A freshwater dwelling, tapering, heterocytous cyanobacterium (strain V13) was isolated from an oligotrophic pond in the Shrirampur taluka, Ahmednagar district of Maharashtra in India. Initial morphological examination indicated that strain V13 belonged to the genus Calothrix. Subsequent molecular and phylogenetic assessment based on 16S rRNA gene, led us to describe the freshwater/terrestrial clade of Calothrix strains without terminal hairs as a new genus Dulcicalothrix gen. nov., with the type species Dulcicalothrix necridiiformans sp. nov. (Strain V13) on the basis of the necridia forming ability of the strain. Also, the 16S-23S ITS secondary structure analysis clearly differentiated strain V13 from the other members of the clade. Past studies and the current state of knowledge makes it imperative to separate the groups Calothrix (marine/freshwater Calothrix), Macrochaete and Dulcicalothrix (freshwater/terrestrial Calothrix) into separate genera in accordance with the International Code of Nomenclature for Algae, Fungi and Plants. Robust phylogenetic evidence and previous reports strongly support the re-erection of the family Calotrichaceae distinct from the existing family Rivulariaceae.

Diatoms inhabiting terrestrial habitats that are affected by thermal activity remain poorly studied, despite significant interest in the biodiversity of hot springs. The Kamchatka peninsula is characterized by the presence of 30 active volcanoes associated with hydrotherms. Our study involved a survey of diatom diversity in soils around the Malki, Upper Paratunka, and Dachnie thermal springs on the Kamchatka peninsula. A total of 49 diatom taxa were found. The genera Pinnularia, Planothidium, Fragilariforma, Epithemia, Halamphora, Gomphonema, Nitzschia, Aulocoseira, Sellaphora, Surirella, and Navicula were the most common. Pinnularia cf. subcapitata and Planothidium lanceolatum were dominant in all springs. Diatom communities in the soils near the thermal springs included both aquatic and terrestrial species, which may reflect the transitional nature of habitats at the borders of hot springs and soils. To gain a better understanding of the diversity of diatom communities in soils near thermal springs, broader worldwide studies are necessary.

Volcanic activity has a great impact on terrestrial ecosystems, including soil algae in general and diatoms in particular. To understand the influence of volcanoes on the biodiversity of diatoms, it is necessary to explore the flora of these microorganisms in regions with high volcanic activity, which includes the Kamchatka peninsula. During the study on diatoms in the soils of Mutnovsky and Gorely volcanoes of Kamchatka, 38 taxa were found. The Mutnovsky volcano diatom flora was more diverse and accounted for 35 taxa. Eunotia curtagrunowii, Humidophila contenta, and Pinnularia borealis were the dominant species. In the Gorely volcano, only 9 species were identified, with Caloneis bacillum and Pinnularia borealis prevailing in the samples. Overall, the genera Pinnularia and Eunotia were the most diverse in the studied area. The diatom flora of the studied volcanoes comprises mostly cosmopolitan small-sized taxa with a wide range of ecological plasticity. Our data confirm the high adaptive potential of diatom algae and add new knowledge about the ecology and biogeography of this group of microorganisms.