BIOTIC INFLUENCES ON PLANKTONIC ALGAE 



229 



both containing 2 percent glucose and y 2 percent 

 peptone. Only Detmer's solution was used for 

 culturing Nitzschia. 



ILLUMINATION OF CULTURES 



A set of four 40-watt daylight fluorescent bulbs 

 was arranged on each of the four shelves in the 

 culture cabinet. Approximately 5 inches above 

 each set of lights was a platform made of glass 

 which had been ground with carborundum to 

 ensure an even dispersal of light and on which 

 were placed the cultures of algae. In these ex- 

 periments, daylight fluorescent lamps were used 

 which produced an illumination on the culture 

 flasks of 375-foot candles as measured by a Weston 

 illumination meter. The cultures were illumi- 

 nated daily from 11 a. m. to 1 a. m. 



TEMPERATURE 



The culture cabinet was placed in a dark con- 

 stant-temperature room. In these investigations 

 :< temperature of 18° ±1.5° C. was maintained. 

 The temperature variation was checked with a 

 maximum and minimum thermometer. Two cir- 

 culators were used to blow a stream of air over 

 the cultures, thus ensuring an even distribution 

 of temperature. 



INORGANIC-PHOSPHATE DETERMINATION 



Phosphate concentrations were determined by 

 the Atkins-Deniges niolybdate method as modified 

 by Wattenberg ( 1937) . Algal cells were removed 

 by filtering the culture medium through a sintered 

 glass filter before making determinations. The 

 concentration of phosphorus is expressed as 

 microgram-atoms of phosphate phosphorus per 

 liter as recommended by the International Asso- 

 ciation of Physical Oceanography (Sverdrup et 

 al. 1942; table 42). 



PREPARATION OF ALGAL CELLS FOR EXPERI- 

 MENTS 



Ketchum (1939) and Pratt (1940) both found 

 that the growth of algae when transferred to fresh 

 medium was influenced by the age of the culture 

 from which the cells were taken. Cells from older 

 cultures had a longer lag period and a slower 

 rate of growth. Therefore, in these experiments 

 C'hlorella cells were always taken from a 6- to 

 7-day-old culture, while Nitzschia cells were taken 

 from a 4- to 5-day-old culture. 



The cells were removed from the culture me- 

 dium in which they were growing by centrifuging 

 at 2,500 r. p. m. for approximately 5 minutes. 

 The cells were resuspended in the same type of 

 culture medium which was being used in the ex- 

 periment. A cell count was made on this con- 

 centrated suspension of cells so that the proper 

 dilutions could be made to give the desired con- 

 centration of cells for the experiment. 



DETERMINATION OF POPULATION SIZE 



After thoroughly mixing the culture medium, 

 0.5 cc. was removed aseptically. This medium 

 was used for making two cell counts with a Levy 

 haemocytometer. The mean of the numbers of 

 cells counted in all like cultures in any one experi- 

 ment was taken as the population size. A care- 

 ful check showed that this method is extremely 

 accurate for determining unialgal population 

 sizes. At the lowest population sizes used in these 

 experiments, this counting method showed a co- 

 efficient of variation of 7.1 percent, while the 

 largest population sizes gave even more accurate 

 results, with a coefficient of variation of 4.(1 per- 

 cent. The accuracy of this method as determined 

 by the author agrees favorably with that found 

 by other investigators (Peaisall and Loose 19:17; 

 Pratl 1940; and Winokur 1948). 



GROWTH RATES AND INTERACTIONS OF 

 CHLORELLA AND NITZSCHIA 



GROWTH CURVE AND DIVISION RATE OF 

 CHLORELLA 



Since it lias been observed that, phosphorus 

 occasionally reaches concentrations as high as 10 

 /igAP/L in natural waters (Chandler and Weeks 

 L945), the growth rate of Chlarella was deter- 

 mined in culture medium containing this concen- 

 tration. Standard culture medium was prepared 

 with Chhrella in a concentration of 70 million 

 cells per liter and placed on the illuminated 

 shelves. At the end of each day cell counts, the 

 pH (as determined with a Beckman pH meter), 

 and phosphorus determinations were made for 

 one culture. 



The Chlorella population (fig. 1) was still in- 

 creasing on the seventh day even though all the 

 measurable phosphorus had been used by the end 

 of the fourth day. Ketchum (1939) had shown 

 that Chlorella pyrenoidosa when grown in a non- 

 nutrient medium divides until the phosphorus 



