Accumulation of motile elongated micro-organisms in turbulence

Abstract

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.

Collision rates of small ellipsoids settling in turbulence

“…Apparently, the tendency to cluster is less strong, indicating the variation in response to changes in the flow field due to the importance of the orientation. Very recently, a similar reduction of clustering has been reported for motile prolate-shaped micro-organisms (Zhan et al 2014). Although these organisms are not heavier than the fluid and do not settle due to gravity, they nevertheless cross the streamlines of the fluid by swimming.…”

Growth and diel vertical migration patterns of the toxic dinoflagellate Protoceratium reticulatum in a water column with salinity stratification: the role of bioconvection and light

“…A prerequisite in this context is that the swimming cells are bottom heavy (like P. reticulatum) and thereby are able to orient themselves by a balance between viscous and gravitational torque (Pedley & Kessler 1992, Karimi & Ardekani 2013, Zhan et al 2014. The viscous torque is due to the vertical shear force formed between the dense falling plume and the upwards swimming cells.…”

“…Bioconvection and fluid mechanics theory have been applied to explain such phenomena (Pedley & Kessler 1992, Vincent & Hill 1996, Ghorai & Hill 2005, 2008, Karimi & Ardekani 2013, Zhan et al 2014, and gyrotaxis is considered to be the main regulatory mechanism for upwardsswimming cells (Kessler 1985, Mitchell et al 1990. Bioconvection therefore describes a type of convection set up by density instability at the surface due to dense cell aggregations formed by upwardsswimming microalgae.…”

Anisotropic particle in viscous shear flow : navier slip, reciprocal symmetry, and jeffery orbit

“…For elongated microswimmers, their flow-driven motion always involves the Jeffery orbit [42]. Recent studies have revealed how their aggregations are affected by different flow types and their motilities in response to possible external stimuli [43][44][45][46], i.e., gyrotaxis, chemotaxis, phototaxis, and so on. All these studies demonstrate that the Jeffery orbit dynamics is a fundamental aspect of the motion of anisotropic objects in viscous flows.…”