8 · “PROTIST–2016” 1 - Lomonosov Moscow State University, Moscow, Russia 2 - Institute for Biology of Inland Waters, Russian Academy of Sciences, Borok, Russia 3 - University of British Columbia, Department of Botany, Vancouver, BC, Canada email@example.com Protists are ubiquitous, but the factors influencing their diversity and biogeography are poorly understood. We use a comprehensive database on the marine benthic heterotrophic flagellate (HF) morphospecies to explore the worldwide patterns in their diversity and distribution in comparison with predictions of the Ubiquity model (UM) and Moderate Endemicity model (MEM). Number of known HF morphospecies was limited (even if considering the rates of descriptions), and local-toglobal diversity ratio was relatively high (10-25%). Regional diversity was highly correlated with the investigation effort, indicating considerable underexploration. Regional endemics were few (not over 19% of total richness), and many morphospecies were widespread or even cosmopolitan. No obvious latitudinal trend in HF diversity was detected. By species composition, the regions were distinctly arranged into three groups according to cold, temperate and warm waters, but not in accordance with geographical distances. This distribution pattern was most likely explained by contemporary climate (temperature) but did not suggest clear geographical barriers for dispersal. Therefore, the HF morphospecies are less concordant with the MEM predictions but closer to the UM than other (larger) protists. (In)consistency between the distributional patterns obtained from genetic- and morphology-based data are briefly discussed. This study was supported by Russian Foundation for Basic Research (grants № 15-04-02245 to AA and № 14-04-00553, 15-29-02518 to DT). MOLECULAR DIVERSITY OF SUMMER PLANKTON PICOEUKARYOTES IN THE WHITE SEA Belevich T.A. 1, Ilyash L.V. 1, Milyutina I.A. 2, Logacheva M.D.2, Goryunov D.V.2, Troitsky A.V.2 1 - Lomonosov Moscow State University, Faculty of Biology 2 - Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University firstname.lastname@example.org Picoeukaryotes (protists < 2 µm) play an important role in marine ecosystems, although knowledge of their diversity and ecosystem functioning is limited. The White Sea is located in the sub-Arctic zone. However, its abiotic conditions are characterized by a number of features typical of the Arctic seas. In this study, the molecular diversity of picoeukaryotes in July 2014 in Kandalaksha Bay (the White Sea) was examined using conventional filter fractionation and Illumina sequencing. In < 2 µm size fraction 186 taxa were revealed that included some nanoplankton (2–20 µm) and even microplankton (>20 µm) taxa. Plankton assemblage was prevailed by autotrophs (>57% of all sequences). Putative picoeukaryotic autotrophs were mostly dominated by three genus: Micromonas sp., Bathycoccus sp. and Ostreococcus sp. (35% of all sequences). Putative heterotrophic picoeukaryote assemblage was more diverse and was presented by marine stramenopiles (MAST, Labyrinthulomycetes), Amoebozoa, Cercozoa and picozoa. Illumina sequencing revealed some new groups and genus of nano- and microplankton organisms which have been never detected in the White Sea. Our study revealed that diversity of picoeukaryotes in summer in the sub-Arctic White Sea corresponds to that of the other Arctic seas, such as the Beaufort and Norwegian Seas. NEW PARAMECIUM SPECIES “CANDIDATUS PARAMECIUM OSSIPOVI” Beliavskaia A., Kiselev A., Rautian M. Saint Petersburg State University email@example.com Among Paramecium species P. putrinum is one of the best identifiable. Most important discriminative features are small cell size (70-140 µm), single big micronucleus (MI) and very special contractile vacuole (CV) represented as set of small vacuoles. We have studied several clones from different collecting sites that were characterized by all features mentioned above but with unusual habitus. Detailed analysis revealed several very specific characteristics of these clones. Usually the cells are bigger up to 120140 µm. Their single MI has got “chromosomal” structure. Contractile vacuole differs from typical P. putrinum one. It has got short channels and is similar to CV of P. nephridiatum, or P. polycarium. Holospora-like bacterium found in P. putrinum do not infect “atypical” strains. Finally the sequence of cytochrome c oxidase subunit I gene has only 86% identity with the one of P. putrinum, which is much higher than difference between different P. putrinum syngens. Taking in account all above, we propose new species “Candidatus Paramecium ossipovi”. Scientific research was performed at the Center for Culturing Collection of Microorganisms and Center for Molecular and Cell Technologies of Research park of St. Petersburg State University.