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 three types of interactions: shading effects from light absorption by the cells, hydrodynamic interactions from their self-generated flows, and chemical interactions from the production and consumption of CO₂ through photosynthesis and respiration. In each case I will aim to make a clear connection between the macroscopic observations and what happens at the micro-scale. When possible I will also try to bring insights on the biological relevance of the phenomenon.