Changes in nanomechanical properties and adhesion dynamics of algal cells during their growth.

Nanomechanical and structural characterisations of algal cells are of key importance for understanding their adhesion behaviour at interfaces in the aquatic environment. We examine here the nanomechanical properties and adhesion dynamics of the algal cells during two phases of their growth using complementary surface methods and the mathematical modelling. Mechanical properties of motile cells are hard to assess while keeping cells viable, and studies to date have been limited. Immobilisation of negatively charged cells to a positively charged substrate enables high-resolution AFM imaging and nanomechanical measurements. Cells were stiffer and more hydrophobic in the exponential than in the stationary phase, suggesting molecular modification of the cell envelope during aging. The corresponding properties of algal cells were in agreement with the increase of critical interfacial tensions of adhesion, determined amperometrically. Cells in exponential phase possessed a larger cell volume, in agreement with the large amount of amperometrically measured displaced charge at the interface. Differences in the kinetics of adhesion and spreading of cells at the interface were attributed to their various volumes and nanomechanical properties that varied during cell aging. Our findings contribute to the present body of knowledge on the biophysics of algal cells on a fundamental level.

Algal cell response to laboratory-induced cadmium stress: a multimethod approach.

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.

Reaction kinetics and mechanical models of liposome adhesion at charged interface.

Dynamics of adhesion of single liposome at the charged mercury interface is analyzed through its amperometric signal using a reaction kinetics model and a mechanical model. We present analytical solutions of the reaction kinetics model for decoupling and identifying temporal evolution of three distinct states: i) the initial state corresponding to an intact liposome, ii) the intermediate state where the liposome is partly deformed, and iii) the final state of a lipid monolayer. The results obtained with this model indicate that all three states simultaneously evolve from the onset of the adhesion process. The new mechanical model provides a physical interpretation of the three states and emphasizes the role of the forces involved in liposome adhesion process. The main conclusion is that the water content of the liposome is released through the pores formed in the membrane rather than through the channels parallel to the electrode. Both models reproduce the measurements well in the wide potential range and offer a complementary insight into the dynamics of single adhesion event, which can find application in studies of cell adhesion and in drug delivery.