Effects of symbiotic Chlorella on the period length and the phase shift of circadian rhythms in Paramecium bursaria

Abstract

Summary Cells of P. bursaria normally contain several hundred cells of the green alga Chlorella in the cytoplasm as endosymbionts. Chlorella -free white cells can be derived easily from natural green cells by rapid growth in the dark. The green cells showed a 3 h longer period of photoaccumulation rhythm in constant light (LL) than white cells, but showed almost the same period length as white cells when photoaccumulation rhythms were assayed in constant darkness (DD) and in LL with a photosynthesis inhibitor (DCMU). The period length was altered in LL according to the number of Chlorella in the cell. Interestingly, white cells can be restored to green ones quickly by the infection of Chlorella isolated from green cells. The phase of photoaccumulation rhythms in re-infected cells was altered in LL depending on the phase of infected Chlorella . The re-infected green cells also showed altered photosensitivity according to the infected Chlorella . On the other hand, white cells of an arrhythmic mutant were rescued by infecting Chlorella isolated from wild type green cells. The period length and the phase of circadian rhythms of P. bursaria were determined in LL in cooperation with symbiotic Chlorella .