Dynamics of fluid mixing in suspensions of swimming Chlamydomonas

Kyriacos C. Leptos1, Jeffrey S. Guasto2, J.P. Gollub1,2, Adriana I. Pesci1, Severin Sasso3, Alison G. Smith3, and Raymond E. Goldstein1

1) Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK 2) Department of Physics, Haverford College, Haverford, PA 19041, USA 3) Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK

In aquatic organisms, the efficient uptake of food and nutrients for survival and growth depends strongly on the mechanisms of transport phenomena in the surrounding fluid, as exemplified by suspension feeding. In this study we attempt to extend the notion of suspension feeding, which is primarily associated with aquatic animals, to the phytoplankton level using Chlamydomonas reinhardtii as a model organism. In particular, we study passive tracers in suspensions of these swimming microalgae, and formulate a quantitative description of their dynamics, reflecting interplay between Brownian motion and advection by flows from the passing swimmers. We found that while the swimmers exhibit ballistic behaviour over short intervals of time, the tracers behave diffusively. They also exhibit a time-dependent but self-similar probability distribution function of displacements consisting of a Gaussian core and distinct exponential tails. We stress the role of flagellar beating in the creation of oscillatory flows which exceed Brownian motion far from each swimmer. Finally, we discuss the possibilities of applying and extending the above model to algal-bacterial interactions.

e-mail address of presenting author: kl344@cam.ac.uk