Phenotyping photokinetic and excitable behaviours of single microswimmers in confinement*

All living organisms are environmentally intelligent. This is the fundamental distinction between life, and other forms of matter. Even unicellular organisms are capable of complex behaviours. Here, we study the detailed motor actions of flagellated algal microswimmers, using motility as a dynamic read-out of whole-organism behaviour. Previous studies have focussed on locomotor transients over short timescales ranging from seconds to minutes. Here we present a novel microfluidic platform which can allow us to monitor single cells over unprecedented timescales. Two representative species of microswimmers were trapped and confined inside circular arenas: a biflagellate which exhibits a form of run-and-tumble, and an octoflagellate which exhibits a distinctive, tripartite behavioural repertoire termed run-stop-shock. Stochastic transitions in swimming gait are projected onto a low-dimensional behavioural state space. Single-cell motility signatures were analysed to reveal species-specific photokinetic and excitable behaviours. Finally, we conducted on-demand pharmacological perturbations within these microenvironments, to shed new light on the physiological basis of excitable flagellar dynamics.