Ptychography of diatoms reveals chemically inhomogeneous biosilica cell walls

Diatoms are a large group of unicellular eukaryotic microalgae, renowned for their beautifully intricate biosilica cell walls (BCWs) and their abundance in sea and fresh water. BCWs exhibit the highest specific strength in all known biological materials and may serve as a powerful armor against parasites and grazers for the ecological success of diatoms. Diatom BCWs exhibit characteristic nanopatterns indicating that silica formation is genetically controlled by silica-forming proteins, presumably functional during silica deposition. Understanding the biomineralization of BCWs is of great interest in biological and ecological studies. Here, we studied the biosilica coordination and the morphology of diatom BCWs using the synchrotron-based x-ray spectromicroscopy (ptychography), with a 10nm spatial resolution. Our preliminary data showed that the BCWs of wild type Thalassiosira pseudonana have three different coordination environments. The girdle band of T. pseudonana has two layers of silica. The outer layer biosilica is less polymerized than the inner layer as the Si K-edge of the outer layer is 0.3 eV lower. The Si environments of valves are heterogeneous with varying degrees of silica polymerization and degrees of long-range ordering.