Phase relationships in phototaxis of microswimmers

Swimming microorganisms have developed versatile strategies to sense and adapt to their living environments. The phototactic microswimmer Chlamydomonas reinhardtii swims in helical trajectories to exhibit directional phototaxis. However, it is not well understood that how Chlamydomonas or phototactic microswimmers in general, switch between positive and negative phototaxis. By experimentally tracking the eyespot and orientation of Chlamydomonas cells during their helical swimming, we show that the frequencies of body rotation and helical swimming are coupled and phase-locked. Our results demonstrate that positive and negative phototaxis of Chlamydomonas displays distinct phase relationships between eyespot and reorientation. They also suggest that the phase relationship may be related to the swimmer's orientation relative to the light source during phototaxis. This study will provide insight about the switching of phototaxis in microswimmers, informing the design and control of light-guided microswimmers.