Phytoplankton Tune Local pH to Actively Modulate Circadian Gravitactic Behavior

Diel vertical migration (DVM) is the daily movement of motile phytoplankton between light‐rich surface waters and deeper nutrient‐rich layers, typically governed by internal clocks. However, many species show irregular patterns that deviate from expected circadian rhythms. Studying Heterosigma akashiwo, a bloom‐forming phytoplankton, we found that cells regulate their vertical movement by modulating local pH, affecting their gravitactic behavior. This self‐regulation creates sub‐populations that are physiologically similar but differ in behavior, remaining vertically separated even in uniform environments. These sub‐populations had similar swimming speeds, growth, and photosynthetic activity, suggesting stable co‐existence rather than environmental differences. Remarkably, vertical separation reappeared when each group was exposed to the other's spent media—an effect not seen with their own. Modeling and imaging showed that these chemical cues subtly alter cell shape, influencing gravitactic stability. Further experiments confirmed that pH shifts, consistent with those in the spent media, could replicate these behavioral changes. Together with nighttime data, results support a circadian model where diurnal pH regulation drives gravitactic divergence. This chemically mediated migration may enhance ecological fitness by promoting division of labor across the day‐night cycle and could refine models of phytoplankton behavior, circadian, diel vertical migration, gravitaxis, microswimmers, modelling, pH particularly in the context of ocean acidification.