Bart Van de Vijver

During the survey on the diatom diversity in mountain streams of the Gorce Mountains National Park (southern Poland), several unidentified taxa were observed. Comparison with available literature and detailed morphological analyses using light microscopy (LM) and scanning electron microscopy (SEM) confirmed a set of distinct valve features in one of the taxa that could not be assigned to any known taxa. As a result, Placoneis montana sp. nov. is described as a new species. A detailed morphological comparison with similar taxa is provided, along with information on the associated diatom assemblage. This new Placoneis species expands knowledge of morphological and ecological variation in the genus, and its lack of visible areola occlusions raises questions about the reliability of that character for separating Placoneis from Witkowskia, pointing to the need for further SEM-based comparison across related taxa. Additionally, observations based on the type locality suggest that the new species is associated with high-altitude, well-oxygenated, low-nutrient environments, emphasising their potential value as indicators of mountain stream conditions.

Gomphonema sarcophagus W. Gregory and Gomphonema productum (Grunow) Lange-Bertalot & E. Reichardt are two well-known but rare species of Gomphonema in Europe that were described in the second half ...

A new species in the genus Chamaepinnularia is described from the Antarctic Continent based on the unique presence of a shallow longitudinal groove bordering the raphe from apex to central area. The new species, Chamaepinnularia attenboroughiana, is further characterised by its lanceolate valve outline, slightly deflected, gradually inflated central raphe endings and small central area forming a subfascia. The species was observed in several layers of a sediment core of Progress Lake, Larsemann Hills, East-Antarctica.

Achnanthes gainii, a marine diatom species, was described as Achnanthepyla gainii in 1921 by Maurice Peragallo from Petermann Island (Antarctic Peninsula area). For over a century, this taxon remained solely known from its original description and illustration of a single raphe valve. More detailed observations of the species are lacking, but a literature search showed that during the last century it was transferred to the genus Achnanthes three times. In this paper, we document and illustrate using both light and scanning electron microscope observations, the morphology of valves from a large population from South Bay, Livingston Island (Maritime Antarctica), matching well the description of this historic taxon. Since the search for the original material in the Peragallo Collection in Bordeaux a couple of years ago proved unsuccessful, in the absence of the original material and slides of A. gainii, the original drawing by M. Peragallo in 1921 is designated as lectotype, and the new population from Livingston Island is designated as epitype to the designated lectotype. The species is separated from other similar taxa, including a taxon recently identified as Achnanthes bongrainii, of which observations and scanning electron microscope illustrations are also provided in this work for comparison, based on populations from South Bay. Finally, several researchers previously commented the similarity of A. gainii to Achnanthes groenlandica. Therefore, the Antarctic records of the latter species were also verified, to the extent possible. This resulted in the discovery that a number of the published illustrations of A. groenlandica from Antarctica represent Achnanthes bongrainii, but not A. gainii.

A revision of the genus Stauroforma is proposed based on the analysis of several historic slides prepared by Grunow to describe Fragilaria virescens var. exigua. The original materials are analysed using both light and scanning electron microscopy together with a critical examination of the original drawings and notes Grunow made on his new taxa. Based on this, the genus description of Stauroforma is emended and the morphological descriptions of S. exiguiformis and S. inermis are updated. Grunow's variety subsalina is raised to species level as Stauroforma subsalina. A new species, Stauroforma subcassubica is described splitting it from S. cassubica, the latter having larger valves. The results of this revision will enable a more exact identification of the different Stauroforma species observed in freshwater and brackish water bodies in Europe.

The genus Nitzschia Hassall is a common and often dominant constituent of the diatom flora in rivers and lakes worldwide (Lange-Bertalot et al. 2017). Nitzschia species are free-living motile diato...