Controlling microalgae populations by phototactic memory

TL;DR

This study demonstrates that microalgae can be guided by structured light landscapes through a mechanism involving short-term memory of light exposure, revealing new insights into their phototactic navigation strategies.

Contribution

It uncovers the role of short-time memory and temporal integration in microalgae phototaxis, advancing understanding of their navigation in complex light environments.

Findings

Microalgae localize in structured light regions.

Short-term memory influences phototactic steering.

Population density decreases due to adaptation.

Abstract

Understanding how microorganisms navigate in complex environments is a central question in active matter and biological physics. Phototaxis - the ability to use light as a navigation cue - is a widespread strategy in motile microalgae to optimise photosynthesis and avoid light-induced stress. The microalga Chlamydomonas reinhardtii is a model system for studying this behaviour, where navigation is classically attributed to a photosensitive organelle named eyespot. While this mechanism enables cells to sense the direction of incoming light, their response to light intensity gradients remains less understood. Here we show that structured light landscapes can guide microalgae populations and localise them in defined spatial regions. By analysing single-cell trajectories, we find that cells actively steer relative to the local light gradient, and a comparison with a minimal theoretical model shows that a short-time memory of light exposure acting on the transition between positive and negative phototaxis is necessary to reproduce the observed accumulation. At longer times, we observe a gradual decrease in cell number density within the trapping region, consistent with phototactic adaptation. Beyond controlling population dynamics, our results reveal new aspects of phototactic behaviour, highlighting gradient-aligned steering together with temporal integration as central mechanisms for navigation in structured environments.