ABSTRACT

Motile unicellular organisms use external stimuli to orient themselves in their habitat in order to find optimal conditions for growth and reproduction. In many species gravity is a prominent stimulus for orientation. In some species the cells movement towards or away from the water surface is mostly likely caused by physical mechanisms. Due to their unequal mass distribution between front and rear end of the cell body the organisms are aligned with respect to the gravity vector by a buoyancy mechanism. This mechanism can be predicted for very small cells in which the mass of the cell is not sufficient to activate mechano-sensitive elements (force <1 pN). Also other physical mechanisms are hypothesized and are summarized in the article. In several other organisms mechanisms of active physiological graviperception have been found. They are based on changes in stimulation of physiological mechano-sensitive elements according to the position of the cell body in the gravity field. In some species gravity is perceived by intracellular statoliths, as in Loxodes, a ciliate which possesses specialized vacuoles (Müller Organelle) filled with barium sulfate crystals as statoliths. In order cellular systems (e.g. Paramecium caudatum) the whole cell body acts as a statolith. Sedimenting onto the lower membrane it activates mechano-sensitive elements (calcium- or potassium channels), which are unequally distributed in the membrane. A similar mechanism is predicted for Euglena gracilis and its close relative Astasia longa. The transduction of the sensor signal which results in the regulation of the flagellar or ciliar beat is still not fully understood. The review gives an overview on the currently discussed mechanisms of signal transduction.