Tetrahymena pyriformis is a small protozoan cell, which has less than 10% of the volume of the well-known Paramecium caudatum. We investigated the graviresponses of Tetrahymena in a search of the lower limits of gravitransduction in ciliates. Equilibrated populations of free swimming cells were enclosed in chambers of 2 mm depth testing the orientational and speed responses with the chambers in horizontal or vertical position. For determinations of gravikinesis, the sedimentation rates of cells immobilized by application of two different procedures were measured. Negative gravitaxis was pronounced after turning the chambers from horizontal to vertical position; it settled, after 1 min, toward an orientation coefficient of 0.2. Gravikinesis did not only neutralize the sedimentation rate (= 22 μms~(-1)) but even exceeded that rate by at least 30%. Tetrahymena is thereby the first cell, in which overcompensation of the sedimentation rate was documented. Biophysical considerations suggest a high gravisensitivity of Tetrahymena with channel gating energy being less than 33 times above the thermic noise level.
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