Backgrounds and aims -In this paper results on Navicula s. str. species from the (sub)littoral zone of the Black Sea are presented. Navicula pinnata var. pontica Mereschk. was originally described from the Black Sea in the early 20th century but, due to mistakes, incorrectly referred to in the literature as Navicula pennata A.W.F.Schmidt var. pontica Mereschk. A second species, Navicula parapontica sp. nov. is described as new for science. Analysis of recent and historical diatom assemblages revealed that Mereschkowsky's taxon occurs abundantly. Nevertheless, it was subject of numerous misidentifications in the past; the same applies for the second species. The aim of our study was to clarify the identity and taxonomic position of these taxa. Methods -This study is based on both historical and recent samples. Recent sampling was performed along the Ukrainian (Crimea) coast. In addition, samples from the Black Sea, originally collected and studied by Proshkina-Lavrenko in 1948-1951 and only recently rediscovered, have been analyzed. Results are based on light and scanning electron microscopy examination. We compared line drawings of Mereschkowsky's taxon with illustrations of similar taxa focusing especially on Navicula pinnata Pant. and N. pennata A.W.F.Schmidt. Key results -Comparison of line drawings of Mereschkowsky's taxon revealed no resemblance to N. pinnata. Micrographs of N. pennata illustrated in our paper indicated that any infraspecific relationship between Mereschkowsky's taxon and N. pennata is unlikely. Hence, we propose a new name, Navicula pontica stat. nov. for Mereshkowsky's taxon. In historical samples N. pontica was accompanied by a similar, however, sufficiently different, species, described as new for science: Navicula parapontica sp. nov. Conclusion -Our study resulted in a status change of a taxon originally described as Navicula pinnata var. pontica. In addition N. parapontica was described as new for science. These results will facilitate future identification of both abundant Black Sea littoral diatoms.

A floristic review of the genus Chaetoceros from Arctic and Antarctic waters is undertaken. Taxonomic composition of the Chaetoceros from the Russian Arctic seas, as well as from some regions of the Antarctic was investigated in both water column and sea ice. The genus is rather diverse in both polar regions: 55 species in Arctic and 34 ones in Antarctic. The regions differ in total number of species, number of species belonging to the subgenera Chaetoceros and Hyalochaete and to different sections. Species of the genus are often dominant and the most abundant in Arctic phytoplankton. However, the genus is not prevailing in number of the dominant species as well as in share of the total cell abundance of Antarctic phytoplankton. The importance of the species in sea ice assemblages of the Antarctic is more significant as compared with the Arctic. The Arctic is characterized by cosmopolitan species and those widely distributed in the Northern Hemisphere, more than half of the Chaetoceros taxa are common to all Arctic seas. The Antarctic has a high percentage of endemic Chaetoceros species. Both polar regions are similar in terms of Chaetoceros species composition mainly due to cosmopolitan species.

The present work is an attempt to summarize, systematize, supplement and clarify the terms used in morphological description of diatoms and their taxonomy (systematics). Two glossaries (Russian and English) include definitions of near 300 key terms (excluding their synonyms) and their Latin translations.

New data on morphology of 32 species of the orders Thalassiosirales, Melosirales, Coscinodiscales and Asterolamprales, including 18 Thalassiosira species, from the East Antarctic waters are presented. The species of the genus Thalassiosira mainly dominate in nearice and plankton assemblages.

First records of diatom species for St. Petersburg, the Leningrad Region and the Volgograd Region, first records of aphyllophoroid fungi for the Volgograd Region and first records of bryophyte species for St. Petersburg, the Novgorod Region and the Republic of Mordovia are provided. The data on their localities, habitats, distribution are provided.