Abstract
Gravitaxis of swimming microorganisms has so far been explained largely in terms of the physical properties of the microorganisms that are assumed not to have any speculative mechanisms of gravity sensation. However gravity-induced sensory input and the subsequent modulation of locomotor activity in Paramecium has been suggested by precise analyses of swimming velocity as a function of swimming direction with respect to gravity. Paramecium appears to modulate its propulsive effort depending on the swimming direction by increasing the propulsive speed in upward and decreasing it in downward directions. This response termed as gravikinesis was obtained from the averaged measurement on a large number of the cell. In the present study we aimed to confirm the results on the basis of the measurement on the swimming of a single cell. Velocities of upward and downward swimming was measured from each single cell, and the velocity of sinking was measured on the same cell which was immobilized by Ni2+. We confirmed the gravikinesis from the analysis on the swimming of single cells. In addition, we found that in the hyper-density medium containing Percoll cells showed opposite gravikinesis to that in the hypo-density medium. These results may suggest the mechanosensitivity of the cell membrane as a physiological sensor underlying the gravity-dependent response in Paramecium.
