Light-regulated swimming motility induces cell aggregation in confinement

A highly concentrated suspension of self-propelled particles can form large-scale concentration patterns, separating into regions of high and low particle concentrations. This phenomenon is induced by the activity of the particles and mediated by their mutual interactions. We observe that a suspension of freely swimming Chlamydomonas reinhardtii cells, a unicellular soil-dwelling microalgae and a model organism of puller-type microswimmers, forms such large-scale aggregations under confinement in specific light conditions. We find that the cell's motility changes under different light conditions. Through active Brownian particle simulations, we show that the change of the motility is sufficient to regulate cell aggregation. Finally, we show evidence that the extent of the photosynthetic activity controls the cell's motility, and consequentially, the aggregation formation.