EPJ E Topical Review: Gyrotactic phytoplankton in laminar and turbulent flows: A dynamical systems approach

Details

Published on 20 March 2019

Biological and geophysical fluids host a sea of microorganisms many of which are motile. An often overlooked aspect of the life of such microorganisms is that the fluids where they are suspended are not still but flowing. In this brief review published in EPJ E, the authors aim to describe some of the interesting phenomena that can emerge due to the modification of the microorganisms' swimming direction by velocity gradients, which affect both the individual motion of microorganisms and their spatial distribution in dilute suspensions.

More specifically, the review focuses on the case study of gyrotactic phytoplankton; bottom heavy, motile cells whose swimming direction is determined by a balance between a buoyancy torque directing them upwards and fluid velocity gradients. Gyrotaxis has become a paradigmatic model for phytoplankton motility in flows, due to the availability of a mechanistic description which has revealed rich and predictive feed-back on the fluid and stochasticity (e.g. in cell orientation).

In this review, the authors consider recent theoretical, numerical and experimental results to show how gyrotaxis can produce inhomogeneous phytoplankton distributions on a wide range of scales, from millimeters to kilometers, in both laminar and turbulent flows. In particular, they focus on the phenomenon of gyrotactic trapping in nonlinear shear flows and in fractal clustering in turbulent flows, demonstrating the usefulness of ideas and tools borrowed from dynamical systems theory.