Nanoscale control of silica morphology and three-dimensional structure during diatom cell wall formation

Abstract

We present a unique approach combining biological manipulation with advanced imaging tools to examine silica cell wall synthesis in the diatom Thalassiosira pseudonana. The innate capabilities of diatoms to form complex 3D silica structures on the nano- to micro-scale exceed current synthetic approaches because they use a fundamentally different formation process. Understanding the molecular details of the process requires identifying structural intermediates and correlating their formation with genes and proteins involved. This will aid in development of approaches to controllably alter structure, facilitating the use of diatoms as a direct source of nanostructured materials. In T. pseudonana, distinct silica morphologies were observed during formation of different cell wall substructures, and three different scales of structural organization were identified. At all levels, structure formation correlated with optimal design properties for the final product. These results provide a benchmark of measurements and new insights into biosilicification processes, potentially also benefiting biomimetic approaches.

Application of AFM in understanding biomineral formation in diatoms

“…An important observation, clearly apparent in the formation of three dimensional silica structures in Coscinodiscus wailesii and Thalassiosira eccentrica, is that distinct structures are formed on different internal (luminal) faces of the SDV [41,42]. This was most recently observed during examination of structural intermediates in valve formation in T. pseudonana [21]. In this species, the circular valve is formed by ribs of silica radiating outward from the center, with the ribs branching to maintain a regular spacing (Fig.…”

“…3a,b). The spaces between the ribs eventually fuse, forming a thin silica base layer with the full outline of the valve but without structural rigidity [21]. Formation of the base layer involves expansion in the x/y axis plane.…”

“…Formation of the base layer involves expansion in the x/y axis plane. After base layer formation, the rim of the valve is built up, followed by additional silica being deposited in the valve center, to form a rigid structure [21]. This involves expansion in the z-axis direction.…”

“…We coined the term "spatial cementing" to describe the initial light deposition of silica to delineate the outline of the final structure in the x/y plane, followed by additional deposition and z-axis expansion to form a more rigid structure [21]. In the examples of C. wailesii, T. eccentrica, and T. pseudonana, z-axis expansion and deposition occurs only in one direction, towards the distal (outer) valve face [21,41,42]. Unidirectional z-axis expansion has been documented in other diatom species as well [7], indicating that the SDV has "sidedness."…”

“…There are three scales of silica structure formation in diatoms, the micro-, meso-, and nanoscale [9,21]. Determinants involved in the formation of micro-and nano-scale structures are at least partially characterized, but we understand little about what generates the mesoscale.…”