Saline and hypersaline waters are one of the most peculiar ecosystems of our planet, characterized by extreme life conditions. Despite their worldwide distribution, the diversity and abundance of protist communities in these ecosystems remain poorly studied. Here, we analyze planktonic communities of protists sampled across 38 saline and hypersaline water environments (2–390‰) from arid climatic zones of the South Urals and Crimea in light of environmental data using high-throughput 18S rDNA amplicon sequencing. A total of 9 eukaryotic supergroups, 34 phyla, 104 classes, 184 orders, 315 families and 548 genera have been identified. We revealed significant differences in the taxonomic structure of protist communities depending on salinity, geographic location and pH. The protist communities demonstrated linear regression of richness and diversity and growth of the percentage of unclassified Eukaryota (up to 43%) with the increase in salinity. Centrohelids demonstrated the ability to inhabit a broad range of salinities, up to 320‰, which is four times higher than previously reported. Centrohelid species Pinjata ruminata and Yogsothoth sp. are assumed to be specifically adapted to salinity of 3–210‰. The obtained results provide insight into the taxonomy and diversity of protists in saline and hypersaline environments and highlight the great potential for the discovery of new taxa due to the large number of unclassified 18S rDNA sequences.

Lake Kislo-Sladkoe is a stratified water body partly isolated from the White Sea. Perennial meromixis in the lake irregularly alternates with mixing events. Taking into account that the protists of Arctic coastal stratified water bodies are understudied, we evaluated for the first time the vertical structure, species richness, and diversity of protists assigned to different taxonomic groups in Lake Kislo-Sladkoe using light, luminescent, and scanning electron microscopy. To test the research hypothesis that a mixing event affects the vertical stratification and species composition of protists in a stratified lake, we compared the protist communities of Lake Kislo-Sladkoe in two extremely different states: strong meromixis vs. full vertical mixing. A total of 97 morphologically distinct phototrophic, heterotrophic, and mixotrophic protists were revealed with the most diverse supertaxa SAR (59), Obazoa (9), and Excavates (14). The hidden diversity of protists (43 species) was a bit less than the active diversity (54 species). A taxonomic list and micrographs of cells for the observed protists are provided. The majority of species revealed are cosmopolitan or widespread in the northern sea waters. The vertical patterns of protist communities were absolutely different in 2018 and 2021. In July 2018, clearly distinct protist communities inhabited different layers of the lake. Bloom of cryptophyte Rhodomonas cf. baltica was detected in chemocline, whereas the maximum density of its grazers was observed in adjacent layers, mainly dinoflagellates Gymnodinium sp. and Scrippsiella trochoidea, as well as a ciliate Prorodon sp. In 2021 due to the recent mixing of lake and seawater, there were no distinct communities in the water column except the superficial 0–1 m layer of fresh water.

In the waterbodies of different types of the steppe zone of the Southern Urals, 19 taxa of silica-scaled Chrysophyceae belonging to the orders Chromulinales and Paraphysomonadales have been recorded. Among them, 16 taxa of the infrageneric rank were recorded for the first time for the studied territory; two species (Lepidochromonas cancellataandPolylepidomonas vacuolata) are new to the flora of Russia. New localities have been revealed of four species rare in the flora of Russia:L. poteriophora,L. stelligera,L. subrotacea, andParaphysomonas caelifrica. For each discovered taxon, data on its morphology, locality, and distribution are provided. All findings are illustrated with microphotographs taken using scanning electron microscopy.