The expression of a circadian rhythm in two strains of Chlamydomonas reinhardii in space.

During the D1 mission the endogenous circadian rhythm of the photoaccumulation response persisted in two strains of Chlamydomonas. The amplitude was about twice as high in space as on the ground indicating that a larger fraction of cells was able to contribute to the expression of the rhythm. On the ground, cells usually enter the light cone of the illuminated area in the recording cuvette on the upper edge and leave it, due to gravity, on the lower one in a pulsating manner. This sometimes produces high frequency oscillations of light extinction on the ground. In space there were no such fluctuations; instead, cells swam into the light and stayed there harvesting more light energy for photosynthesis than did control cells. This probably enhanced the survival rate and increased the fraction of motile cells which contribute to the photoaccumulation. A more sophisticated evaluation technique allowed determination of the phase in the short period strain; it was delayed by two hours compared to the control. In an acetate free wildtype sample a rhythm with a period of about 24 hours was also detected.