Data for: Hexagonal patterns in diatom silica form via a directional two-step process

Organisms are able to control material patterning down to the nanometer scale. This is exemplified by the intricate geometrical patterns of the silica cell wall of diatoms, a group of unicellular algae. Theoretical and modelling studies proposed putative physical and chemical mechanisms to explain morphogenesis of diatom silica. Nevertheless, direct investigations of the underlying formation process are challenging because this process occurs within the confines of the living cell. Here, we develop a method for in situ 3D visualization of silica development in the diatom Stephanopyxis turris, using electron microscopy slice-and-view techniques. We document the formation of an isotropic hexagonal pattern made of nanoscale pores. Surprisingly, our data reveal a directional process that starts with elongation of silica rods along one of the three equivalent orientations of the hexagonal lattice. Only as a secondary step, these rods are connected by crisscrossing bridges that give rise to the complete hexagonal pattern. These in situ observations combine two known properties of diatom silica, close packing of pores and branching of rods, to a unified process that yields isotropic patterns from an anisotropic background. This calls for future research into diatom morphogenesis to focus on the elongating and branching rods as the key for pattern formation.