Responses of the photosynthetic flagellate, Euglena gracilis, to microgravity.

Motility and orientation have been studied in the unicellular photosynthetic flagellate. Euglena gracilis, using real time image analysis capable of tracking up to 200 cells simultaneously before, during and after a sounding rocket (TEXUS) experiment. The cells orient negative gravitactically under 1 g conditions and the degree of orientation increases with time when kept in a closed system. Under micro-g conditions the cells orient randomly and display no cooperativity between the individuals. Under 1 g conditions the velocity, which has been determined simultaneously to the orientation vectors, depends on the direction of swimming with respect to the gravity vector. Under micro-g conditions the cells swim significantly faster. After retrieval of the rocket the cells returned to normal gravitaxis and motility as observed before the space flight.