Collective Cycling Behavior and Bioconvection in Spirostomum ambiguum

Spirostomum ambiguum are among the largest free-living protists, reaching up to 4 mm in length and 150 µm in diameter. These unicellular, rod-like ciliates can contract up to 75% of their body length within 5 ms, generating rapid hydrodynamic disturbances in their surroundings. Beyond these extraordinary single-cell mechanics, populations of S. ambiguum exhibit striking emergent behaviors. Here, we investigate how dense collections of S. ambiguum self-organize within Hele–Shaw confinement and generate collective fluid-like motion. We observe the spontaneous formation of a continuous front of cells that ascends along the chamber walls, followed by the emergence of large-scale fluid circulation driven by falling clusters of cells in initially quiescent fluid, exhibiting an active flow pattern reminiscent of bioconvection. We quantify the onset conditions, velocity fields, and characteristic length scales of these emergent flows using high-speed videography, macrophotography and particle image velocimetry (PIV). Our results reveal a new regime of self-organized motion in a millimeter-scale active system, bridging unicellular biomechanics and multifield active hydrodynamics, wherein orientation, density, and flow fields are tightly coupled through reciprocal feedback.