The swimming cell and its world: Structures and mechanisms of orientation in protists

The properties of motility and sensory organisation of unicellular organisms suggest that cells were able to utilise the resources of their fluid environment before they had developed senses for its exploration. Two principles of locomotion, helical swimming and abrupt responses of turning (tumbling, reversal) are used by both prokaryotes and protists to provide access to multiple sites in the biosphere. There is no indication that motility alone can establish any dimensionality to the world of cells. The primary sensorimotor mechanism in bacteria and protists, kinesis, enables them to locate nutrients etc. by their concentration gradients using the dimension of time. Increases in cell size of protists necessitated highly sensitive mechanoreceptors to overcome passive sedimentation due to gravity. The polar and/or gradient-type arrangement of membrane receptors, deformed through gravitational forces, allows the determination of the gravity vector and thereby establishes the most important dimension of the biosphere, its vertical axis. Responses are modulations of swimming speed and frequency of reversals (both kineses). Gravikinesis uses, presumably for the first time in evolution, spatial information without a need for temporal input. The rise of physiologically guided, directly orientating taxes in protists (e.g. phototaxis) is associated with spatial assessment of a vector-type-stimulus (radiant light) as well. Taxes exploit only a single dimension of space (bright/dark; up/down). The first indication of inclusion of a second spatial dimension comes from advanced ciliates (e.g. Paramecium), which are able to fully neutralise gravikinesis in the horizontal position. The world of protists is thereby, at best, a sheet spread between the vertical and horizontal axes.