Biophysics of Collective Phototaxis of Euglena gracilis

Abstract

High-density suspensions of actively swimming cells in fluids often self-organize into complex dynamic patterns due to the interplay between microscopic force generation and macroscopic hydrodynamics. Though various biophysical models have been developed to describe the dynamics of these systems, experimental verification has been difficult to obtain. This is due to the complex interaction rules which govern the swimmer behavior, and the lack of tools for delivering controlled perturbations to a defined subset of cells inside such a suspension. Here, we consider concentrated suspensions of the unicellular, photoresponsive algae, Euglena gracilis. We propose an integrated experimental and theoretical platform for predicting the collective behavior of Euglena under light stimulation. By varying the spatiotemporal light patterns, we are able to finely adjust cell densities and achieve arbitrary non-homogeneous distributions, including compression into high-density aggregates of varying geometries. Our models were able to account for the light-induced aggregation that result from a complex interplay between negative phototaxis, hydrodynamic interactions, and shading effects between the cells. This work will deepen our general understanding of how relatively simple mechanical feedback interactions can generate multi-cell behaviors in microswimmer suspensions, and how to bridge such understanding over different length and time scales. Furthermore, it may also aid the engineering of microfluidic devices and advance concepts in active matter physics.