  We study the effect of turbulence on marine life by performing numerical
simulations of motile microorganisms, modelled as prolate spheroids, in
isotropic homogeneous turbulence. We show that the clustering and patchiness
observed in laminar flows, linear shear and vortex flows, are significantly
reduced in a three-dimensional turbulent flow mainly because of the complex
topology; elongated micro-orgamisms show some level of clustering in the case
of swimmers without any preferential alignment whereas spherical swimmers
remain uniformly distributed. Micro-organisms with one preferential swimming
direction (e.g. gyrotaxis) still show significant clustering if spherical in
shape, whereas prolate swimmers remain more uniformly distributed. Due to their
large sensitivity to the local shear, these elongated swimmers react slower to
the action of vorticity and gravity and therefore do not have time to
accumulate in a turbulent flow. These results show how purely hydrodynamic
effects can alter the ecology of microorganisms that can vary their shape and
their preferential orientation.
