80 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



tion of 44 MgAP/L. Nitzschia cells were added to 

 this medium in sufficient quantity to give a popu- 

 lation of 18.5 X 10 7 cells per liter and the culture 

 was placed in the culture cabinet in the light. 

 Counts for the radioactive phosphorus content 

 of cells and colorimetric analysis for the inorganic- 

 phosphorus content of the medium were made 

 periodically. This colorimetric analysis also in- 

 cluded the radioactive phosphorus in the medium, 

 but there was such a small number of radioactive 

 atoms that little, if any, change was caused by 

 their presence or absence. 



In figure 3, the amounts of radioactive and 

 nonradioactive, or inorganic, phosphorus ab- 

 sorbed by the cells are shown. Since both radio- 

 active and inorganic phosphorus are plotted on 

 the vertical scale so that a certain percentage of 

 one is equal to the same percentage of the other, 

 the determinations for radioactive phosphorus 



also represent microgram-atoms of phosphorus. 

 The cells continued to absorb phosphorus from 

 the medium until it was reduced below a concen- 

 tration detectable with the colorimetric method. 

 However, radioactive determinations for phos- 

 phorus made at the end of this experiment showed 

 that a trace of phosphorus still remained in the 

 medium. This may be due to radioactive deter- 

 minations being many times more sensitive than 

 colorimetric determinations. It is believed that 

 contamination can be ruled out. The author 

 suggests that this trace of radioactive phosphorus 

 in the medium is due to exchange from the cells. 

 From the two curves in figure 3 it can be seen 

 that at first the absorption of radioactive phos- 

 phorus by the cells was greater than absorption 

 determined by the decrease of inorganic phos- 

 phorus in the medium. This difference must be 

 the result of exchange or of errors in measurement 



z 



to 

 a 



z 

 < 



z> 

 o 



I 



z 



V) 



o 

 o 



If) 



IT 

 O 

 I 

 Q. 

 </) 



o 



I 

 0. 



V) 



2 

 o 



2 

 < 



IX 



o 

 o 

 <r 

 o 



TIME IN HOURS 



Figure 3. — Phosphorus absorption by Nitzschia cells as determined by both colorimetric and radioactivity measure- 

 ments. Circles show amounts of inorganic phosphorus measured colorimetrically; dots show amounts of inorganic 

 phosphorus from radioactivity observations. 



