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SUMMARYThe specific growth and feeding rates of Tetrahymena pyriformis GL grown axenically in proteose-peptone yeast-extract medium and monoxenically in suspensions of the bacterium Klebsiella aerogenes were studied. The relation between the initial concentration of substrate, whether bacteria or soluble organic complexes, and the maximum yield of ciliates was linear, although some inhibition was noted at higher substrate concentrations. The effective yields of Tetrahymena are 9-1 % (carbon to carbon) in axenic cultures and 50% (dry-weight bacteria to dry-weight ciliate) in monoxenic cultures. The maximum growth rates at 25" in axenic and monoxenic cultures were 0.20 and 0.22 hr-l, respectively. Carbon balance studies on axenic cultures suggested that of the carbon utilized during growth 36.5 % was incorporated into the Tetrahymena and 69 % was respired. The removal rates of K. aerogenes from suspension by T. pyriformis were studied and there was evidence which suggested that the individual feeding rate of a ciliate was governed by the concentration of ciliates as well as the concentration of bacteria present. From these observations a model for ciliate feeding was derived.

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