A comparison of cryo- versus chemical fixation in the soil green algae Jaagiella.

Chemical fixation protocols provided unsatisfactory preserved material for ultrastructural studies on Jaagiella alpicola Vischer (Chlorophyta). Instead, several methods of rapid freeze fixation followed by freeze substitution were applied. For fast freeze fixation, two methods were tried: plunge immersion freezing in liquid propane using a home-made device, and projection against a copper block cooled by either liquid nitrogen or liquid helium. Each method furnished well fixed material. The quality of the fixed samples was quite similar whether propane or the cryoblock cooled with liquid nitrogen was used. Liquid helium, however, provided superior results. After fixation the samples were cryosubstituted, using acetone or methanol as organic solvent with a chemical fixative added. Acetone gave better results than methanol as a substitution solvent when high temperature embedding was performed. The best cryosubstitution for ultrastructural studies was that in which osmium tetroxide or a mixture of osmium tetroxide and urany acetate was used.