Phototaxis in the flagellate, Euglena gracilis, under the effect of microgravity.

Positive phototaxis was analyzed in the unicellular photosynthetic flagellate, Euglena gracilis, under the conditions of microgravity during a parabolic rocket flight (TEXUS experiment). The fluence rate at which positive phototaxis changes to negative phototaxis was the same in a sample which had previously been exposed to microgravity as in a sample which had not been exposed to microgravity. During weightlessness the precision of positive phototaxis is higher than at 1 g at the same fluence rate. The swimming velocity of the cells is higher at 0 g than at 1 g confirming previous results that under terrestrial conditions the cells are subject to simultaneous sedimentation while they swim upward. Tracks of organisms recorded at 0 g show much more frequent deviations from the straight path than those at 1 g.