BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



235 



3 4 



TIME IN DAYS 



Figuke 9. — Comparison of division rates of Chlorella 

 Krown in Chlorella culture and in mixed culture in 

 aerated medium. Dots represent division rate in 

 Chlorella culture; circles, division rate in mixed culture. 



marked increase in size when grown in aerated 

 mixed culture over that obtained when it was 

 grown without aeration, it still reached only about 

 one-half the population size of aerated cultures 

 of Chlorella grown alone. The division rates in 

 aerated cultures of Nitzschia were also larger than 

 in unaerated cultures. The division rate of 

 Chlorella when grown alone was greater than 

 when grown in the mixed culture on every day ex- 

 cept the sixth and seventh days when the division 

 rates were about the same (fig. 9). It now ap- 

 pears beyond doubt that this inhibition of growth 

 of Chlwella in mixed cultures was due not to a 

 change in pH but to the presence of Nitzschia. 



Nitzschia reached a total population size sig- 

 nificantly larger when grown alone than when 

 grown in the presence of Chlorella. The division 

 rates for Nitzschia grown alone were generally 

 higher than in mixed cultures (fig. 10), although 

 the difference was perhaps not significant except 

 on the fourth day. The relatively great depres- 

 sion of the division rate for Nitzschia in mixed 

 culture on the fourth day may have been due to 

 the large increase of Chlorella which occurred on 

 the same day (fig. 9). The pH in the Nitzschia 

 culture fluctuated only from 7.2 to 7.5 (fig. 8, B), 



and the growth of Nitzschia was significantly im- 

 proved by aeration. 



On solid agar : 



Since solid medium has been used extensively 

 for testing the antagonistic action of one organism 

 on the growth of another, this experiment was 

 designed to test the effect of chlorellin on 

 Nitzschia when grown on agar. After much ex- 

 perimenting, it was found that Chlorella and 

 Nitzschia grew well on agar medium prepared 

 with Detmer's solution, diluted to two-thirds its 

 normal concentration and containing 1 percent 

 agar, 2 percent glucose, and y 2 percent peptone. 

 Since Chlorella is much less motile than Nitzschia 

 when grown on agar, it was transferred to fresh 

 agar slants which were then placed in a horizontal 

 position 63 centimeters below a 50- watt Mazda 

 lamp in a constant-temperature room. 



After Chlorella bad grown on the agar for 10 

 days,- a small piece from an agar slant on which 

 Nitzschia was growing was transferred to the agar 

 slant on which Chlorella had been growing, being 

 placed midway between the clump of Chlorella 

 and the opposite end of the agar slant (fig. 11). 

 It is a well-known fact that, compared with other 

 organisms, algae grow very slowly on agar. Also 

 algae require light to grow and the heat from the 

 light shortens the length of time an agar slant can 

 be used by speeding up dehydration of the agar. 



3 4 



TIME IN DAYS 



Figure 10. — Comparison of division rates of yitxschia 

 grown in Nitzschia culture and in mixed culture in 

 aerated medium. Dots represent division rate in 

 Nitzschia culture; circles, division rate in mixed cul- 

 ture. 



