Examination of the type material of some diatom species related to Achnanthidium minutissimum (Kutz.) Czarn. (Bacillariophyceae)

Diatoms of the Achnanthidium minutissimum species complex are widely distributed taxa believed to occur in different freshwater habitats from unpolluted oligotrophic to highly polluted hypereutrophic waters. However, the taxonomy and nomenclatural history of the species complex are complicated and rather confusing, because the species within the group often lack an adequate description and an exhaustive differential diagnosis drawing clear morphological and ecological species boundaries. To fill in this gap we examined the type materials of five different Achnanthidium species using light and scanning electron microscopy: Achnanthidium catenatum (J. Bilý et Marvan) lange-Bert., A. dolomiticum Cantonati et lange-Bert., A. eutrophilum (lange-Bert.) lange-Bert., A. saprophilum (H. KoBayasi et MayaMa) round et BuKHt. and A. straubianum (langeBert.) lange-Bert. Frustular morphology of the five taxa was studied in order to provide complementary information on the variability and ultrastructure of the nomenclatural types related to the A. minutissimum species complex. Our observations are mostly consistent with the original descriptions. Comparisons of morphological characters observed with the SEM showed that all species are closely related having many morphological features in common; however, they differ in the shape and variability of areolae. The SEM investigations demonstrated that ultrastructural characters on their own do not distinctly discriminate among the taxa. Morphometric comparisons indicated that the only distinct morphological features that could separate the Achnanthidium species studied are valve outline, striation pattern and valve dimensions.