We examined the response of algal cells to laboratory-induced cadmium stress in terms of physiological activity, autonomous features (motility and fluorescence), adhesion dynamics, nanomechanical properties and protein expression by employing a multimethod approach. We develop a methodology based on the generalized mathematical model to predict free cadmium concentrations in culture. We used algal cells of Dunaliella tertiolecta, which are widespread in marine and freshwater systems, as a model organism. Cell adaptation to cadmium stress is manifested through cell shape deterioration, slower motility and an increase of physiological activity. No significant change in growth dynamics showed how cells adapt to stress by increasing active surface area against toxic cadmium in the culture. It was accompanied by an increase in green fluorescence (most likely associated with cadmium vesicular transport and/or beta carotene production), while no change was observed in the red endogenous fluorescence (associated with chlorophyll). To maintain the same rate of chlorophyll emission, the cell adaptation response was manifested through increased expression of the identified chlorophyll-binding protein(s) that are important for photosynthesis.Since production of these proteins represents cell defence mechanisms, they may also signal the presence of toxic metal in seawater. Protein expression affects the cell surface properties and therefore the dynamics of the adhesion process. Cells behave stiffer under stress with cadmium and thus the initial attachment and deformation are slower. Physicochemical and structural characterizations of algal cell surfaces are of key importance to interpret, rationalize and predict the behaviour and fate of the cell under stress in vivo.

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

Microalgae are considered an accurate indicator of ecosystem perturbations induced by global climate change. The present work aims to investigate the alteration of temperature on surface properties and behaviour of three algal species using the complementary surface methods (electrochemical and atomic force microscopy). The results showed that the temperature-induced response of algae is species-specific due to the structural features of the cell envelope. Wall-less algae experience the largest nanomechanical and chemical change, while algae with silicified walls show the pronounced chemical change in the degree of hydrophobicity. Alterations of surface properties suggest a molecular modification of the algal barrier and cytoskeletal rearrangements due to a change in cell size, while algal morphology reveals no change. The physiological activity of cells showed a different organisation of released extracellular substances in the form of fine fibrillar structures, aggregated particles, and dense networks. Both types of algal responses, physiological activity, and molecular modification of the cell barrier determine the cell adhesion and motility. This study highlights the role of surface properties in cell-substrate and cell–cell interactions, which is important for the understanding of algal behaviour at natural interfaces and the mechanism of algal biofilm and aggregate formation in aquatic systems under the stress. Graphical abstract

The present work aims to develop a fast and reliable procedure for motility analysis of a short-term effect of heavy metal cadmium on the algal cell response in laboratory conditions. Three unicellular motile species similar in cell length, while differing in the cell wall and the flagellar system are used as model algae. We quantitatively characterise motility in terms of swimming speed and search radius following addition of 1 mg Cd L −1 . Both swimming speed and search radius determined in control algal cultures reflect morphological features of the corresponding flagellated system. After 1 h of cell exposure to a toxic concentration of cadmium, a statistically significant decrease in swimming speed is determined with predominant erratic cell movement on the spot in all examined cultures. After 3 h of cell exposure to cadmium, swimming speed in most of the examined cell cultures recovered close to the control value, indicating quick cell adaptation to elevated cadmium concentration. The results support the implementation of swimming speed and search radius as motility parameters for direct screening of cell physiological state, which is applicable to ecotoxicological studies providing insight into the mechanism of cell adaptation under stress, as well as a better understanding of the spatial distribution of algal cells in aquatic systems.