Single-cell and emergent collective phototaxis in the micro-alga Chlamydomonas reinhardtii – PHOTOFOCUS

Many motile photosynthetic micro-organisms have the ability to orient themselves in light fields, a property called “phototaxis”. This phenomenon, usually mediated by a specialised organelle called the eyespot (composed of photo-sensitive molecules), is still poorly understood. Modelling such process is difficult because we lack information on how the temporal light signal received at the eyespot as the cell rotates along its swimming axis is interpreted and converted to a modulation of the flagellar beating (to impose a torque on the cell body and reorient in the correct direction). Here we propose to investigate this still open question in the model micro-alga Chlamydomonas reinhardtii by quantifying single-cell trajectories in well-controlled illumination landscapes while imaging the location of the eyespot. This will allow correlating the cell motion with the inferred temporal signal received at the eyespot along the trajectory. In addition we will investigate phototaxis at the population level, where light-matter interaction (absorption) with the cell population feedbacks on the phototactic behavior of the cells and can lead to emergent phenomena, such as collective modes of phototaxis. By bringing further knowledge on single-cell and collective phototaxis we will aim to finely control the individual and collective phototactic transport of the cells under dynamic light patterns. This will constitute a proof of concept that light can be exploited in concrete applications, such as micro-algal biomass harvesting in industrial bioreactors, step that is the current energetic and financial bottleneck in the micro-algal industry (which aims to produce low-carbon footprint biofuels, food, drugs, etc). In addition, building on existing models of phototaxis will help to better describe phytoplanktonic population dynamics in natural environments where phototaxis plays a crucial role.