Significance of photosynthetic products of symbiotic Chlorella to establish the endosymbiosis and to express the mating reactivity rhythm in Paramecium bursaria

Paramecium bursaria contain several hundred cells of the green algae Chlorella as endosymbionts and are designated green. Chlorella-free white cells can be obtained from natural green cells by rapid growth in constant darkness (DD). Chlorella were isolated easily from their host cells and re-infected. The infection of Chlorella was restrained by a photosynthesis inhibitor (DCMU). This result can be related with the fact that symbiotic Chlorella release their photosynthetic products. Furthermore, when green cells were cultured in DD, the number of endosymbiotic Chlorella decreased and the density of host cells increased. On the other hand, the mating reactivity rhythm of green cells disappeared in DD. The photosynthetic products of symbiotic Chlorella, maltose and oxygen, induced the rhythms of their host cells in DD, but they could not shift the phase of the rhythms. Moreover, arrhythmic mutant whits cells reverted to rhythmicity after infection with Chlorella isolated from wild type green cells. Thus the photosynthetic products released by endosymbiotic Chlorella have important roles in the establishing of the endosymbiosis and the expression of circadian rhythms in P. bursaria cells.