232 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



> 



3 4 



TIME IN DAYS 



Figure 4. — Division rate of Nitzschia in standard culture 

 medium. 



INTERACTIONS OF CHLORELLA AND NITZSCHIA 



To test whether or not one species of phyto- 

 plankton may inhibit the growth of another, the 

 following experiment was run in which Chlorella 

 and Nitzschia were grown together in mixed cul- 

 ture. The same procedure was followed in this 

 experiment as described in the foregoing experi- 

 ments, except that both Chlorella and Nitzschia 

 were added initially to each flask. 



Chlorella grown in mixed culture under the con- 

 ditions of this experiment reached a population 

 size only 60 percent of that attained when grown 

 alone in the same type of medium (fig. 5) ; also 

 the division rate was significantly less than when 

 grown alone (fig. 6). There was considerable dif- 

 ference in the division rates on the second day 

 and Chlorella grown in mixed culture continued 

 to divide at a slower rate for the remainder of the 

 experiment. Nitzschia in mixed culture reached 

 a population size which was not significantly dif- 

 ferent from that obtained in the cultures in which 

 Nitzschia was grown alone (fig. 7), and the divi- 

 sion rates of Nitzschia under these conditions 

 varied only slightly. 



Even after taking into consideration the error 

 of counting and the ordinary fluctuation occurring 

 between two different cultures, it is evident that 

 the results for Chlorella are significant. It was 

 also found later that Chlorella can inhibit the 

 growth of Nitzschia under certain conditions. 

 Other investigators have shown that several algae 

 inhibit their own growth, while in this experi- 

 ment it has been demonstrated that a species of 

 alga can also inhibit the growth of another 



species. Pratt and Fong (1940) reported that 

 Chlorella produces and liberates into the culture 

 medium an antibiotic substance which they named 

 chlorellin. This author believes that Nitzschia 

 also produces an antibiotic substance which in- 

 hibited the growth of Chlorella in the experiment 

 described here. Further experiments supporting 

 this conclusion are presented later. 



Referring again to figure 5, it is seen that the 

 phosphorus was depleted by the end of the fourth 

 day in the mixed culture. However, this also 

 occurred in the Chlorella culture (fig. 1). The 

 pH increased a little more rapidly and was slight- 

 ly higher in the mixed culture than in either the 

 Chlorella or the Nitzschia cultures. Therefore 

 the possibility must now be considered that the 

 observed inhibition of Chlorella was due to either 

 a depletion of nutrients or an unfavorable change 

 in pH concentration instead of the production of 

 an antibiotic substance by Nitzschia. 



In initially enriched medium : 



To determine whether a lack of nutrients had 

 limited the growth of either or both Chlorella and 

 Nitzschia, experiments similar to the preceding 

 ones were run again with twice the initial concen- 

 tration of nutrients in the culture medium. In the 

 cultures in which Chlorella and Nitzschia were 

 grown alone and in mixed cultures, the sizes of 



Figure 5. — Comparison of growth curves of Chlorella in 

 Chlorella culture and in mixed culture prepared with 

 standard culture medium. Dots represent growth curve 

 in Chlorella culture; circles, growth curve in mixed 

 culture; X's, phosphorus concentration; triangles, pH. 



