Multisensory navigation enables upstream swimming in Chlamydomonas reinhardtii

Chlamydomonas is a model eukaryotic alga widely used to study how microswimmers sense and respond to external cues in fluid environments. It exhibits phototaxis, an active steering behavior in response to light, as well as rheotaxis, hydrodynamic alignment in flow. However, it remains unknown how these navigation mechanisms interact with each other. Here, combining experiments and theory, we discover that Chlamydomonas is enabled to swim upstream through the synergy of phototaxis and rheotaxis. Utilizing microfluidic channels that enable the control of flow rate and illumination, we track individual cells under combined light and flow stimuli. By systematically varying both environmental cues, we identify distinct navigation modes and demonstrate the transition between light and flow dominated navigation regimes. These findings provide new insights into multisensory navigation in microswimmers and can inspire control strategies for microrobotics in microfluidic, ecological, and biomedical contexts.