An investigation into the morphology and genetics of Cyclotella comensis and closely related taxa

The planktonic freshwater diatom Cyclotella comensis Grunow is widely used as an indicator of oligotrophic lake conditions. Taxonomic confusion arose with the discrimination of the morphologically similar species Cyclotella pseudocomensis Scheffler and Cyclotella costei Druart & Straub. In this study, we used morphological and molecular data to investigate the relationship between C. comensis, C. pseudocomensis and C. costei using 11 strains from eight lakes in Germany and Austria. Morphological variability was high among the three morphospecies such that they could not be separated unambiguously using morphological traits. DNA sequence analysis revealed only minor differences in 18S V4 (<0.25%), 5.8S rDNA (all sequences identical), LSU D2/D3 (<0.8%), rbcL (<0.4%) and cox1 (<0.79%) between the cultures. Cyclotella comensis, C. pseudocomensis and C. costei are indistinguishable on morphological and DNA sequence data, suggesting that they might comprise one taxon. However, it cannot be completely excluded that they are recently diverged, closely related species.