  As for many motile micro-algae, the freshwater species Chlamydomonas
reinhardtii can detect light sources and adapt its motile behavior in response.
Here, we show that suspensions of photophobic cells can be unstable to density
fluctuations, as a consequence of shading interactions mediated by light
absorption. In a circular illumination geometry this mechanism leads to the
complete phase separation of the system into transient branching patterns,
providing the first experimental evidence of finite wavelength selection in an
active phase-separating system without birth and death processes. The finite
wavelength selection, that can be captured in a simple drift-diffusion
framework, is a consequence of a vision-based interaction length scale set by
the illumination geometry and depends on global cell density, light intensity
and medium viscosity. Finally we show that this active phase separation shields
individual cells from the deleterious effects of high light intensity,
demonstrating that phototaxis can efficiently contribute to photoprotection
through collective behaviors on short timescales.
