Circadian Rhythm Changes in Autotrophic Euglena Induced by Organic Carbon Sources*

SYNOPSIS. Acetate added to autotrophic Euglena cultures changed the period length of the circadian rhythm of phototaxis. Phase shifts were induced by acetate pulses. Since transition from one metabolic state to another (autotrophic/mixotrophic) caused a phase shift or a period change, such effects possibly result from switching metabolic pathways. As suggested (Brinkmann, K., 1966. Planta 70 , 344–89), differences in the temperature responses of the rhythm in mixotrophic and autotrophic cells might also be caused by participation of different metabolic pathways with different Q¹⁰ values, e.g. dark reactions vs photochemical reactions. However the Q¹⁰ of a given dark reaction, e.g. protein synthesis, can differ in the 2 states. Therefore temperature experiments alone do not suffice for deciding whether the pathways include photochemical reactions, dark reactions, or both.