Adaptation-induced oscillatory phototaxis and emergence of ordered density bands in the microswimmer Chlamydomonas reinhardtii

Biological microswimmers exhibit versatile taxis behaviors and switch between multiple behavioral states to navigate the environment and search for physiologically favorable regions. Here, we report a striking oscillatory phototaxis observed in Chlamydomonas reinhardtii, where cells swim back-and-forth under a constant, unidirectional light stimulus due to alternation between positive and negative phototaxis. This oscillatory phototaxis at the individual cellular level further leads to the emergence of a highly ordered, propagating band structure formed by high density Chlamydomonas cells collectively. We experimentally verify a unified phototaxis mechanism that couples light detection, light adaptation, flagella dynamics and cell reorientation, showing that transition between phototaxis modes is achieved by switching of flagella waveforms and modulation of flagella phase difference. Oscillatory phototaxis emerges as a semi-stable state in an overlapping light intensity regime for positive and negative phototaxis, where adaptation shifts the light intensity thresholds over times. This adaptation mechanism over multiple time scales enables phototactic microswimmers to effectively expand the survival range of light intensity and provide collective photoprotection for the colonies through the formation of dynamic band structures with high density.