  The motility of microorganisms is often biased by gradients in physical and
chemical properties of their environment, with myriad implications on their
ecology. Here we show that fluid acceleration reorients gyrotactic plankton,
triggering small-scale clustering. We experimentally demonstrate this
phenomenon by studying the distribution of the phytoplankton Chlamydomonas
augustae within a rotating tank and find it to be in good agreement with a new,
generalized model of gyrotaxis. When this model is implemented in a direct
numerical simulation of turbulent flow, we find that fluid acceleration
generates multi-fractal plankton clustering, with faster and more stable cells
producing stronger clustering. By producing accumulations in high-vorticity
regions, this process is fundamen- tally different from clustering by
gravitational acceleration, expanding the range of mechanisms by which
turbulent flows can impact the spatial distribution of active suspensions.
