Structural anisotropy of biogenic silica in pennate diatoms under fourier transform polarized infrared microspectroscopy

Abstract

Exoskeletons of diatoms are constituted of amorphous silica and organic matter, called frustules. They are composed of two valves fit each other like a petri-dish, and connected by girdle bands. In this study, structural analyses of the silica-based compound in frustules were carried out using Fourier transform polarized infrared microspectroscopy (pol-mFTIR). Unpolarized spectrum of diatoms show strong peaks at v≈ 1100 cm-1 and small peaks at 800cm-1 in the region of v= 700-1500cm-1. The feature is similar to those of low-quartz and fuzed silica glass indicating the existence of the SiO4 network structure. The slight difference of three band shapes may indicate different linkage ordering of SiO4 tetrahedra in these materials. Polarized IR spectra of a pennate diatom show an anisotropy of absorption band, showing the existence of two Si-O bonds with different distances. Polarized IR spectra of Si-O band with long distances shows a sharp peak in the region v= 1051-1081 cm-1. When the long-axis of pennate diatoms was set parallel to IR light oscillation, the absorbance took a maximum. Whereas the polarized IR spectra of short Si-O bonds shows a broad peak at v≈ 1227cm-1. When the long-axis of pennate diatoms was set parallel to IR light oscillation, the absorbance took a minimum value. These results indicate that Si-O bonds with long distances exists along the long-axis of the diatom and short Si-O bonds exist along the short-axis. We applied models suggested by Scifert et al. (1982) to interpret Si-O-Si angle in the frustules. Si-O-Si angle from long Si-O bonds was smaller than ones from short Si-O bonds. The structural distortion generated with the anisotropic SiO4 network may be stabilized by the presence of organic matters in diatoms. The pol-mFTIR technique used in this study has a possibility for the application to investigate structural anisotropy in other amorphous materials that show a broad amorphous profile in X-ray diffraction method.