||Microalgae have attracted substantial attention for their ability to produce various products utilizing carbon dioxide. Many efforts have been made to investigate the various biological functions using a model microalga, Chlamydomonas reinhardtii. Despite the importance, unlike other nutrients, the chemotaxigenesis of HCO3− is still unknown. In this study, we demonstrated the chemotaxis of C. reinhardtii towards HCO3− using a hydrogel-based convection-free microfluidic device. We observed the chemotaxis of C. reinhardtii towards HCO3− for the first time and found the concentration for the strongest response (~30mM). Furthermore, we verified several factors influencing on CCM(CO2 concentrating mechanism) caused significant change in the chemotactic behavior.