103 Assessing Microalgal Morphology from Century‐Old Herbarium Sheets Using SEM

Abstract

Traditionally, type specimens have not been deposited for most unicellular algae. There are few known holotypes and those that are available are often of limited use. Samples from taxa described before 1900 commonly consist of materials dried down on herbarium sheets or mica. Dried algal samples can be rehydrated, but preservation is very poor and little structure can be determined from them. Whether wet or dry, the amount of information that can be derived from such samples using conventional light microscopy is limited. However, identification of microscopic algae from herbarium samples may be necessary when the original descriptions and/or illustration(s) are inadequate, questionable or when a taxon is under revision. We were able to derive high‐resolution images from dried algal samples using a JEOL 6300 field emission SEM. For example, we have imaged samples of Euglena spirogyra that were dried on mica chips in October of 1899 and stored in the Michigan State University herbarium. The overall shape of the cell body and the arrangement and ornamentation of the pellicle strips were clearly visible. Because of this success we decided to examine a variety of algal samples that had been preserved on mica. Algae such as Staurastrum, Scenedesmus, or Mallomonas (particularly the scales), retained the most structure while others such as Pediastrum and Pandorina, retained sufficient structure to allow their identification. We suggest that FESEM should be more broadly used for assessing structure in dried herbarium samples.