Role of Bacteria in Mineralization of Phosphorus in Lakes 659 



phosphate from the water when the mud control was unable to 

 do so in the presence of bacteria, indicates the affinity of Erio- 

 caulon for phosphate. The control curve in Figure 3 A is an ex- 

 treme illustration of another phenomenon which was common 

 to most experiments. While a control mud-water system seemed 

 to reach equilibrium after three to eight days, a gradual loss of 

 radiophosphorus in water continued over a period of weeks imtil 

 practically none remained in the water. This decline is attributed 

 to a fall-out of radiophosphorus in the bodies of bacteria. 



In summary, when bacteria and higher plants compete for 

 inorganic phosphorus the bacteria get there first and change a 

 part of it to organic forms which are apparently unavailable to 

 plant use. In the absence of bacteria the aquatic plants tested take 

 up enormous quantities of P^^ within 12 hrs, indicating a rapid 

 exchange and the establishment of an equilibrium with most of 

 the phosphorus in the plant body. 



A large fallout of plankton following a bloom might be ex- 

 pected to affect bacterial activity at the mud surface of a lake 

 and thus alter the exchange of phosphorus. Plankton was netted 

 from Grand Lake, autoclaved, and approximately 0.3 g was 

 pipetted into duplicate mud-water svstems from the same lake. 

 The plankton sedimented immediately to the mud surface. Equal 

 quantities of dead plankton were placed in two antibiotic 

 treated glass blanks, omitting mud. Results are shown in Figure 

 3B. Obviously sterile dead plankton, of itself, is unable to absorb 

 significant quantities of phosphate by a colloidal mechanism or 

 by other means. The addition of dead plankton to a mud-water 

 system resulted in an enormous and rapid loss of P^^ from the 

 water. This greater uptake at the mud surface, as compared with 

 the control in Figure 3A, was evidently caused by increased ac- 

 tivity there, due to the large amount of readily decomposable 

 organic matter. An uptake of phosphate by bacteria attached to 

 falling organic matter in lakes is probably of importance in the 

 natural cycle. 



The first experiment described showed tliat bacteria in \\\c 

 water convert a large part of inorganic P^^ to the organic form in 

 a few hours. The question arises: do bacteria serve as necessary 

 intermediates in uptake by invertebrates? To test this, experi- 



