Circadian rhythm of gravitaxis in Euglena gracilis

Euglena gracilis, a unicellular, photosynthetic flagellate is a model syste m for environmentally controlled behavioral reactions. One pronounced react ion is the orientation with respect to gravity. In synchronized cultures wi th no cell growth a distinct circadian rhythm of negative gravitactic orien tation could be observed. The main maximum of sensitivity was detected 5 h after the beginning of the subjective day, the main minimum 5 h before the beginning of the subjective day. Transferring synchronized cultures to cont inuous light resulted in an almost instantaneous loss of rhythmicity In con trast, after transfer to permanent: darkness cells exhibited a circadian rh ythm with a progressive shortening of the period for more than 5 days. Thes e findings are in contrast to the circadian rhythm of phototaxis in Euglena , where a free-running period of 24 h was observed. Parallel measurements o f negative gravitactic orientation, velocity, cell shape as well as cAMP co ncentration in synchronized cultures revealed a circadian rhythm of all rea ctions. The results are discussed with regard to the possible role of cell shape and cAMP in gravitactic orientation.