Diatoms are unicellular algae that make cell walls out of silica with highly ornate features on the nano- to microscale. The complexity and variety of diatom cell wall structures exceeds those possible with synthetic materials chemistry approaches. Understanding the design and assembly processes involved in diatom silicification should provide insight into patterning on the unicellular level, and information for biomimetic approaches for materials synthesis. In this report we examine the formation of distinct cell wall structures (valves and girdle bands) in the diatom Cyclotella cryptica by high resolution imaging using SEM, AFM, and fluorescence microscopy. Special attention was paid to imaging structural intermediates, which provided insight into the underlying design and assembly principles involved. Distinct stages in valve formation were identified, indicating a transition from a fractally organized structure to a dynamic pathway-dependent process. Substructures in the valves appeared to be pre-positioned prior to complete silicification, suggesting that organics responsible for these structures were pre-assembled and put in place. Microtubules and microfilamentous actin play significant roles in the positioning process, and actin is also important in the pathway-dependent expansion of the front of silicification. Our results indicate that even though all silica structures in C. cryptica are made of assemblies of nanoparticulate silica, control of meso- and microscale structure occurs on a higher order. It is apparent that diatoms integrate bottom up and top down control and synthesis mechanisms to form the diversity of structures possible.
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