PHOSPHORUS EXCHANGE IN PHYTOPLANKTON 



89 



tion in the medium so that any radioactive phos- 

 phorus exchanged from cells to medium jan be 

 measured before being reabsorbed by the cells. 

 Thus it can be concluded that while phosphorus 

 absorption and regeneration can be followed in 

 nature, as shown by Hutchinson and Bowen, the 

 measurement of exchange is limited to the labora- 

 tory where conditions can be more closely con- 

 trolled. 



SUMMARY 



1. Phosphorus exchange between culture medium 



and marine planktonic algae was demon- 

 strated. 



2. The amount of phosphorus exchanged by 



Nitzschia cells when filter-washed varied 

 with the concentration of phosphorus in the 

 medium in which the cells were grown and 

 with the physiological condition of the cells. 



3. The percent of radioactive phosphorus ex- 



changed from cells depends upon the length 

 of time the cells are grown in the presence of 

 radioactive phosphorus. It was found that 

 cells grown over longer periods of time con- 

 verted more radioactive phosphorus into 

 organic or nonexchangeable compounds. 



4. A redistribution of intracellular phosphorus 



between the inorganic and organic fractions 

 occurred when cells were placed in the light 

 in medium containing only a trace of phos- 

 phorus. Cells kept in the dark converted 

 very little intracellular inorganic phosphorus 

 into the organic state. 



5. Exchange between photosynthesizing and divid- 



ing cells and culture medium was demon- 

 strated by placing cells containing radioactive 

 phosphorus in nonactive medium and meas- 

 uring the radioactive phosphorus exchanged 

 from the cells to the medium. 



6. Exchange between cells and medium was shown 



while the cells were not photosynthesizing 

 and dividing. 



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