Light-regulated cell aggregation in confinement

Photoactive microbes live in complex environments with spatially and temporally fluctuating light conditions. They have adapted to such habitats by switching their metabolic activity from photosynthesis to aerobic respiration in unfavorable light conditions. We demonstrate that under confinement this adaptation in a suspension of soil-dwelling Chlamydomonas reinhardtii cells leads to a spontaneous separation into regions of high and low cell densities. We show that the inhibition of the photosynthetic machinery is necessary but insufficient to generate the observed aggregation. Microfluidic experiments, simulations, and mean-field theory approaches demonstrate that the emergence of microbial aggregations is governed by the oxygen concentration field inside the microhabitat. In fact, in regions where the energy production is completely arrested by both, the photosynthetic and respiratory systems, the cell speed decreases resulting in an aggregation, which thus takes the form of the underline oxygen field.