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 the model microswimmer Chlamydomonas reinhardtii. These emerging phenomena arise from different types of interactions between the cells, whether from hydrodynamic origin or from phototactic cell behavior. These self-organizing phenomena therefore question the way single cells navigate complex light fields, to which I will also try to bring insights through dedicated model experiments.