Some marine bacterial species form mucosal layers, called veils, on sulfidic marine sediment. The bacteria attached to this veil actively swim and exert force on the surrounding fluid. The bacteria can break free of the veil and swim, chemotacting back to the veil. Over time the veil forms holes arranged in a hexagonal pattern. Motivated by this system, we present a simplified model to describe pattern formation induced by force-generating bodies embedded in a layer surrounded by fluid. When the bacteria break free of the layer, they are advected by the flow and diffuse. Competition between the fluid flow generated by the embedded bacteria and diffusion of the swimmers leads to a novel instability that drives bacterial aggregation. Analytic and numeric analysis of this system correctly defines the length scale and developmental timescale for the biological system. Similar flow dynamics may also play a role in other biological systems such as encrusting bryozoan colonies.
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