Phototactic Decision-Making by Micro-Algae

TL;DR

This study investigates how the unicellular alga Chlamydomonas reinhardtii makes phototactic decisions when exposed to competing light stimuli, revealing a geometrical averaging law and population-level behaviors.

Contribution

It uncovers a novel geometrical law governing phototactic responses and links decision-making to photoreceptor distribution and symmetry breaking in micro-algae.

Findings

Cells follow an intensity-weighted average of light directions.

Subpopulations exhibit different swimming behaviors at large angles.

Photoreceptor distribution influences decision-making and symmetry breaking.

Abstract

We study how simple eukaryotic organisms make decisions in response to competing stimuli in the context of phototaxis by the unicellular alga $Chlamydomonas~reinhardtii$. While negatively phototactic cells swim directly away from a collimated light beam, when presented with two beams of adjustable intersection angle and intensities, we find that cells swim in a direction given by an intensity-weighted average of the two light propagation vectors. This geometrical law is a fixed point of an adaptive model of phototaxis and minimizes the average light intensity falling on the anterior pole of the cell. At large angular separations, subpopulations of cells swim away from one source or the other, or along the direction of the geometrical law, with some cells stochastically switching between the three directions. This behavior is shown to arise from a population-level distribution of photoreceptor locations that breaks front-back symmetry of photoreception.