Gravitaxis in Flagellates and Ciliates

Abstract

Motile microorganisms such as flagellates and ciliates use the gravity vector of the Earth to adjust their position in the water column. Oriented movement by gravity is called gravitaxis and can be positive (downward swimming) or negative (upward swimming). In addition, some ciliates modify their velocity according to the swimming direction (gravikinesis). Earth-bound research and experimentation under simulated and real microgravity have revealed that a heavy mass such as a statolith or the whole cell content presses onto a gravireceptor which perceives the signal. In some cases mechanosensitive ion channels have been identified as gravireceptors. The activation of the receptor results in a cascade of reactions which amplify the signal and result in a steering response changing the direction of movement.