  Large ensembles of interacting, out-of-equilibrium agents are a paradigm of
active matter. Their constituents' intrinsic activity may entail the
spontaneous separation into localized phases of high and low densities. Motile
microbes, equipped with ATP-fueled engines, are prime examples of such
phase-separating active matter, which is fundamental in myriad biological
processes. The fact that spontaneous spatial aggregation is not widely
recognized as a general feature of microbial communities challenges the
generalisation of phase separation beyond artificial active systems. Here, we
report on the phase separation of populations of Chlamydomonas reinhardtii that
can be controlled by light in a fully reversible manner. We trace this
phenomenon back to the light- and density-dependent motility, thus bridging the
gap from light perception on the single-cell level to collective spatial
self-organization into regions of high and low density. Its spectral
sensitivity suggests that microbial motility and phase separation are regulated
by the activity of the photosynthetic machinery. Characteristic fingerprints of
the stability and dynamics of this active system paint a picture that cannot be
reconciled with the current physical understanding of phase separation in
artificial active matter, whereby collective behavior can emerge from inherent
motility modulation in response to changing stimuli. Our results therefore
point towards the existence of a broader class of self-organization phenomena
in living systems.
