A simple method for making transverse cleavages of diatom frustules for scanning electron microscopy and its application

The surface structures of diatom valves have been primarily studied using scanning electron microscopy (SEM), while the frustule shape and internal cellular structures have been elucidated in cross-sections by transmission electron microscopy (TEM). However, ultrathin sections can show neither the part of the whole frustule that is cut nor reflect the relationship between the sectioned portion and the surrounding structure. Therefore, we developed a simple method for obtaining a clean cleavage surface of the frustule. In this method, a drop of diatom suspension was frozen on a custom-made bundle of metal joints pre-soaked in liquid nitrogen, followed by sample preparation for SEM. The three-dimensional (3D) structure of the entire frustule and cross-sectional features, such as the thickness of the valve and bands, the angle of projection, and the internal structures of the areolae, raphe systems, and isolated pores, were visible within each half of the cleaved frustule. Applying this method shortly after cell division revealed differences in the silica deposition profiles between parental and newly forming valves at the corresponding sites. The utility of the information obtained from the cleaved frustule is discussed.