Oscillatory phototaxis of Chlamydomonas reinhardtii

Biological microswimmers typically switch between positive and negative taxis under favorable and unfavorable environmental stimuli, respectively. However, it remains largely unclear how the cell transition between taxis of opposite signs. Here, we focus on investigating the transition between positive and negative phototaxis of Chlamydomonas reinhardtii. In particular, we observe that Chlamydomonas cells exhibit oscillatory phototaxis at an intermediate light intensity, where the cells oscillate between positive and negative phototaxis which occur at low and high light intensity, respectively. By experimentally tracking the Chlamydomonas cells at different phototaxis modes, we find that oscillatory phototaxis is mediated by tuning the phase relationship between the eyespot and the cell orientation as well as their flagellar beats. We develop a hydrodynamic model to capture how the cell tunes the phase relationship and how they respond to light stimuli. This study will lead to a general understanding on how swimming microorganisms transition between their positive and negative taxis behaviors.