Intracellular morphogenesis of diatom silica is guided by local variations in membrane curvature

Silica cell wall formation in diatoms is a showcase for the ability of organisms to control inorganic mineralization. The process of silicification by these unicellular algae is tightly regulated within a membrane-bound organelle, the silica deposition vesicle (SDV). Two opposing scenarios were proposed to explain the tight regulation of this intracellular process: a template-mediated process that relies on preformed scaffolds or a template-independent self-assembly process. The present work points to a third scenario, where the SDV membrane is a dynamic mold that shapes the forming silica. We use in-cell cryo-electron tomography to visualize the silicification process in situ, in its native state, and with a nanometer-scale resolution. Significantly, this revealed that the plasma membrane interacts with the SDV membrane via physical tethering at membrane contact sites, where the curvature of the tethered side of the SDV membrane mirrors the intricate silica topography. We propose that silica growth and morphogenesis result from crosstalk between membranes and their biophysical properties.

Methods
Thalassiosira pseudonana cells were cryo-preserved at various stages during cell wall formation and cryo-electron tomography data sets of cryo-FIB milled thin lamellae were collected.