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SummaryThe species composition and morphology of centrohelid heliozoa collected from mire water bodies of different types in the North of the European part of Russia were studied. Eighteen species from five genera and four families, two species with an uncertain systematic position and some unidentified Heterophrys-like organisms were found (Arkhangelsk Region -9 species, Republic of Karelia -7, Vologda Region -9). Three species (Pterocystis paliformis, P. striata, P. tropica) are new for Russia. Nine species are new for Arkhangelsk Region (Acanthocystis penardi, A. trifurca, A. turfacea, Choanocystis symna, P. pinnata, P. tropica, Raineriophrys echinata, R. kilianii). Eight species are new to the centrohelid diversity of the Vologda Region (Acanthocystis lyra, A. aff. takahashii, A. trifurca, Polyplacocystis symmetrica, Pterocystis pinnata, P. tropica, Raphidiophrys intermedia). Four species are new for the Republic of Karelia (Pterocystis paliformis, P. striata, P. tropica, Raphidiophrys minuta). The species composition of the mires of the Arkhangelsk Region and the Vologda Region has much more in common in comparison with the centrohelid diversity of the Republic of Karelia. The most favourable conditions for the presence of high centrohelid species diversity occurred in the minerotrophic mires (15 species, incl. 5 -in mire streams, 7 -in space between hummocks, 9 -in flarks of the aapa mires), in comparison with the water bodies of ombrotrophic mires (4 species, incl. 2 -in laggs and 2 -in hollows of the raised bogs).

Species diversity of centrohelid heliozoans in different types of freshwater bodies (forest lakes, floodplain lakes, sphagnum bogs, and river) in the Middle Russian forest-steppe was studied. The morphology of cell coverings (scales and spicules) were observed using scanning and transmission electron microscopy. A total of 20 species of centrohelid heliozoans from five families (Acanthocystidae, Pterocystidae, Raphidiophryidae, Raphidocystidae, and Choanocystidae) and six genera (Acanthocystis, Raineriophrys, Pterocystis, Raphidiophrys, Raphidocystis, and Choanocystis) as well as an unidentified Centrohelea species and a Heterophrys-like organism were found. Morphological descriptions and electron micrographs are provided. The most common species were Acanthocystis nichollsi, Raphidiophrys capitata, and a Heterophrys-like organism. The highest species diversity was observed in terrace forest lakes, whereas the lowest was seen in sphagnum bogs. Four species (Acanthocystis costata, A. elenazhivotovae, A. mikrjukovii, and A. spinosa) were found for the first time since their original description. Three species (Acanthocystis spinosa, Raphidiophrys capitata, and Pterocystis pulchra) are new records for Russia. The distribution of the observed morphospecies confirms the cosmopolitanism of many centrohelids and highlights the importance of the type of water body and microbiotope in forming the species composition. The obtained data on the morphology of studied scales supplements our knowledge of the intraspecific variability of centrohelid heliozoans.

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Telonemia is a poorly known major phylum of flagellated eukaryotes with a unique combination of morphological traits. Phylogenomics recently revealed the phylogenetic position of telonemids as sister to SAR, one of the largest groups of eukaryotes, comprising Stramenopiles, Alveolata and Rhizaria. Due to this key evolutionary position, investigations of telonemids are of critical importance for elucidating the origin and diversification of an astounding diversity of eukaryotic forms and life strategies. To date, however, only two species have been morphologically characterized from Telonemia, which do not represent this genetically very diverse group. In this study, we established cultures for six new telonemid strains, including the description of five new species and a new genus. We used these cultures to update the phylogeny of Telonemia and provide a detailed morphological and ultrastructural investigation. Our data elucidate the origin of TSAR from flagellates with complex morphology and reconstruction of the ancestral structure of stramenopiles, alveolates and rhizarians, and their main synapomorphic characters. Since telonemids are a common component of aquatic environments, the features of their feeding, behaviour and ecological preferences observed in clonal cultures and the results of global metabarcoding analysis contribute to a deeper understanding of organization of microbial food webs.

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A new genus and species of centrohelid heliozoans, Triangulopteris lacunata gen. et sp. nov. (Pterocystidae Cavalier-Smith and Heyden, 2007), from four geographically remote locations (the Crimean Peninsula, the Dnieper Lowland (the East European Plain), Franz Josef Land, and the Kolyma Lowland (North–Eastern Siberia) was examined using light and electron microscopy. The novel centrohelid is characterized by round shape, 4.3–16.3 μm in diameter, covered with two types of scales: 1.06–4.54 μm long triangular spine scales and 1.22–2.05 μm oval plate scales. Studied centrohelid heliozoan possesses a unique spine scale morphology. The base of scales is represented by a horse hoof-shaped basal plate. The inner surface and lateral wings of spine scales have numerous radial ribs with two ‘pockets’ that are located on both sides of the spine shaft. These pockets are formed by the lateral wings and ends of the basal plate. The cyst formation and transition to a spicules-bearing stage were noted. Additionally, phylogenetic tree was constructed based on SSU rRNA sequences including the strain HF-25 from the permafrost of Kolyma Lowland. The resulting phylogeny recovered it within the clade Pterista, while forming a separate sister lineage to H2 clade, which only had included freshwater environmental sequences.

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