Machemer-Röhnisch S, Bräucker R, Machemer H (1998): Relaxation of graviresponses of the ciliate Didinium following step transition to the weightless condition.

Equilibrated, free swimming specimens of Didiniumnasutum were exposed to a step transition from normal acceleration to 10 s of microgravity in a 500 m drop shaft for free-fall experiments. Prior to microgravity, the orientation and swimming speed of the cells were registered by video microscopy in the horizontal and vertical position of the experimental chamber, and recording continued beyond the end of the weightless condition. At normal gravity horizontally swimming cells showed constant speed and did not prefer any direction. Turning the chamber to the vertical position induced Didinium to be oriented upwards (negative gravitaxis) and so raise speed in the down ward direction. Gravitaxis persisted until the end of microgravity. For about 1 s of microgravity upward swimming rates exceeded the downward rates. During the entire weightless period, the median speed of all cells was below the value of cellular propulsion as unaffected by gravity. A detailed analysis of the events during microgravity revealed a gradual rise in downward swimming rates and decline of upward swimming rates. The apparent gravikinetic paradox (gravikinesis persisting in the weightless condition), and the slow kinetics of gravikinesis are discussed at the basis of the established mechano- and gravisensory organization of Didinium and an model of visco-elastic linkage between the sensitive anterior cell end and the insensitive posterior cell end.