Application of the filter-transfer-freeze technique to permanent slide preparation

Abstract

Cleaning and mounting diatom valves and frustules are necessary to observe their fine morphological structures under a light microscope. However, the presence of salt in the sample (seawater) and/or small numbers of diatom cells make the process laborious and time-consuming. To solve this problem, we propose to use a modified filter-transfer-freeze (FTF) technique, first proposed by Hewes & Holm-Hansen (1983), to prepare permanent slides. The protocol involves 1) concentrating diatom cells by filtering, 2) cleaning and rinsing diatom valves and frustules on the filter, and 3) recovering the cleaned material to a cover slip via the FTF technique. In terms of the number of species, diversity index, and the ratios of intact valves, no significant difference was observed between the slides prepared via the FTF and conventional centrifugation methods. Our results suggest that the modified FTF technique is a promising approach for the qualitative analysis of diatoms because it facilitates the rapid preparation of permanent slides.