Shedding light on micro-algae: phototaxis-induced collective phenomena

Motile micro-algae modify their environment by absorbing light, consuming and releasing chemical compounds and generating flows. Flows, light, and chemicals in turn influence their motion. These complex interactions can drive the formation of patterns at macroscopic scales. In this talk, I will explain how phototaxis, the ability to detect light and to respond by moving away from or towards it, can be harnessed to induce different kinds of collective behaviors in populations of Chlamydomonas reinhardtii. These emerging phenomena arise from different types of interactions: shading effects from light absorption by the cells leading to phase separation and branching patterns, hydrodynamic interactions from their self-generated flows giving rise to pearling or zig-zag instabilities in "active jets", and chemical interactions from the production and consumption of CO2 through photosynthesis and respiration producing self-sustained aster-like patterns. In each case I will try to make a clear connection between the macroscopic observations and what happens at the micro-scale.