New methods for preparing, imaging and typifying desmids (Chlorophyta, Zygnematophyceae), including extended depth of focus and 3-D reconstruction

Species- and genus-level taxonomy of desmids depends largely on shape and detail of the cell wall and chloroplast morphology. The depth of most desmid semicells, relative to the focal depth of conventional light microscopes, means that morphological characteristics are usually illustrated by drawings, made from material that is mounted in water to allow reorientation of specimens to different aspects of shape and pattern. Though a productive approach for two centuries, this has the disadvantages that features not initially detected or thought irrelevant are not recorded, drawing quality is variable, and individual specimens are rarely retained for further study. We describe methods for making permanent preparations of desmid cell walls and using these to produce extended depth of focus summary images and three-dimensional (3-D) reconstructions. Together with World-Wide Web dissemination of image stacks, these advances make it practical to make a desirable change from typification via drawings to typification via single or multiple preserved specimens. They will also facilitate standardization of taxon concepts and identification.