Relaxation and activation of graviresponses in Paramecium caudatum

The kinetics of gravitaxis and gravikinesis in Paramecium caudatum were investigated by employing (1) step transitions from normal gravity (1 g) to weightlessness (microgravity) and (2) turns of the experimental chambers from the horizontal to the vertical position at 1 g. The transition to microgravity left existing cell orientations unchanged. Relaxation of negative gravitaxis under microgravity took longer than 10 s and may be described by the time constant of the decay of orientation coefficients. Gravitaxis was started at 1 g by turning the experimental chamber from a horizontal to a vertical position. Gravitaxis activated rapidly during the turning procedure and relaxed to an intermediate level after the turning had stopped. Gravity-induced regulation of swimming speed (gravikinesis) at 1 g had reached a steady state after 1 min; at this point, gravikinesis counteracted the effects of sedimentation (negative gravikinesis). A step transition to microgravity initially reversed the sign of the gravikinesis (positive gravikinesis). The relaxation of this kinetic response was not completed during 10 s of microgravity. The data suggest that gravikinesis is functionally unrelated to gravitaxis and is strongly affected by the rate of change in acceleration. We present a model explaining why gravikinesis reverses sign upon the onset of a step from 1 g to microgravity.