Cell Size, Macromolecule Composition, Nuclear Number, Oxygen Consumption and Cyst Formation During Two Growth Phases in Unagitated Cultures of Acanthamoeba castellanii1

SYNOPSIS. The size, composition and physiology of average cells have been studied in cultures of Acanthamoeba castellanii during the phases of logarithmic growth and population growth deceleration (PGD). Most of the features examined were relatively constant during log phase, but had significant changes during PGD. Average cell volume increased about 60% and total dry mass about 15–20% during the latter period. Total protein content remained constant thruout both growth phases, but cytochrome oxidase doubled during PGD. DNA, RNA and glycogen levels began to change during late log phase. DNA decreased about 50% and RNA increased about 75%. Glycogen decreased 50% during the RNA build‐up and then increased to a plateau above the log phase level. A final decrease in glycogen followed an increase in the relative numbers of cysts in late PGD. It was found that PGD begins when O 2 becomes limiting and evidence that the subsequent changes in macromolecule composition are related to encystation is discussed.