Trajectory analysis of single microswimmer behaviour*

Movement trajectories can be an informative measure of the behaviour and physiology of biological active matter at all scales. Even single cells can display complex motility patterns in order to navigate their surroundings and respond to environmental cues. Recent advances in tracking and analysis frameworks enable behavioural characteristics to be revealed from microscopy data. Here, we combine droplet microfluidics, high-speed imaging and computational analysis to achieve unprecedented long-term monitoring of individual cells of two algal species that display highly distinct behavioural signatures (namely the ‘run-and-tumble' like motility of the freshwater biflagellate Chlamydomonas reinhardtii and the ‘run-stop-shock' motion of the marine octoflagellate Pyramimonas octopus). We study the distributions of single-cell swimming speed, motility state transitions, and perform probability flux analysis to reveal the emergence of steady-state flux cycles in confined microswimmers. We propose novel motility measures to deduce how swimming behaviour changes and adapts in response to increasing physical confinement, light stimulation and sudden chemical perturbations. Our results emphasize the need for recording long-time statistics and trajectories for revealing non-equilibrium and non-stationary features in organismal behaviour.