Laboratory-evolved multicellular organisms overcome diffusion limit by metabolically-driven flow

Large size can confer many benefits to multicellular organisms. However, transport of nutrients via diffusion is limited to relatively small length scales, leaving cells starving in the interior of large groups. While this obstacle can be overcome by complex circulatory systems, it is unclear what evolves first: large size or the ability to assemble a complex circulatory systems. Nevertheless, a lab-evolved multicellular organism, snowflake yeast, can sustain exponential growth to millimetric size, far past the diffusion limit, without a specialized circulatory system. We found that their metabolism induces a buoyancy-driven flow, which transports nutrients farther than diffusion alone. However, it is not clear if buoyancy-mediated flows occur under very specific conditions or for a broad range of parameters. 

We simulate fluid dynamics coupled with nutrient uptake, enabling us to determine how flow speed depends on snowflake yeast material properties and nutrient uptake rate. Group size is then analyzed to identify the range of parameters necessary for exponential growth to large size. The results will help revealing if bouyancy-driven flow is exceptional, or if it could potentially be observed in other simple multicellular organisms.