Microalgae are living organisms that play a key role in the environment and can also be used for the sequestration of CO2. At LPENS we aim at understanding their collective motion as well as their individual behavior in response to light (phototaxis). We propose a M2 research internship and a PhD thesis aiming to probe the internal phototactic states dynamics of the model unicellular micro-alga Chlamydomonas reinhardtii. These algae can perform both positive and negative phototaxis (towards higher or lower light intensities resp.), but the decision-making process between these two states is still rather poorly understood, although essential for understanding the ecological dynamics of these globally important micro-organisms. Recent results in the group have established that this switch involves memory effects that can be used to localize cells in specific location of structured light landscapes and that we aim to quantitatively probe with the use of dynamical light patterns using a Spatial-Light-Modulator that allows to create any light pattern and to modulate them at relevant frequencies. In this project, the student will use video-microscopy, optics, image analysis, microalgal culturing techniques, microfluidic techniques and data analysis. The internship can be followed with a PhD (funded through a doctoral scholarship). During the PhD the student will also have the opportunity to work on collective behaviours induced by phototaxis in populations of these micro-algae.