pH induced motility pattern change in a marine dinoflagellate

Flagella-driven motility is a conserved feature across eukaryotic lineages, from unicellular plankton to mammals. In marine dinoflagellates, such as Lingulodinium polyedra , motility underlies diel vertical migration (DVM), a key adaptive strategy that enables access to spatio-temporally segregated resources in the water column. To investigate how pH influences motility, we used L. polyedra and two other dinoflagellates as a model and used a multi-particle tracking algorithm to monitor and quantitatively analyze cellular motility. Under normal pH conditions, L. polyedra displays linear, random motility with variable speeds. Upon CO _² -induced acidification, we observed a dose-dependent decrease in motility speed accompanied by a striking behavioral shift: within minutes of pH reduction, over 90% of the cells transitioned to spiral motility. This effect was both reversible and reproducible, including when pH was modified chemically rather than via CO _² . The rapid onset of these changes suggests a non-genomic mode of regulation, an area that remains largely unexplored in phytoplankton. We hypothesize that external pH modulates flagellar dynamics in dinoflagellates. Our findings offer new insights into the link between environmental pH and flagellar motility and provide a platform for investigating non-genomic responses to ocean acidification in marine phytoplankton.