The dynamics of unicellular swimming organisms

Swimming microorganisms, such as motile algal cells, are oriented passively by gravity and also by velocity gradients of the fluid which surrounds them. They may actively orient themselves as well, in response to sensory signals such as illumination. These axial alignments are degraded by molecular collisions which generate rotational Brownian motion and by endogenously generated random turning behavior. The geometry of the track traced out by an individual cell's swimming is governed by the joint competitive action of these orienting influences. These determinants of the cell trajectories are considered. It is also shown that, although gravity may not directly interact with cell metabolism, it can affect the energy consumption accompanying taxis, by suppressing pathway tortuosity and by supplying an orientational bias. This paper also reviews the role of gravity in organizing patterns--collective modes of the fluid and of the suspended swimmers which energize the dynamics.