditions in which they were tested inhibit the 

 growth of the other species more than the species 

 producing them. Finally, it has been determined 

 that these substances can be removed from the me- 

 dium by filtering and/or washing with Norit A 

 and autoclaving. Lefevre et al. (1949) found 

 that algastatic substances secreted into the me- 

 dium by one species of alga which caused other 

 species of alga to divide at slower rates could be 

 destroyed by heat. 



INHIBITORY EFFECT ON CHLORELLA AND 

 NITZSCHIA OF P,4M>ORiW/4-CONDITIONED POND 

 WATER 



During the time the experiments with condi- 

 tioned media were being conducted in the labora- 

 tory, fluctuations in the phytoplankton popula- 

 tions of Belmont Hill Pond were being followed. 

 This pond was fertilized during the last week of 

 July 1949 and within a period of 3 days a bloom 

 of Pandorina appeared with a population of 73 

 million cells per liter. After this alga had been 

 growing in the pond for a period of '2 weeks and 

 the population had dropped to 46 million cells 

 per liter, a sample was collected and Chlorella and 

 Nitzschia cultures were prepared as in the pre- 

 vious experiment testing conditioned medium. 



Using Pandorina -conditioned pond water the 

 population size reached by Chlorella after 7 days' 

 growth in medium which had been Berkefeld 

 filtered only was 81 percent of that obtained in 

 similar medium which had also been washed with 

 Norit A and autoclaved (table 1). Similarly, 

 Nitzschia, after 5 days' growth in Berkefeld- 

 filtered pond water, reached a population size only 

 70 percent of that obtained in culture medium 

 which had also been washed with Norit A and auto- 

 claved (table 1). From these observations it can 

 be seen that a substance was present in the pond 

 water which inhibited the growth of both 

 Chlorella and Nitzschia. Also Lefevre et al. 

 (1949) have grown several species of algae in 

 culture medium prepared with filtered medium in 

 which Pandorina had previously grown. Of the 



