Versatile phototactic behaviors of the chiral microswimmer Euglena gracilis

The various taxis strategies of microorganisms regarding external stimuli highlight interesting solutions to control-feedback tasks bounded by biophysical constraints. Here we study the versatile phototactic behaviors of the microswimmer Euglena gracilis. These cells scan the surrounding light by rolling around their long axes while swimming along helical paths, thereby pointing their single directional photoreceptor periodically into various directions. How this detected time-varying light signal is coupled to active reorientation of the cell to achieve various phototaxis tasks effectively is not fully understood. We develop a biophysical model that describes the change of the cell’s pitch axis orientation and strength in dependence of the light intensity, which leads to a unified account of various phototactic behavioral states of these cells. We experimentally test this model using different spatiotemporal light stimuli and show how a simple monotonous dependence of the pitch axis orientation on light intensity can naturally evoke the appropriate taxis behavior given lighting conditions. This work has implication for taxis strategies of other natural and synthetic chiral micro-swimmers.