Isabelle Eisenmann

Excess of light can be hazardous for photosynthetic organisms. When intensity is too high, the motile micro-algae Chlamydomonas reinhardtii reorients itself to swim away from the incident light. We recently discovered that a collection of a few millions such migrating cells is unstable, leading to the formation of a branched pattern which retracts within a few minutes into a very dense and dynamical “drop”. Here, dense areas create a shadow toward which neighboring algae tend to swim. This coupling between cell density and light fields can amplify initial density fluctuations and quickly trigger the complete phase separation of the suspension. A simple drift-diffusion model captures the destabilization of the system for critical control parameters and finely reproduces experimental data. On the physiological side we measured photodamage with Sandrine Bujaldon and Benjamin Bailleul (IBPC), showing that cells inside the dense phase are protected from the light stress. On short timescales, phototaxis thus efficiently contributes to photoprotection through non-trivial reponses at the population level.

Phototaxis in Chlamydomonas reinhardtii is poorly understood, yet it allows those motile photosynthetic cells to navigate a highly variable light environment. This new instability is very sensitive to subtle changes in the phototactic sensitivity of individual cells. Any modification will result in a dramatically different macroscopic pattern. We are currently using this setup to explore the interplay between phototaxis and other biological functions such as photosynthesis.

We currently investigate instabilities in active fluid jets, in collaboration with Eric Lauga (Cambridge). Phototaxis allows to precisely control millions of cells, at temporal and spatial resolutions much higher than other aligning stimuli such as gravitaxis or chemotaxis. Over the past few years, many groups have predicted instabilities in suspensions of aligned micro-swimmers. Here, we verify it experimentally thanks to the fine control of Chlamydomonas reinhardtii by light. We show two new kind of instabilities in an active cell jet : its breaking into droplets and its buckling into waves. Those patterns arise from hydrodynamic interactions between individual micro-swimmers as shown by simulations.

• Collective protoprotection through light-induced phase separation in a phototactic micro-algae (work in progress) – Eisenmann, Lhomme, Bujaldon, Bailleul, Le Saux, Lahlou, Desprat, Jeanneret
• Fluorescence to measure light intensity (submitted) – Lahlou, Tehrani, Coghill, Shpinov, Plamont, Aujard, Niu, Mahou, Suppato, Beaurepaire, Eisenmann, Desprat, Croquette, Jeanneret, Nedbal, Lazár, Le Saux, Jullien