658 



Marine Microbiology 



radiophosphate and reaches an equilibrium at which 10 per cent 

 of the P^^ is in the water ( bottom curve, F'ig. 2 ) . The phosphorus 

 turnover time for Eriocaulon, in the absence of bacteria, was cal- 

 culated to be 3.0 days. The effect of bacteria (middle curve. Fig. 

 2) is to hold phosphorus in the water and prevent the large loss 

 to the bottom plants. This is because bacteria hold large quanti- 

 ties of phosphorus in their cells and produce soluble organic radio- 

 phosphorus, which the plant presumably cannot assimilate. The 

 experiment was repeated with the plants Sphagnum and Utricii- 

 laria with similar results. 



An experiment was done with a layer of sediment at the 

 bottom of the bottle and with added plants and bacteria, thus 

 placing all three components in competition. As Figure 3 shows, 

 the addition of Eriocatdon to the control resulted in a loss of 70 

 per cent of the radiophosphate from the water after six days. 

 The ability of the plant to remove so great a quantity of radio- 





^ 



-•— • 



DEAD PLANKTON 

 a ANTIBIOTIC, 

 WITHOUT MUD 



X. 



DEAD PLANKTON 



ADDED 



\, 



dayf; 



Fig. 3. A. Amount of P"^- remaining in an aerated Grand Lake mud-water 

 system to which 2 g of Eriocaulon sprigs or 30 mg of tetracycline was 

 added. B. Effect of adding dead plankton in the presence and absence of 

 bacteria. Comparing the lowest curve of A with the top one of B it is shown 

 that mud is necessary for the inorganic exchange of P^- and that sterile 

 plankton will not serve as a substitute for mud. 



