AFM for Studying Geometrical Constraints of Diatoms Silica Cell Wall

Unicellular organisms are known to exert tight control over their cell size. In the case of diatoms, abundant eukaryotic microalgae, two opposing notions are widely accepted. On the one hand, the rigid silica cell wall is thought to enforce geometrical reduction of the cell size by the need to fit any new silica element into the previously formed structure. On the other hand, numerous exceptions that include long-term culturing without noticeable size changes cast doubt on the generality of the geometrical size reduction theory.
To gain a deeper insight into the growth mechanism of the diatom rigid silica cell wall in various regions, namely Valve and Gridle band, we have employed the AFM technique to study their flexibility. To accurately calculate the shell wall Elastic modulus, it is necessary to take into account the shell geometry. As opposed to classical contact mechanics models (i.e Hertz model), where the deformation measured is solely the indentation of the tip into the material, hollow cylindrical shells can bend, buckle or collapse. By using a thin shell cylindrical model that takes into account the geometry of the shell we show that the primary factor contributing to the higher deformability of the girdle bands is their distinct geometry, characterized by a thinner shell wall. These results show that the mechanical properties of Stephanopyxis turris girdle bands are flexible enough to accommodate geometrical fluctuations that can override the deterministic prediction of the geometrical model.1

¹D. de Haan, N.-H. Ramos, Y.-F. Meng, R. Rotkopf, Y. Addadi, I. Rosenhek-Goldian and A. Gal, New Phytologist n/a. https://doi.org/10.1111/nph.19743