Nanomechanical and structural characterizations of algal cells are of key importance for understanding their adhesion behavior at interfaces in the aquatic environment. We examine here the nanomechanical properties and adhesion dynamics of the marine, unicellular, green flagellate Dunaliella tertiolecta in laboratory culture. Mechanical properties of motile cells are hard to access while keeping cells viable, and studies to date are limited. Immobilization of negatively charged cells to a positively charged substrate enables high-resolution imaging and nanomechanical measurements. Cells in exponential phase possess a large cell volume as imaged by AFM, in agreement with the large amount of amperometrically measured displaced charge at the interface. Cells are stiffer and more hydrophobic in exponential than in the stationary phase. Differences in the critical interfacial tensions of adhesion of cells in exponential and stationary phase show the analogy with adhesion of hydrophobic droplets of organic liquids. Differences in the kinetics of adhesion and spreading of cells at the interface are attributed to their volume and nanomechanical properties that vary during cell aging. In the future, cell mechanical properties could be considered as a marker for environmental stress in order to better understand viability and adaptation strategies of algal populations in aquatic systems.

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.

Please cite this article as: Nadica Ivošević DeNardis, Jadranka Pečar Ilić, Ivica Ružić, Galja Pletikapić, Cell adhesion and spreading at a charged interface: Insight into the mechanism using surface techniques and mathematical modelling, Electrochimica Acta http://dx.(N. Ivošević DeNardis) 1 ISE member Highlights  Kinetics of adhesion and spreading of the algal cell at a charged interface is explored. Amperometric signals are analyzed using extended methodology and the reaction kinetics model.  The model reconstructs and quantifies individual states of the three-step adhesion process. Adhesion kinetics of the algal cell is slower than that of its plasma membrane vesicle. Slow spreading of organic film at the interface could be due to the attenuated effect of the potential. AbstractWe study the kinetics of adhesion and spreading of an algal cell and its plasma membrane vesicle at the charged interface. A simple system of an isolated plasma membrane vesicle without internal content has been developed and characterized by atomic force microscopy (AFM). We extend the methodology based on the reaction kinetics model and empirical fitting for the analysis of amperometric signals, and demonstrate its validity and pertinence in a wide range of surface charge densities. Adhesion kinetics of the algal cell is slower than that of its plasma membrane vesicle. Isolated plasma membrane contributes about one quarter to the cell contact area. The model reconstructs and quantifies individual states of the three-step adhesion process of the algal cell and makes it possible to associate them with various features of amperometric signal. At the time of current amplitude, the ruptured state predominates and the cell contact area is larger than its initial area as well as the contact area of the plasma membrane vesicle. These results suggest that a major structural disruption of the cell membrane, collapse of cytoskeleton and leakage of intracellular material could appear close to the time of current amplitude. Further, kinetics of the organic film spreading at the interface to its maximal extent is considered as the rate determining step, which could be a consequence of the attenuated effect of potential at the modified interface, stronger intermolecular interactions and reorganization of molecules in the film. Our findings offer an insight into the mechanism of algal cell adhesion and spreading at charged interfaces, relevant for electroporation based studies.

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