Konrad Schultz

The southern Baltic Sea coastline is a dynamic system, often bordered by coastal peatlands. Artificial dunes and dykes commonly separate these peatlands from the sea. Given their potential as carbon sinks and natural buffer zones against sea-level rise and flooding, interest in their restoration is increasing. Microphytobenthos, particularly diatoms, are key primary producers in coastal and peatland environments and serve as valuable bioindicators. While their use as bioindicators has focused on freshwater systems, their application in coastal contexts also holds promise. In contrast to their freshwater counterparts, the ecology, taxonomy and diversity of benthic diatoms along the southern Baltic Sea coast, particularly in coastal peatlands, remain poorly understood. Despite being separated from the Baltic Sea by a dune dyke, the coastal peatland in the nature reserve “Heiligensee und Hütelmoor” is periodically exposed to saltwater inflow during storm surges. Following a storm surge in January 2019, brackish water entered the peatland, providing a rare opportunity to study a coastal peatland with periodic salt water input. Over one year, diatom community compositions, biomass, and key environmental variables were monitored at two peatland sites and an adjacent marine site to assess spatial and temporal patterns along a marine–terrestrial gradient. Overall, 198 diatom taxa were found and the share between the Baltic Sea and the peatland was only 4% despite the recent saltwater intrusion. Diatom community compositions showed distinct seasonal and spatial variation within the peatland area. The main drivers of community differences in the dataset were salinity, temperature, pH and nitrogen-based nutrients.

In this study on diatoms, which are unicellular microalgae with a siliceous cell wall, images obtained by electron microscopy were used to compare three taxa from East Asia that were referred as Cyclotella hubeiana, Discostella guiyangensis and “Cyclotella fottii” in the original studies. Morphometrical and morphological analyses indicate that these three taxa are conspecific. The distinctive morphological features of this species are the following: one rimoportula located on the valve mantle within the ring of marginal fultoportulae, marginal fultoportulae situated on costae with internally two satellite pores positioned circumferentially (laterally) and externally long tubular opening, biseriate striae and branching costae. A comparison of these morphological features with those of Discostella and Cyclotella species and a re-assessment of the existing molecular data suggest that this species could be separated from both these genera. On this basis, we describe the new genus Discocyclus gen. nov. with Discocyclus hubeianus comb. nov. as the type species.

The centric diatom genus Discostella is common in freshwater communities worldwide. Sixteen extant species are currently known, many of which are morphologically very similar. In applied taxonomy mostly broader concepts of D. stelligera and D. pseudostelligera are used. To assess the species diversity of the genus, 58 strains were cultivated and investigated using morphometric and molecular methods, obtaining the first multi-gene phylogeny of Discostella. Up to 22 operational taxonomic units were identified. For the taxonomic assignment of the OTUs a detailed morphological analysis including several type materials was performed. Up to 15 OTUs did not fit any existing taxon, revealing previously undetected diversity in the genus. A comparison of the different molecular markers revealed hints of a more complex intrageneric structure possibly due to reticulate evolutionary processes. The identity and generic placement of D. glomerata is resolved and D. angainor sp. nov. is described.

The taxonomic situation of the species complex around Stephanodiscus minutulus/parvus is controversial. Insufficient species description and fossil type material contribute to persistent confusion. Håkansson & Kling (1990) described S. binatus as another morphologically similar species in this group. Following the reinvestigation of the type materials of S. minutulus and S. parvus by Houk et al. (2014), it seems that these names are often falsely applied to S. binatus. Monoclonal strains of the S. minutulus/parvus/binatus species complex from European water bodies were established for morphological and molecular analyses. Most of the strains belonged to S. binatus, which seems to be very common in meso- to eutrophic waters in middle Europe. Differing in the barcoding markers, the remaining strains represented a new species, S. neglectus sp. nov., morphologically similar to S. binatus. Stephanodiscus minutulus and S. parvus were not found. Seven strains of S. neglectus sp. nov., seven strains of S. binatus and two strains of Cyclostephanos invisitatus were subjected to detailed morphometric investigation. Four strains of Stephanodiscus hantzschii, Stephanodiscus neoastrea, Cyclostephanos dubius were also included in the molecular analysis. The morphological differential diagnosis of S. neglectus sp. nov. from S. binatus is difficult, but the molecular results show that the species are distinctly different. Using three different loci the distance between the new species and S. binatus equals the distance between S. binatus and S. hantzschii.

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.