# Re-enterant efficiency of phototaxis in Chlamydomonas reinhardtii cells

## TL;DR

This study investigates how cell density influences phototactic efficiency in Chlamydomonas reinhardtii, revealing a density-dependent enhancement due to slowed cell movement that improves light sensing.

## Contribution

It uncovers a re-entrant efficiency phenomenon in phototaxis, demonstrating collective effects and modeling the response as a density-dependent torque on active particles.

## Key findings

- Phototactic efficiency has a minimum at a specific cell density.
- Above this density, collective behavior enhances phototaxis performance.
- Slowing down of cells at higher densities improves light sensing.

## Abstract

Phototaxis is one of the most fundamental stimulus-response behaviors in biology wherein motile micro-organisms sense light gradients to swim towards the light source. Apart from single cell survival and growth, it plays a major role at the global scale of aquatic ecosystem and bio-reactors. We study photoaxis of single celled algae Chalmydomonas reinhardtii as a function of cell number density and light stimulus using high spatio-temporal video microscopy. Surprisingly, the phototactic efficiency has a minimum at a well-defined number density, for a given light gradient, above which the phototaxis behaviour of collection of cells can even exceed the performance obtainable from single isolated cells. We show that the origin of enhancement of performance above the critical concentration lies in the slowing down of the cells which enables them to sense light more effectively. We also show that this steady state phenomenology is well captured by a modelling the phototactic response as a density dependent torque acting on an active Brownian particle.