Molecular phylogenetic analyses place Ardissonea crystallina (C. Agardh) Grunow and all Toxariids among the bi- and multipolar centric diatoms, almost always recovered as a derived lineage sister to Lampriscus. In all centrics where sexual reproduction has been documented, oogamy, with larger immobile eggs and smaller flagellated sperm has been observed. We were able to initiate both homothallic and heterothallic reproduction in A. crystallina. The heterothallic reproduction turned out to be non-oogamous; gametes were more or less equal in size but no flagellated cells were detected. At the same time, two mating types (“male” and “female”) were recognized by the distinct morphology and behaviour of the gametes. While no flagella were observed, periodically thin cytoplasmic projections arose on the surface of the “male” gametes. These projections similar to those found in some pennate diatoms facilitated contact with the “female” cells. In each gametangial cell, regardless of the mating type, only one gamete was formed. Thus, the Toxariids may represent a unique evolutionary group, at least in respect to their reproductive biology. The hypothesis discussed is that non-oogamous mode of reproduction could have evolved in Ardissonea (and possibly in other Toxariids) independently of the pennate lineage of diatoms.

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.

Strains of Mallomonas mangofera var. mangofera and M. mangofera var. foveata from Russia and Vietnam were studied. Their phylogenetic position was revealed using the SSU rDNA + rbcL cpDNA dataset. The ultrastructure of scales and bristles was studied using transmission and scanning electron microscopy. Based on these studies, a new combination was proposed: Mallomonas foveata comb. et stat. nov. The problems associated with the identification of Mallomonas mangofera and determination of the scale morphotype belonging to this taxon are discussed, and a new emended formal description and holotype are proposed. Based on the literature data and metabarcoding results, the distribution of Mallomonas mangofera and Mallomonas foveata was revised. Mallomonas mangofera has been recorded mainly in the temperate zone of Eurasia, North America and Japan. Mallomonas foveata has a wider distribution and is more often observed in regions with a warm climate.