Boundary conditions determine emergence and dynamics of density patterns in microbial suspensions

Motility is vital for many microorganisms to explore and identify favorable habitats and living conditions. Self-propelled microbes exhibit a range of unique phenomena, including the separation into phases of high and low cell densities. We showed that dense suspensions of motile Chlamydomonas reinhardtii cells aggregate in 2D confinement in response to a reduction of the light intensity. At low light conditions the cells photosynthetic activity is suppressed and a metabolic switch from photosynthesis to respiration creates an inhomogeneous oxygen field in the compartment. The oxygen concentration is directly linked to the motility of the cells, thus creating localized patterns of low and high density.
Here, we explore this phenomenon in 3D and study the emergence of a rich variety of patterns, including dense rings in cylindrical chambers. We also observe the formation of bioconvection patterns even for shallow suspensions with depths below the reported threshold value. This work investigates the interplay of pattern formation and bioconvection for different chamber geometries, light intensities and boundary conditions determining the oxygen concentration field within the compartments.