Nanostructure and nanomechanics of live Phaeodactylum tricornutum morphotypes

The ultrastructure and mechanical properties of the fusiform, triradiate and ovoid morphotypes of Phaeodactylum tricornutum were investigated using atomic force microscopy. Using topographic imaging, we showed that the surface of the ovoid form is rougher than those of the two other specimens, and coated with an outer layer of extracellular polymers. Using spatially resolved force-indentation curves, we found that the valve of the ovoid form is about five times stiffer (Young modulus of approximately 500 kPa) than those of the other forms (approximately 100 kPa), a finding fully consistent with the fact that only the ovoid form has a silica valve, whereas the valves in the other two consist mostly of organic material. Notably, the girdle region of both fusiform and ovoid forms was five times softer than the valve, suggesting that this region is poor in silica and enriched in organic material. For the triradiate form, we showed the arms to be softer than the core region, presumably as a result of organelle localization. Last, we observed mucilaginous footprints of moderate stiffness (approximately 100 kPa) in the vicinity of ovoid diatoms, which we believe are secreted extracellular polymers.