Mathematics

To optimize foraging in a viscous environment, aquatic microorganisms use chemotactic motility or generate feeding flows. Some form colonies, possibly improving prey uptake through hydrodynamic interactions. In this study, we investigate how a group of Stylonychia lemnae ciliates interacts with food patches. As cells aggregate around a patch, they generate a chaotic collective flow that redistributes food within the population. We measure and predict the flow’s properties, showing that it enlarges the food patch faster than individual swimming would, thereby accelerating food detection by starving cells. We interpret the behavior of the cluster as cooperative in terms of hydrodynamic interactions and emphasize the importance of accounting for the stochastic motions of individual cells that drive the chaotic flow. Aquatic microorganisms typically inhabit a heterogeneous resource landscape, composed of localized and transient patches. To effectively exploit these resources, they have evolved a wide range of feeding strategies that combine chemotactic motility with active feeding flows. However, there is a notable lack of experimental studies that examine how these active flows shape resource fields to optimize feeding. In particular, the suspected cooperative hydrodynamics provided by groups of cells remains largely unexplored due to the difficulties in visualizing these dynamic three-dimensional flows. Here, we experimentally investigate how Stylonychia lemnae ciliates form feeding clusters of independent cells around food patches. Individual feeding flows interact hydrodynamically to create a chaotic collective flow at the population scale. Using a combination of experimental and numerical techniques, we measure and predict the entire collective flow, enabling us to assess its remarkable mixing and dispersion properties. We show that the active spreading of the food patch accelerates its detection by starving cells. As many fitness advantages provided by collective flows can be envisioned, we propose that this feeding cluster represents a form of intraspecific by-product cooperative behavior.