Valve ultrastructure of some confusing Fragilariacease

Abstract

The study of diatom taxonomy most frequently uses light'microscopy to distinguish the silicous cell wall components of different species. The structures that give a characteristic appearance in the light microscope are usually patterns that reflect the ultrastructure of the cell wall that cannot be resolved adequately with this technique. Scanning electron microscopy provides a more complete concept of diatom valve structure, especially for diatoms that are less than i0 ~m in length. While examining a collection of epipelic diatoms from Lake Kathrine, Michigan, during 1979 and 1980, a number of small Fragilariaceae were encountered which could not be easily identified with the light microscope. These diatoms were cIeaned in nitric acid, washed in distilled water, dried on aluminum stubs, and coated with 12 nm gold-paladium before examination. Distinct differences were found in the structure of the striae in a group of diatoms that are similar in shape and size. Fra~ilaria elli~tica Schumann (Fig. i) has striae composed of lineate puncta that become orbicular on the valve mantle. Fra~ilaria brevistriata var. elli~tica H~ribaud (Fig. 2) has striae composed of a single punctum. Opephora mart[ii H~ribaud (Fig. 3) has striae with puncta that are entirely lineate. An unknown species of Fra~ilaria (Fig. 4) has striae that are clavate in shape and composed of numerous puncta, a pattern entirely new to the literature. The elliptical members of the Fragilariaceae described are smaller than i0 ~m and the detaiis of the striae cannot be resolved with the light microscope. Fra~ilaria elli~tica (Fig. i), identified by Haworth (1975) has striae that appear thin, leaving a narrow central area. With F. brevistriata var. elliptica (Fig. 2) described by H~ribaud (1903), the individual puncta appear as refractile points and the central area is wide. In Opephora mart~ii (Fig. 3), also described by H~ribaud (1902), the striae appear thick and the central area is narrow. The unknown Fra@ilaria species appears similar to Opephora mart[ii under the light microscope. All of the organisms described are difficult to distinguish with the light microscope, making the correct assignment of these diatoms to species tedious and often inaccurate.