Geotaxis in Motile Micro-Organisms

ABSTRACT It is suggested that the principal cause of geotaxis in many motile micro-organisms is a gravity-induced hydrodynamic torque, the size of which depends upon the shape of the organism. The nature of the torque has been investigated using small scale models. An expression for the distribution of organisms in long, vertical columns has been derived. The distribution of Paramecium in long, vertical columns has been determined, and it has been shown that the strength of the hydrodynamic interaction is sufficient to account for the negative geotaxis usually exhibited by this organism. The induction of positive geotaxis in certain circumstances has been considered. The survival value of negative geotaxis for free-swimming organisms in search of food may explain the characteristic shape of many protozoa.