Light-induced circular swimming of Chlamydomonas reinhardtii

Curvature modulation is a crucial strategy for swimming microorganisms responding to external stimuli, which can be caused by hydrodynamic interactions due to wall boundaries or periodic environmental stimulation. However, it remains largely unclear whether microorganisms can actively tune their trajectory curvature and adapt their swimming strategies in response to environmental changes. Here, we investigate how swimming trajectory of Chlamydomonas reinhardtii is influenced by light perpendicular to its motion. Chlamydomonas cells tend to swim in counterclockwise circular trajectories at low light intensities and clockwise circular trajectories at high light intensities. High-speed imaging reveals that this handedness switching results from the asymmetry between the cells' two flagella in response to varying light conditions. This swimming strategy enables the cells to tune their exploration region in response to light intensity change. Our findings provide new insights into locomotion strategies of swimming microorganisms.