  A suspension of gyrotactic microalgae Chlamydomonas augustae swimming in a
cylindrical water vessel in solid-body rotation is studied. Our experiments
show that swimming algae form an aggregate around the axis of rotation, whose
intensity increases with the rotation speed. We explain this phenomenon by the
centripetal orientation of the swimming direction towards the axis of rotation.
This centripetal focusing is contrasted by diffusive fluxes due to stochastic
reorientation of the cells. The competition of the two effects lead to a
stationary distribution, which we analytically derive from a refined
mathematical model of gyrotactic swimmers. The temporal evolution of the cell
distribution, obtained via numerical simulations of the stochastic model, is in
quantitative agreement with the experimental measurements in the range of
parameters explored.
