We discover that S. cerevisiae (baker’s yeast) growing on a viscous liquid behave like “active matter”: They metabolically generate fluid flows many times larger than their unperturbed colony expansion speed, and that flow, in turn, can dramatically impact the colony growth and morphology. We show that yeast cells generate fluid flows by consuming surrounding nutrients and decreasing the local substrate density. This leads to misaligned fluid pressure and density contours in the colony vicinity that ultimately generates convection. As the viscosity of the substrate is lowered and the self-induced flow intensifies, we observe three distinct morphologies: At the highest viscosity, cell proliferation and movement produces compact circular colonies; intermediate viscosities give rise to compact colonies with unusual “fingers” that break into smaller cell clusters; at the lowest viscosity, the expanding colony fractures into many mutually repelling fragments that can colonize an entire 94-mm-diameter Petri dish within 36 hours.
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