Diatoms, unicellular algae with silica-rich cell walls that are intricately ornamented, are important ecological indicators of water quality and nutrient status, with individual species adapted to particular conditions. Their use as indicators depends upon a reliable classification of the species, using stable morphological features of the cell wall. Diatom cell walls are easily preserved as fossils, for example in lake-bed cores, and make possible the analysis of very long-term changes in species composition and abundance, a task again dependent upon reliable taxonomic markers. Dr. Eugene Stoermer of the University of Michigan is studying freshwater diatoms of the genus Tabellaria to assess the limits of morphological variability within and between species of this group. Tabellaria species are found throughout the northern hemisphere, and about 80 species have been named and described over the years, mostly briefly and in many cases on the basis of unreliable, environmentally induced variations. Dr. Stoermer is developing morphometric procedures using Fourier transformations for the rapid measurement, under the light microscope, of cell outline and other cell-wall features, to delimit those aspects of size and shape that are less prone to modification by transient water conditions, and can thus be used as stable markers of particular species. Long-term evolutionary trends among diatom species, with a presumed genetic basis, can then be studied, uncomplicated by variations locally induced by the environment. The morphometric methods developed for size and shape analysis should be useful with other groups of diatoms and other unicellular organisms.
