Disentangling the Taxonomic History of the Widespread and Overlooked Centric Diatom Stephanodiscus makarovae and Its Transfer to Cyclostephanos

Stephanodiscus makarovae, a taxon originally described from Russia, is morphologically similar to several other taxa within Cyclostephanos, namely C. invisitatus, C. delicatus and C. tholiformis. However, it has not yet been transferred into Cyclostephanos, perhaps due to the difficulty in identifying it, as its original description is available only in the Russian language. To investigate its morphology, a detailed morphological comparison of S. makarovae and C. invisitatus was done from 286 SEM micrographs of 12 monoclonal strains. We performed a three-gene phylogenetic analysis with strains from eight additional taxa to independently confirm the position of S. makarovae. The morphology of S. makarovae shows key features of the genus Cyclostephanos and this attribution is supported by the phylogeny. Here we propose the transfer of the taxon S. makarovae to Cyclostephanos, considering the morphological and molecular data. According to both the molecular and morphological data, C. delicatus has a unique position within the genus; S. makarovae and C. invisitatus are morphologically very similar but genetically distinct. Furthermore, based upon the results, it was possible to reassign the authority of the transfer of S. delicatus into Cyclostephanos.