Silica-scaled heterotrophic protists: Rotosphaerida, Thaumatomonadida and Centroplasthelida in Arctic waters of Russia.

Maintaining high levels of biological diversity in various ecosystems is necessary for stable functioning of the Earth's biosphere. The article describes diversity and ecology of heterotrophic siliceous protists - rotosphaerids, colourless free-living thaumatomonad flagellates, and centrohelid heliozoans - in Arctic waters located of Asian Russia. Samples were collected in the mouths of the Olenyok, Yana, Indigirka, Kolyma Rivers - and small freshwater ponds near Settlement of Tiksi in Yakutia. Based on electron microscopy, 35 centrohelids (Heliozoa), 11 rotosphaerid species as well as four thaumatomonad flagellate species were found in the region. Seven species were recorded in Russia for the first time: Rabdiaster multicosta, Rabdiaster reticulata, Turriplaca denticulata, Choanocystis cf. cordiformis parvula, Raineriophrys pteromorphos, Pseudoraphidocystis glutinosa and Pseudoraphidiophrys formosa. For 43 species, the study area is the most northern location they were described. Morphological details of scales are discussed for selected species, particularly for widespread species of rotosphaerids - Pinaciophora fluviatilis. The literature on hydrochemical conditions was analyzed to find taxa with high sensitivity towards environmental changes. Such species could be further used for monitoring plankton, recovering evolutionary changes, and reconstructing past environments.