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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



phosphorus exchanged depended on the amount of 

 phosphorus in the inorganic fraction in the cells. 

 Whether or not the small amount of phosphorus 

 remaining in the medium at the end of the uptake 

 experiment was due to exchange is not known. For 

 this trace of phosphorus to be the result of ex- 

 change would require that some phosphorus always 

 remain in an inorganic state in the cells. 



To prepare cells with the inorganic fraction 

 reduced to a minimum necessitates growing them 

 in the light in medium containing no phosphorus. 

 This was the method used to prepare phosphate- 

 deficient cells in the previous experiment. Under 

 these conditions the cells reduce the intracellular 

 phosphorus to a point where further cell division 

 is impossible. Theoretically, under these condi- 

 tions the inorganic fraction lias been reduced to as 

 small an amount as possible. 



The rearrangement of intracellular phosphorus 

 was traced in Nitzschia cells by following the 

 movement of radioactive phosphorus. To medium 

 containing 14.7 X 10 7 cells per liter were added 

 8.75 microcuries of radioactive phosphorus. The 

 same amount of radioactive phosphorus was 

 added again the following day. On the second day 

 the cells were centrifuged from this active medium 

 and resuspended in medium with less than 0.5 

 MgAP/L of nonactive phosphorus. This medium 

 with cells containing radioactive phosphorus was 

 divided into two cultures. One culture was kept 

 in the light while the other was placed in the dark. 

 Ketchum (1939 b) has shown that nondeficient 

 cells similar to these will not absorb phosphorus 

 from the medium in the dark, but it was not shown 

 whether changes occur in the distribution of 

 intracellular phosphorus. 



The distribution of intracellular radioactive 

 phosphorus between the organic and inorganic 

 fractions was determined at the time the cells 

 were suspended in the new medium and again on 

 the sixth, tenth, and fourteenth days. The cells 

 were fractionated with trichloracetic acid, as 

 described in Materials and Methods, and radio- 

 activity counts were made on each fraction. The 

 results of this experiment are shown in table 4. 

 The cells kept in the light continued to combine 

 inorganic phosphorus into the organic fraction 

 for most of the 14 days. At the end of this time, 

 14.7 percent of the radioactive phosphorus still 

 remained in the inorganic state. Under the 

 conditions of this experiment, cell division had 



stopped by the sixth day. It can be seen in 

 table 4 that little inorganic phosphorus was 

 converted into the organic fraction after cell 

 division stopped. The ceasing of cell division 

 could not have been due to the lack of some other 

 nutrient since the medium contained sufficient 

 amounts of all nutrients except phosphorus. 

 The occurrence of phosphorus in the inorganic 

 fraction at the end of 14 days may indicate that 

 some inorganic phosphorus is always present in 

 Nitzschia cells and that division is stopped not 

 by a complete disappearance of inorganic phos- 

 phorus but rather by the reduction of the inorganic 

 phosphorus below a threshold amount. From 

 these findings it appears that the trace of phos- 

 phorus remaining in the medium in the phos- 

 phorus-uptake experiment can be attributed to 

 exchange between the cells and medium. 



Table 4. — Distribution of intracellular radioactive phos- 

 phorus in Nitzschia cells suspended in medium containing 

 only a trace of nonradioactive phosphorus 



1 Determined before dividing cells into two cultures. 



There was not as much change in the distribu- 

 tion of phosphorus between the inorganic and 

 organic fractions in the cells kept in the dark as 

 in those grown in the light, yet there was a 

 slight decrease in the inorganic fraction except on 

 the tenth day (table 4). This one determination 

 was undoubtedly in error. From the data for 

 cells grown in the dark, it can now be surmised 

 that a slight metabolic activity is going on which 

 requires the redistribution of intracellular 

 phosphorus. 



EXCHANGE BY CELLS SUSPENDED IN CULTURE 

 MEDIUM 



Exchange in the light 



The difficulty of detecting phosphorus exchange 

 between cells and medium when phosphorus is 

 being absorbed rapidly by the cells has been shown 

 previously. However, if large amounts of radio- 

 active phosphorus can be incorporated into the 



