PHOSPHORUS EXCHANGE IN PHYTOPLANKTON 



83 



entering and exchanged by the phosphate-deficient 

 cells, the same as for the nondeficient cells. 



Comparison with the nondeficient cells of the 

 preceding experiment (table 1) shows that phos- 

 phate-deficient cells grown in low concentrations 

 of phosphorus absorbed more phosphorus than 

 nondeficient cells grown in a similar concentration 

 of phosphorus. Since the initial population sizes 

 were approximately the same in this and the 

 previous experiment, it can be assumed that the 

 metabolic rate of the phosphate-deficient cells was 

 higher than that of the nondeficient cells. Also, 

 less phosphorus was exchanged by deficient cells 

 when filter-washed even though they had absorbed 

 more phosphorus. This was probably due to more 

 of the phosphorus taken into the cells during this 

 period of time being converted into the organic 

 itate. 



Table 2. — Exchange by phosphate-deficient cells grown in 

 low and high phosphorus concentrations for 20 hours 

 before filter-washing 



[Initial cell count was 45 X 10' cells per liter. Specific activity was 0.3538 

 jiC/jigAP in both medial 



Effect of exposure time on exchange 



The smaller amounts of phosphorus exchanged 

 by phosphate-deficient cells as compared with that 

 exchanged by nondeficient cells in the previous 

 experiment was due to an altered physiological 

 condition. This difference in phosphorus ex- 

 change in the two different types of cells was 

 expressed as the percent of the total radioactive 

 phosphorus contained in the cells. Even though 

 exchange between cells in the same physiological 

 condition and the medium remains constant it 

 would appear to decrease with time if expressed 

 as a percent of the total radioactive phosphorus 

 in the cells. This would be due to the amount 

 of labeled phosphorus in the nonexchangeable frac- 

 tion increasing during a longer growing period. 

 This experiment only demonstrates the effect of 

 time upon the amount of phosphorus exchanged 

 when computed in the above manner and empha- 

 sizes the importance of considering the conditions 

 of the experiment when exchange is expressed as 

 a percent of total radioactive phosphorus. 



The cells were taken from a culture similar to 

 that used in the previous experiment with non- 

 deficient cells. The cells were suspended in two 

 cultures, the one with a low and the other with a 

 high phosphorus concentration, and grown for 7 

 days in the culture cabinet in the light. The initial 

 population of cells in these cultures was 10 X 10 7 

 cells per liter, a much smaller number of cells than 

 was used in the previous experiments. This 

 smaller initial population was used in order that 

 the cells could continue to divide and convert 

 inorganic phosphorus into organic-phosphorus 

 compounds. After 7 days in the culture cabinet, 

 the cells were filter-washed as in the two previous 

 experiments. 



The cells grown in the high concentration of 

 phosphorus absorbed more phosphorus than those 

 grown in the low concentration, as in the previous 

 experiments (table 3). Similarly, the cells grown 

 in the high concentration of phosphorus exchanged 

 more phosphorus with the medium when filter- 

 washed. Since the cells used in this experiment 

 came from the same type of culture as those cells 

 used in the first of these filter-washing experi- 

 ments, a comparison of the results is possible. As 

 would he expected, more radioactive phosphorus 

 was absorbed by the cells during the 7-day period 

 than by the cells grown for only 20 hours. Since 

 more radioactive phosphorus entered the cells 

 during this longer period, more was converted into 

 organic compounds. Consequently, when filter- 

 washed, a smaller percent of radioactive phos- 

 phorus was lost by the cells. 



Table 3. — Exchange by nondeficient cells grown in low 

 and high phosphorus concentrations for 7 days prior to 

 filter-washing 



(Exposure to radioactive phosphorus was also for the 7 days. Initial ell 

 count was 10 x 10 ; cells per liter. Specific activity was 0.2923 pc/wgAP m 

 both media] 



REDISTRIBUTION OF INTRACELLULAR 

 PHOSPHORUS 



Phosphorus exchange is difficult to detect in a 

 culture with a growing population; however, in 

 the filter-washing experiments, exchange was 

 shown. It was further found that the quantity of 



