Abstract
For the first time, the three-dimensional (3D) internal structure of naturally produced Didymosphenia geminata frustules were nondestructively visualized at sub-100 nm resolution. The well-optimized hierarchical structures of these natural organisms provide insight that is needed to design novel, environmentally friendly functional materials. Diatoms, which are widely distributed in freshwater, seawater and wet soils, are well known for their intricate, siliceous cell walls called ‘frustules’. Each type of diatom has a specific morphology with various pores, ribs, minute spines, marginal ridges and elevations. In this paper, the visualization is performed using nondestructive nano X-ray computed tomography (nano-XCT). Arbitrary cross-sections through the frustules, which can be extracted from the nano-XCT 3D data set for each direction, are validated via the destructive focused ion beam (FIB) cross-sectioning of regions of interest (ROIs) and subsequent observation by scanning electron microscopy (SEM). These 3D data are essential for understanding the functionality and potential applications of diatom cells.
Highlights
Diatoms (Bacillarophyceae) are unicellular, eukaryotic and photoautotrophic organisms that inhabit aquatic and terrestrial environments
An important fact is that focused ion beam (FIB)–scanning electron microscopy (SEM) must be conducted in vacuum, whereas nano-XCT, which is conducted at multi-keV photon energies, does not require vacuum conditions
This paper presents a novel approach for the high-resolution imaging of internal structures of diatom frustules via nano X-ray computed tomography complemented by SEM imaging of FIB cross-sections through the regions of interest (ROIs), to nondestructively study diatoms and their biomineralization kinetics
Summary
The visualization of the diatom frustule was conducted on Didymosphenia geminata, a biraphid pennate diatom. SEM images (Fig. 1) illustrate girdle (A) and valve (B) views of the D. geminata frustule. Nano-XCT studies of the frustules of D. geminata show both surface and internal structures of the frustules in a nondestructive manner based on a single tomographic data set. From the 3D tomographic imaging, the radiograph shows all surface and internal structures; more details are distinguishable in individual cross-sections based on nano-XCT. Both X-ray and SEM images provide information regarding the geometry of the D. geminata frustule. Spaulding (2010) claims a much larger size range for the valves of D. geminata: from 65 μm to 161 μm in length and from 36 μm to 41 μm in width[30].
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