This study examines how salinity reduction triggers the response of three marine microalgae at the molecular and unicellular levels in terms of chemical, mechanical, and behavioral changes. At the lowest salinity, all microalgal species exhibited an increase in membrane sterols and behaved stiffer. The glycocalyx-coated species Dunaliella tertiolecta was surrounded by a thick actin layer and showed the highest physiological activity, negatively affecting cell motility and indicating the formation of the palmella stage. The lipid content of membrane and the hydrophobicity of cell were largely preserved over a wide range of salinity, confirming the euryhaline nature of Dunaliella. The species with calcite-encrusted theca Tetraselmis suecica exhibited the highest hydrophobicity at the lowest salinity of all cells examined. At salinity of 19, the cells of T. suecica showed the lowest growth, flagellar detachment and the lowest cell speed, the highest physiological activity associated with a dense network of extracellular polymeric substances, and a decrease in membrane lipids, which could indicate develepment of cyst stage. The organosilicate encrusted species Cylindrotheca closterium appeared to be salinity tolerant. It behaved hydrophobically at lower salinity, whereas becoming hydrophilic at higher salinity, which might be related to a molecular change in the released biopolymers. This study highlighted the interplay between chemistry and mechanics that determines functional cell behavior and shows that cell surface properties and behavior could serve as stress markers for marine biota under climate change. Graphical Abstract

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