@article {1435923,
	title = {Microbial range expansions on liquid substrates},
	journal = {Physical Review X},
	volume = {9},
	number = {2},
	year = {2019},
	abstract = {
	Despite the importance of flow for transporting and organizing populations, few laboratory\&nbsp;systems exist to systematically investigate the impact of advection on their spatial evolutionary\&nbsp;dynamics. To address this problem, we study the morphology and genetic spatial structure of\&nbsp;microbial colonies growing on the surface of a nutrient-laden fluid 104 to 105 times more viscous\&nbsp;than water in Petri dishes; the extreme but finite viscosity inhibits undesired thermal convection and\&nbsp;allows populations to effectively live at the air-liquid interface due to capillary forces. We discover\&nbsp;that S. cerevisiae (baker{\textquoteright}s yeast) growing on a viscous liquid behave like {\textquotedblleft}active matter": they\&nbsp;metabolically generate fluid flows many times larger than their unperturbed colony expansion speed,\&nbsp;and that flow, in turn, can dramatically impact their colony morphology and spatial population\&nbsp;genetics. We show that yeast cells generate fluid flows by consuming surrounding nutrients and\&nbsp;decreasing the local substrate density, leading to misaligned fluid pressure and density contours\&nbsp;\&nbsp;which ultimately generates vorticity via a thresholdless baroclinic instability. Numerical simulations\&nbsp;with experimentally measured parameters demonstrate that an intense vortex ring is produced below\&nbsp;the colony{\textquoteright}s edge. As the viscosity of the substrate is lowered and the self-induced flow intensifies,\&nbsp;we observe three distinct morphologies: at the highest viscosity, cell proliferation and movement\&nbsp;produces compact circular colonies with, however, a stretched regime of exponential expansion;\&nbsp;intermediate viscosities give rise to compact colonies with {\textquotedblleft}fingers" that are usually monoclonal\&nbsp;and then break into smaller cell clusters; and at the lowest viscosity, the expanding colony fractures\&nbsp;into many genetically-diverse, mutually repelling, island-like fragments that can colonize an entire\&nbsp;94 mm-diameter Petri dish within 36 hours. We propose a simple phenomenological model that\&nbsp;predicts the early colony dynamics. Our results provide rich opportunities to study the interplay\&nbsp;between fluid flow and spatial population genetics for future investigations.

},
	url = {https://journals.aps.org/prx/abstract/10.1103/PhysRevX.9.021058},
	author = {Atis, Severine and Weinstein, Bryan T. and Andrew W. Murray and David R. Nelson}
}
