The diversity of chloroplast forms, and their number and cellular location, as well as pyrenoid structure, distinguishes diatoms from other groups of heterokont algae. The fine chloroplast structure is considered to be informative for taxonomic and phylogenetic studies of diatoms. Six species of diatoms belonging to different classes have been examined with transmission electron microscopy. New data on the chloroplast structure have been obtained. Characteristics of the pyrenoid ultrastructure of diatoms belonging to various phylogenetic clades have been defined more precisely. The results specify the ultrastructure of pyrenoids for different phylogenetic clades of diatoms and contribute to the previously obtained data.

Determination of the taxonomic position of microorganisms associated with diatoms was carried out using microscopic, microbiological, and phylogenetic methods. Examination of the cultures of the fresh-water diatom Synedra acus grown in Lake Baikal water by epifluorescence and electron microscopy revealed that bacterial cells colonized the mucilage surrounding the cells of living microalgae and their cell surfaces and penetrate into the frustules of dead diatoms. A total of 13 strains of heterotrophic bacteria were isolated in pure cultures and described. Based on the results of analysis of their morphological, physiological, and biochemical properties, as well as on data obtained by 16S rRNA gene analysis, the following strains were identified to the species level: Sphingomonas sp., Variovorax paradoxus, Pseudomonas fluorescens, Microbacterium trichothecenolyticum, Rhodococcus sp., Caulobacter vibrioides, and Brevundimonas vesicularis.

The important role of the cytoskeleton in the morphogenesis of siliceous frustule components, which are synthesized within the diatom cells, has been revealed due to experiments with microtubule inhibitors. It has been shown that colchicine entering the diatom cell inhibits polymerization of tubulin, the main protein of microtubules, thereby disrupting the normal processes of biogenic silica deposition and daughter valve morphogenesis. In this study, experiments with a synchronized culture of the pennate diatom Synedra acus have been performed to determine the timing and duration of the formation of various valve components and analyze the effect of colchicine at a subtoxic concentration on the structure of daughter valves at different stages of their morphogenesis. Electron microscopic analysis has revealed several types of micro- and nanoscale anomalies in daughter valve morphology, with their frequency varying depending on the time of colchicine treatment. Laser scanning microscopy of preparations vitally stained with Tubulin Tracker Green has shown that polymerized tubulin at early stages of valve morphogenesis is localized along the periphery of the developing valve. This is evidence for an important role of microtubules in the horizontal growth of the valve at the stage when its general structural pattern is established, including its shape and arrangement of basic micro- and nanostructures. Treatment with a microtubule inhibitor at a certain stage of valve morphogenesis makes it possible to obtain new forms with a specific structure of siliceous components that hold promise for use in nanotechnologies.

Interest in studies concerning the effect of organic carbon sources on the growth of diatoms is largely aimed at subsequent physiological changes occurring in their cells. There are no data on structural changes in the cytoplasm and their relationship with changes in the composition of fatty acids in the course of mixotrophic culturing of freshwater diatoms. To elucidate the role of lipids in the growth of diatom cells in autotrophic and mixotrophic cultures, it is necessary to obtain information on the distribution of fatty acids among intracellular compartments and on possible ultrastructural changes in the cells. In this study, the results demonstrated that Synedra acus cells cultured in the presence of 80 mM glycerol contained lipid bodies of increased size, while cells from cultures supplemented with 40 mM glucose accumulated polysaccharide (chrysolaminarin) granules. An increase in the relative amounts of palmitic and stearic acids was revealed at the exponential growth phase of S. acus in the medium with 80 mM glycerol, which was indicative of the accumulation of fatty acids contained in triacylglycerols. Synedra acus subsp. radians have an ability to proliferate in the presence of high concentrations of organic substances and their transport into cells is evidence for its high adaptation potential, which, along with other factors, accounts for the dominance of this diatom in the spring-summer plankton of the oligotrophic Lake Baikal.

Fragilaria and Ulnaria are two closely related diatom genera for which the delimitation and circumscription of several species is unclear. We studied strains isolated from Lake Baikal and compared them with the species from freshwater reservoirs in Europe and Asia using phylogenetic and species delimitation methods, microscopy and interclonal crossing experiments. The results of the phylogenetic analyses of the fragments of rbcL and 18S rRNA genes revealed that baikalian F. radians clade was independent from the representatives of the genus from other localities. Among Ulnaria we found the following 18S rRNA phylogenetic tree groups at species level: U. acus, U. ulna and U. danica. Genetic distance between genera varied between 3.9–10.2% substitutions in rbcL gene and 3.2–11.5% in 18S rRNA. The boundary between intraspecies and interspecies polymorphism for studied species of Ulnaria and Fragilaria in these marker genes was around 0.8% substitutions. Morphometric characters of individual strains showed their variability and division into F. radians, U. acus and U. ulna together with U. danica. Strains of U. acus and U. danica from different localities of Europe and Asia were sexually compatible inside the species. Sexual reproduction has never been observed in monoclonal cultures, either between this species or with strains of the Fragilaria.

Diatoms are capable of accumulating substantial amounts of triacylglycerides in their cells, which differ in the composition of fatty acids depending on the conditions of cultivation, making them attractive subjects in biotechnology. In the present study, we characterized the structural features of lipid bodies in the diatom Entomoneis cf. paludosa (W. Smith) Reimer strain 8.0727-B and revealed the peculiarities of fatty acid composition in cultures during the stationary and exponential growth phases. Laser scanning confocal microscopy revealed an increased number of lipid bodies in the cytoplasm during the stationary phase of culture growth. Electron microscopy of ultrathin sections showed that an extreme increase in the number and size of plastoglobules in the cells occurs in the stationary phase of culture growth. The gas chromatography with mass spectrometric detection method revealed differences in the fatty acid composition depending on the growth phase. The studied strain can be recommended as a source of hexadecanoic and octadecanoic fatty acids from the culture during the stationary growth phase, as well as eicosapentaenoic fatty acid from the culture during the exponential growth phase.