238 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



both the Berkefeld-filtered medium which had 

 been Norit washed and the medium which was only 

 Berkefeld filtered. Determinations of pH at this 

 point showed that both the Norit-washed, Berke- 

 feld-filtered and the Berkefeld-filtered media had 

 a pH of 7.2 ±0.1. This was the pH concentration 

 of culture media used in previous experiments. 

 Chlorella cultures with an initial concentration of 

 70 million cells per liter were prepared with both 

 the Berkefeld-filtered and Norit-washed media. 

 Daily cell counts were made for each culture for 

 a period of 7 days. More Nitzschia-conditioned 

 medium was prepared but this time both the 

 Norit-washed, Berkefeld-filtered and the Berke- 

 feld-filtered, 7Wte.se/wa-conditioned media were 

 used for the preparation of Nitzschia cultures 

 with an initial concentration of 10 million cells 

 per liter. These cultures were placed on the 

 illuminated shelves and daily cell counts were 

 made for all cultures for a period of 5 days. 



Chi or ell a-condit\onzd medium was prepared in 

 a way similar to that described for Nitzschia 

 except that the Chlorella cultures were started 

 with a concentration of 70 million cells per liter, 

 and were grown for 7 days. Culture medium pre- 

 pared with this Chlorella-condit\oned medium was 

 used for the preparation of Nitzschia cultures with 

 a concentration of 10 million cells per liter, and cell 

 counts were made daily for 5 days. More Chlo- 

 reKa-conditioned medium was prepared in the 

 same way, and to the culture medium prepared 

 with this conditioned medium, Chlorella cells were 

 added in sufficient quantity to give a concentration 

 of 70 million cells per liter. These cultures were 

 placed on the illuminated shelves and counted 

 each day for 7 days. 



As previously stated, standard culture medium 

 was used in the preparation of both Nitzschia- 

 conditioned and c?A?o/'eZ7a-conditioned media. 

 Also, the same amounts of nutrients were again 

 added to the conditioned culture media to be used 

 in further growth studies. Thus, Chlorella and 

 Nitzschia cells growing in Chi or ell a-conditioned 

 and in Nitzschm-conditioned media were being 

 supplied with concentrations of nutrients ranging 

 between 1 and 2 times the concentration used in 

 the standard culture medium. However, in ex- 

 periments in which Chlorella and Nitzschia were 

 grown in concentrations of nutrients 1 to 2 times 

 the concentration contained in the standard cul- 



ture medium, there was no significant differ- 

 ence in the total size of the population or in 

 the division rate. Further experiments showed 

 that the growth of neither Chlorella nor Nitzschia 

 was changed significantly if distilled water which 

 had been washed with Norit A by bringing the 

 water to a boil was used in the preparation of 

 culture media with the same concentration of nu- 

 trients. Therefore no substance which would 

 change the division rate was added to the condi- 

 tioned medium by washing it in Norit A. 



The size of the Chlorella populations reached 

 at the end of the seventh day in Berkefeld-filtered 

 and Norit-washed Nitzschia- and Chlorella-condi- 

 tioned medium was not significantly different 

 from that obtained in cultures prepared with dis- 

 tilled water and the same concentration of nu- 

 trients. The Chlorella grown in culture medium 

 prepared from Nitzschia- and Chi or ell a-condi- 

 tioned medium at the end of 7 days had not 

 reached population sizes as large as Chlorella 

 grown in similar medium which in addition had 

 been washed in Norit A and autoclaved (fig. 14). 

 Chlorella grown in Chlorella-conditioned medium 

 reached a population size larger than when grown 

 in culture medium prepared from Nitzschia-condi- 

 tioned medium. Pratt and Fong (1940) have 

 shown in somewhat similar manner that the 

 growth of Chlorella is inhibited when grown in 

 culture medium prepared from C hi orel la-condi- 

 tioned medium 



The division rate of Chlorella was not as high 

 in culture medium prepared from Berkefeld- 

 filtered Nitzschia- and Chlorella-condit\onzd me- 

 dium as in the same type of medium which in ad- 

 dition had been washed in Norit A. The division 

 rate of Chlorella in Norit-washed Nitzschia- and 

 Chi or ella-conditioned medium was not signifi- 

 cantly different from growth obtained in standard 

 culture medium. It can thus be concluded that 

 either the heating of the medium or the washing 

 with Norit A destroyed or absorbed the antag- 

 onistic substances formed by these algae. The 

 division rate of Chlorella in culture medium pre- 

 pared from jV^2s<?A?'a-conditioned Berkefeld- 

 filtered medium, which was not washed in Norit 

 A, is less than that obtained in culture medium 

 prepared from Chi or ell a-conditionzd, Berkefeld- 

 filtered medium and not washed in Norit A for 



