PHOSPHORUS EXCHANGE IN MARINE PHYTOPLANKTON 



By Theodore R. Rice, Fishery Research Biologist 



The labeling of planktonic algae with radio- 

 active phosphorus was investigated as one phase 

 of a study of the food of filter-feeding inverte- 

 brates. The phytoplankton first had to be grown 

 under conditions that would result in the cells 

 absorbing and retaining large quantities of active 

 phosphorus before it could be used as labeled 

 food. This required that the processes of phos- 

 phorus exchange in phytoplankton be analyzed. 



The relative abundance of phosphorus has been 

 known to be a factor limiting the growth of 

 phytoplankton in the sea since the time of Brandt 

 (1899, 1902). Studies in the laboratory with 

 pure cultures, as well as correlations resulting 

 from work in the field, have proved phosphorus 

 to be of critical importance. Its abundance in 

 natural bodies of water previous to rapid increases 

 in population sizes and its seasonal depletion with 

 an increase of phytoplankton have been shown 

 many times. 



A decrease in the phosphorus concentration of 

 the medium following an increase in the phyto- 

 plankton population has been relatively easy to 

 demonstrate in the laboratory. Detection of 

 phosphorus exchange was not possible until 

 tracer techniques for active phosphorus were 

 developed. By using a culture medium con- 

 taining a mixture of radioactive and nonradio- 

 active phosphorus, a measurement of phosphorus 

 exchange is possible. The normal movement of 

 phosphorus can be traced by following the move- 

 ment of radioactive phosphorus, since it is gen- 

 erally accepted that cells cannot distinguish 

 between the two isotopes of phosphorus and 

 that isotopic effects can be disregarded. Further- 

 more, the addition of small amounts of radio- 

 active phosphorus, in these studies, has not 

 changed the physiology of the cells through 

 radiation, nor the rate of uptake of phosphorus 

 by them. 



Using radioactive phosphorus, Gest and Kamen 



Note.— This study was part of a project carried on under a cooperative 

 agreement between the United States Fish and Wildlife Service and the 

 United States Atomic Energy Commission. 



(1948) and Goldberg, Walker, and Whisenand 

 (1951) have demonstrated that phosphorus is 

 exchanged by planktonic algae. In both of these 

 investigations, exchange was tested for only short 

 intervals of time by either centrifuging or filter- 

 washing the cells containing radioactive phos- 

 phorus. The appearance of radioactive phos- 

 phorus in the washings demonstrated exchange. 

 Although filter-washing also has been used in 

 this investigation to determine factors influencing 

 exchange, emphasis has been placed on measuring 

 exchange over longer periods of time while the 

 cells were maintained under various environ- 

 mental conditions. 



MATERIALS AND METHODS 



CULTURE MEDIUM 



The addition of nutrients to natural sea water 

 is still apparently the best method of preparing 

 culture medium for marine planktonic algae. 

 One disadvantage of this method is that precipi- 

 tates often form when the enriched sea water is 

 autoclaved. It was important that no precipi- 

 tates form in the medium used in these experi- 

 ments, since the radioactive phosphorus must be 

 contained either in the cells or in the medium. 

 Sea water enriched with Miquel's solutions, as 

 modified by Ketchum and Kedfield (1938), was 

 used since Nitzschia closterium grows well in this 

 medium. It was found that their solutions A 

 and B caused precipitates to form when added to 

 sea water and autoclaved. By adding iron as 

 Fe(NH 4 )(S0 4 ) 2 instead of as FeCl 3 in solution B, 

 and by removing the phosphate compound from 

 solution B and adding it as solution C, the for- 

 mation of precipitates was avoided. The same 

 concentration of phosphorus, 56 /zgAP/L added 

 as KH 2 P0 4 instead of as Na 2 HP0 4 - 12H 2 0, was 

 used to prepare solution C. The important 

 change is that solution C was not added until after 

 the sea water containing solution A and the 

 modified solution B had been autoclaved and 

 allowed to cool. If solution C was added to the 



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