Light-Switchable Collective Behavior in Photoactive Microalgae

In their natural habitats, many biological microorganisms are exposed to variable light conditions throughout the day. For light-sensitive microbes, these can be of paramount importance since their biological functions change with the illumination conditions. Here we present that Chlamydomonas, a unicellular soil-dwelling microalgae, form regions of high and low cell density in the absence of light, while in a uniform illumination the active suspension becomes homogeneous. We find that the emerging pattern follows the shape of the compartment that encloses the motile cells by staying a typical distance away from the compartment wall. As result, we find that for circular confinement, the pattern transitions from a disk to a ring morphology depending on the compartment size and cell density. The effect is completely reversible and we explore the spatial characteristics of the patterns as well as the time scales associated to the emergence of this collective behaviour. Our results suggest that the mechanism of this light-switchable collective behaviour is unrelated to phototaxis but rather linked to cells' metabolic and/or chemotactic activity in certain light conditions.