Asterocapsa thermalis sp. nov. from the Unique European Continental Geyser in Sapareva Banya (Bulgaria)

Thermal algae are extremophilic organisms that live in one of the harshest environments in the world and thrive in waters with temperatures of up to 90 °C. They have gained attention due to their special ecological adaptations, their great biotechnological potential and their recently recognised role in combating global climate change and achieving sustainable development. However, the biodiversity of these algae is far from being fully explored. The article presents the first finding of the prokaryotic genus Asterocapsa (Chroococcales, Cyanophyceae, Cyanoprokaryota/Cyanobacteria) in thermal waters and describes a new species from the fountain basins in the thermal system of the only continental European geyser (101 °C) in the town of Sapareva Banya (south-west Bulgaria). This species is not only one of the few aquatic representatives of this generally aeroterrestrial genus, but is also characterised by its extremophilic lifestyle and differs clearly from the type species and other aquatic species of the genus due to its morphological characteristics. These include the smaller dimensions of the cells and colonies, as well as the colourless, transparent, but always lamellar and regularly verrucous mucilage envelopes. The unique locality of this alga is highly endangered and was included in the first Red List of Bulgarian wetlands. Due to human activities and changes in the geyser system, we have detected some unfavourable changes in the algal habitat and therefore propose to add the newly described species to the Red List of Bulgarian Microalgae with the status Critically Endangered.