Interaction of the mechanosensitive microswimmer Paramecium with obstacles

In this work, we report investigations of the swimming behavior of Paramecium tetraurelia, a unicellular microorganism, in micro-engineered pools that are decorated with thousands of cylindrical pillars. Two types of contact interactions are measured, either passive scattering of Paramecium along the obstacle or avoiding reactions, characterized by an initial backward swimming upon contact, followed by a reorientation before resuming forward motion. We find that avoiding reactions are only mechanically triggered about 10% of the time. In addition, we observe that only a third of all avoiding reactions triggered by contact are instantaneous while two thirds are delayed by about 150 ms. These measurements are consistent with a simple electrophysiological model of mechanotransduction composed of a strong transient current followed by a persistent one upon prolonged contact. This is in apparent contrast with previous electrophysiological measurements where immobilized cells were stimulated with thin probes, which showed instantaneous behavioral responses and no persistent current. Our findings highlight the importance of ecologically relevant approaches to unravel the motility of mechanosensitive microorganisms in complex environments.