A slide preparation technique for light microscopy analysis of ciliates preserved in acid Lugol's fixative

Abstract

A slide preparation method for seawater samples preserved in acid Lugol's is presented here as an alternative to the traditional Utermöhl settling chamber method for microplankton analysis. This preparation maintains the integrity of fragile cells, such as ciliates, resolves issues associated with the transience of samples prepared in settling chambers, and allows the use of automated image acquisition methods. Samples are filtered onto polycarbonate membranes and analyzed with transmitted light microscopy. The visibility of pore outlines is minimized by using mounting oil (Cargille Series A immersion oil, Certified Refractive Index, nD25°C 1.5840 ± 0.0002) with a refractive index matching that of the membrane material. We assessed the efficacy of this new method by comparing abundance and biomass estimates for ciliates in settled and filtered samples. Acceptable results were found for the most delicate of samples stored long-term in acid Lugol's. Some cell shrinkage occurred during the filtration and brief drying steps. Therefore, corrections for ciliate length and width measurements in filtered samples were determined to counteract this effect on total cell biovolume. Overall, the method provides a simple and stable alternative to settling chamber analysis for ciliates preserved in acid Lugol's. © 2014, by the American Society of Limnology and Oceanography, Inc.