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Article

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

by
Maya Stoyneva-Gärtner
1,
Georg Gärtner
2 and
Blagoy Uzunov
1,*
1
Department of Botany, Faculty of Biology, Sofia University “St. Kliment Ohridski”, 8 Blvd. Dragan Tsankov, 1164 Sofia, Bulgaria
2
Institut für Botanik der Universität Innsbruck, Sternwartestrasse 15, 6020 Innsbruck, Austria
*
Author to whom correspondence should be addressed.
Microbiol. Res. 2025, 16(9), 204; https://doi.org/10.3390/microbiolres16090204
Submission received: 22 June 2025 / Revised: 2 September 2025 / Accepted: 11 September 2025 / Published: 13 September 2025
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Abstract

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.

1. Introduction

Thermal algae live in aquatic habitats with high water temperatures—harsh biotopes that are inimical for many other organisms [1,2,3,4,5]. These geothermally heated ecosystems with a specific ionic water content are also geographically isolated, closed and stable, preventing species exchange, which creates the conditions for the development of a unique microbiota [6,7,8]. Many thermal species are rare, often endemic and as such threatened and important for nature conservation (e.g., [9,10,11]). As typical extremophiles [12,13] that are well adapted to their specific biotopes [14], thermophilic algae are also gaining increasing attention as suppliers of novel valuable bioactive compounds in modern environmentally friendly biotechnologies (e.g., [15,16,17,18]). Currently, thermophilic algae are also considered important for sustainable development and helpful in combating global climate changes [8]. However, even after the application of modern genetic methods, the taxonomy and biodiversity of thermal algae have not been fully explored worldwide [8,19,20,21].
Important components of this biodiversity, of which many species and genera have been described from various hot springs, are prokaryotic blue-green algae (Cyanoprokaryota/Cyanobacteria) (e.g., [10,22,23,24,25,26,27]). They are ancient phototrophs that are the pioneer colonisers of these biotopes and promote the further colonisation of other organisms [24]. Blue-greens thrive at temperatures above 70 °C [8,20,26,28,29,30,31,32,33] and, more rarely, can even live at 90 °C [34]. The best-studied thermophilic complexes to date are those from Yellowstone National Park, which harbour a wide variety of prokaryotic algae [6,20,21,33,35,36].
The ability to survive at high temperatures is polyphyletic within the Cyanoprokaryota [37,38]. This is reflected in the great diversity of thermophilic species with different morphological types—from coccoid unicellular and colonial forms to various branched or unbranched, non-heterocytic or heterocytic filamentous algae. The best-studied coccoid cyanoprokaryotes mostly belong to the genera Synechococcus and Thermosynechococcus, which form two very deep branches near the base of the phylum [19,39,40]. The taxonomically relatively poorly studied genus Asterocapsa, comprising 31 species and four varieties, is sparsely distributed in various parts of the world on rock surfaces [41] or, much more rarely, in the benthos of stagnant or slow-flowing freshwaters (e.g., [41,42,43,44,45]), but has never been recorded in thermal habitats. The present paper reports the first finding in the thermal system of the only European continental geyser (101 °C), located in the town of Sapareva Banya in Bulgaria, and provides a description of the new species, i.e., Asterocapsa thermalis sp. nov. Due to its unique character, this geyser has been included in the first Red List of Bulgarian wetlands with the status Vulnerable [46,47]. Considering the rapid remodelling of its thermal system by the local authorities and the resulting changes in the species’ habitat, we propose to classify the newly described species in the Critically Endangered category and to include it in the Red List of Bulgarian microalgae [48].

2. Materials and Methods

The geyser Saparaeva Banya is situated in south-west Bulgaria (Figure 1) at the foothills of Rila Mt. (42°17′16″ N 23°15′27″ E) at an altitude of 741 metres above sea level. It is described under number IBW 8810 in the Inventory of the Bulgarian Wetlands [47].
This active continental geyser, unique in Europe, with a temperature of 101 °C erupted after an earthquake in September 1999 and since then has been pulsating regularly every 5–6 s with steam and water rising 3–5 m high. This natural geyser existed in its original state for less than a year [47]. It was then captured and turned into a park attraction in the form of a fountain, where the water and steam partially evaporate through a tube that crosses two metal fountain lavers one above the other (Figure 2a,b). Some of the steam and hot water were channelled into pipes located in a small room below the main fountain and used for domestic purposes and the municipal heating system (Figure 2c,d). The geyser was seen in this state by two of the authors of this paper (MSG and GG) on 21 May 2003 and later, on 11 May 2006, by all authors when the material discussed in this paper was collected (Figure 2b). On our next visit on 21 July 2008, we found a newly constructed “theatre” around the geyser and a remarkable remodelling of both geyser parts: (1) in the upper part, a circular stone foundation and an “ancient” statue were installed, the old metal pipe and fountain lavers were removed and replaced by a new spherical stone for steam and water release; (2) in the lower part, more pipes and complicated technical constructions could be seen (Figure 2e–h). On 6 August 2024, we found the geyser installation redesigned again, with a newly installed metal structure on a new thick, circular stone base, in which a main metal pipe crosses two new metal fountain lavers (Figure 2i,j).
The samples for this work were collected on 11 May 2006 from the bottom of the lowest and largest geyser fountain laver (Figure 2b) and from the underwater part of the stone bottom of a nearby fountain, situated about 25 m away, with flowing water fed by the geyser (Figure 3a). These surfaces were covered with similar dark green slimy algal mats that were completely and permanently covered by hot water (Figure 3b). Qualitative samples from the mats were taken by scraping with a scalpel [50], which was washed in 70% ethanol each time to avoid contamination of the samples.
The collection was performed for floristic purposes by scraping these mats from both thermal sites (Table 1) with a scalpel, which was washed each time in 70% ethanol for disinfection. The samples were immediately fixed in 2–4% formalin and are still in this state in the algological collection of the Department of Botany of the University “St. Kliment Ohridski” in Sofia. On 21 July 2008 and 6 August 2024 (Table 1), the site was revisited to collect live material, and despite the observed changes in the thermal system, samples were taken from the algal mats in the geyser fountain basin and the closely connected fountain (Figure 3c).
The light microscopic processing was carried out immediately in 2006 and 2008. It was repeated on the same material in 2012 using non-permanent slides and staining with Gentian Violet on a Motic BA 400 microscope (Motic, Xiamen, China). In this study, the same samples and the material from 2024 were analysed in the same way using the Motic B1 microscope equipped with a Moticam 2.0 mp camera (Motic, Xiamen, China). For photographic documentation, the Motic Images 2 Plus software programme was used, without additional processing of the images.
All species identification methods are standard, based on the classical algological taxonomic literature on Cyanoprokaryota [51,52,53,54,55,56,57,58,59] with searches in relevant papers on the description and findings of Asterocapsa (e.g., [42,43,44,45,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]) and the globally recognised Algaebase [41].
Assignment of the conservational status followed the method proposed for sozological assessment of microalgae, which allows quantitatively based classification of microalgae using the total sum (T) of the assessment of seven criteria (labelled by the capital letters A-G, each with values from 1 to 4) to the main conservationally important categories—Extinct, Critically Endangered, Endangered, Near Threatened and Data Deficient [77].

3. Results

The algal mats consisted mainly of the fine, straight or slightly coiled filaments identified under the light microscope as Leptolyngbya thermalis Anagnostidis (Figure 4a), with some other cyanoprokaryotes lying between or above them.
In all samples collected in 2006 from the main geyser fountain and in one of the samples, taken on the same day from the nearby geyser-fed fountain with flowing waters, one of these additional components was relatively more abundant. This was a small, coccal colonial cyanoprokaryote whose cells were embedded in distinct, delimited, firm, colourless, transparent mucilage envelopes (Figure 4b–l). The mucilage was lamellate and covers each cell, as well as the entire colony (Figure 4b–l). The surface of the mucilage layers was covered with numerous short, rounded and evenly distributed projections. This warty structure was better visible after staining the microscopic slides with Gentian Violet (Figure 4j,l–p and Figure 5a–h).
Cells were solitary (Figure 4b and Figure 5a,e) or formed few-celled colonies up to 15–25 µm in diameter (Figure 4f,j–l). Sometimes the colonies were accumulated in larger agglomerations (Figure 4c,g–i,m,p). The cells were regularly and relatively densely arranged in the colonies (Figure 4l,p).
The cells were blue-green, oval, round when viewed from the top, 2–2.5 (3)x4 µm (Figure 4b–o and Figure 5a–e). At lower magnifications, the cell content looked homogenous, but under immersion, large cyanophycin (?) granules were seen in the cells (Figure 5a–h). The division was in twos (Figure 5f,h), running in a consecutive manner (Figure 5g).
Due to its peculiar warty mucilage structure, the alga strongly resembled the distinct genus Asterocapsa, which was described from wet rocks in China [59]. However, the specific ecology of the alga from Sapareva Banya, which was found in the benthos of thermal waters with temperatures of 90–100 °C, contrasted with the original description of this aerophytic genus. Therefore, despite the lack of living material and relevant molecular-genetic data, considering the combination of the morphology typical of the genus with the extremophilic lifestyle, which differs from the other species of the genus, we found it appropriate to assign our material to the genus Asterocapsa, but to describe a new species, Asterocapsa thermalis, emphasising its thermophilic character. The new species, like the genus, belongs to the order Chroococcales, class Cyanophyceae, and phylum Cyanoprokaryota. The combination of the most important diagnostic features (cell size, mucilage colour, sculpture and organisation, and habitat type [55]) clearly distinguishes it from the few other species of the same genus described from aquatic habitats [42,72] and from the type species [59]—see Table 2. The same applies to other characters such as cell colour, shape and content, arrangement of cells in colonies, size of colonies and predominant consecutive mode of reproduction (Table 2). Furthermore, regarding the rarity of the species and the vulnerability of its unique habitat, we propose to include it in the Red List of Bulgarian microalgae [48] with the Critically Endangered conservation status, as presented in the following text.

3.1. Species Diagnosis

Latin diagnosis:
Asterocapsa thermalis Stoyneva-Gärtner M. P., Gärtner G. et Uzunov B., sp. nov.
Cellulae ovales, rotundae desuper, 2–2.5 (3)x4 µm, inclusae in involucris mucilaginis auctis stratificatis bene conspicuis, superficies mucillaginis verrucas gerit, post tinctionem melius conspicuae. Cellulae caeruleo-viridis, cum granulis magnis (?cyanophycinum), sub immersio olei conspicuis. Divisio est per fusionem binariam, consecutive repetitam, ita parvas colonias formans, mucilago communi obducta, quae saepe multiplices agglomerationes multiplices accumulant. Species descriptioni generis respondet membrana involucrorum mucilaginosorum verrucosorum, sed a ceteris speciebus generis differt praecipue vita extremophilica, magnitudine cellularum et coloniarum minoribus, et praesentia involucrorum mucilaginosorum incolorum, pellucidorum, projectionibus brevibus et rotundatis (verrucis) tectorum.
Iconotypus: Fig. nost. 5 (a,g,h).
Typus locus: Lacus aquae systematis thermalis (cum temperaturis 90–100 °C) geyseris Sapareva Banya, Bulgariae, Europae (42°17′16″ N 23°15’27″ E) in altitudine 741 metrorum supra mare.
Hab.: In aquis thermalis, coloniae benthicae.
Collectores: Stoyneva-Gärtner M. P., Gärtner G. et Uzunov B.
Collectio diem: XI Maii MMVI
Etymologia: Species epitheti thermalis ad peculiarem modum vivendi in aquis thermis.
Species in collectione exemplorum algarum Universitatis Sofiensis “St. Clementis Ohridski” deposita est (exempla GSP1-GSP6/06052006).
English diagnosis
Asterocapsa thermalis Stoyneva-Gärtner M. P., Gärtner G. et Uzunov B., sp. nov.
Description: Cells oval, round when viewed from above, 2–2.5 (3)x4 µm, embedded in stratified well-visible enlarged mucilage envelopes, the surface of mucilage layers bears warts, better visible after staining. Cells are blue-green, with large cyanophycin (?) granules, better visible under oil immersion. Division is through binary fusion, repeated consecutively, thus forming small colonies, covered with a common mucilage, often clustering together to form large, complex agglomerations. The species corresponds to the genus description by its warty mucilage envelope membranes but differs from the other species of the genus mainly by its extremophilic lifestyle, smaller cell and colony sizes and the presence of colourless, transparent mucilage envelopes covered with short and rounded projections (warts).
Iconotype: Figure 5a,g,h.
Type locality: Water basins of the thermal system (with temperatures 90–100 °C) of the geyser Sapareva Banya, Bulgaria, Europe (42°17′16″ N 23°15′27″ E) at height 741 m a.s.l.
Habitat: Benthos of thermal waters.
Collection date: 11 May 2006.
Collected by: Stoyneva-Gärtner M. P., Gärtner G. and Uzunov B.
Etymology: The species epithet thermalis refers to the specific mode of life in thermal waters.
Species is deposited in the algal sample collection of Sofia University “St Kliment Ohridski” (samples GSP1-GSP6/06052006).

3.2. Suggested Conservation Status

The conservation status of the species was assessed using the seven criteria relating to localities, habitats and general ecology of the species (A–G) and their numerical expression (4–1) proposed for the assessment of the endangered status of microalgae [77]. As the species was found in a single location (A4) in a threatened habitat (B4), belonged to a single ecological group of Thermophyton (C4), but did not occur in an area of conservation importance, such as a national park (D1), and was classified as a local endemic (E4) and globally rare species (F4) according to current knowledge, giving it a high expert weight (G4), it received 25 points (T25). These points classify it as Critically Endangered with the sozological formula CR—A4 B4 C4 D1 E4 F4 G4 T25.

4. Discussion

The discovery of a coccal prokaryotic alga with a strikingly peculiar morphology and warty mucilage envelope in a unique habitat, such as the only continental geyser in Europe, allows us to describe a new species of the genus Asterocapsa, previously known only from conventional aerophytic and freshwater biotopes. Besides the warty mucilage, reasons for the affiliation of the material to Asterocapsa were found in the coccal morphology, the oval cells, the colonial organisation with individual cell polysaccharide lamellar envelopes and the common colonial mucilage with a distinct margin, as well as in the consecutive division of the cells with the formation of colonies, usually consisting of a small number of cells, but which can accumulate in groups and form larger agglomerations between the filamentous cyanoprokaryotic mats.
It should be noted that the lack of data on Asterocapsa from thermal waters prevented immediate identification. Therefore, after collection and initial identification in 2006, all samples were separated for further processing and more literature on the subject was checked. Six years later, after repeated microscopic examination of the same samples from 2006, the material was again assigned to the genus Asterocapsa, but for the same reasons its identification down to species level was postponed. In the meantime, we also investigated another thermal system in Bulgaria [78] and conducted two more samplings in 2008 and 2024 in the geyser system to try to collect living colonies. However, we did not find them again. The literature search confirmed previous data on the occurrence and distribution of most species of the genus as epiliths on wet or dry rocks (e.g., [41,60,64,69]), walls of caves, buildings or temples [62,63,70,71,72,74,75] or as epiphytes on tree bark [67]. Fewer species have been found in slow-flowing or stagnant, temporary or permanent freshwater bodies such as swamps, lakes and channels (e.g., [41,42,43,44,45,73,76]).
Although the cells and colonies found in the geyser system resemble the genus and its type species due to the warty mucilage, they clearly differ in most main and secondary diagnostic characteristics from the type species Asterocapsa gloeocystifrmis, described from wet rocks of China [59] (Table 2). The material found by us among the underwater Leptolyngbya mats, resembled Asterocapsa submersa Azevedo, Sant’Anna, Senna, Komárek et Komárková by the benthic mode of life on algal mats formed by the filamentous cyanoprokaryote genus Phormidium in periodically flooded shallow channels in Brazil [42]. However, the mucilage of this Brazilian species with larger cells and colonies varied from colourless to pink and golden brown, smooth to slightly granulated, whereas the cells and colonies in our material were smaller and the mucilage was always colourless and transparent, warty. In addition, the cells in our material differ greatly in colour, cell content organisation and arrangement of the cells in the colonies (Table 2). A similar mode of life in benthic cyanoprokayote mats, but in oligotrophic marshes in northern Belize, was outlined for Asterocapasa belizensis Komárek et Komárková-Legnerová and A. stagnina Komárek et Komárková-Legnerová, as well as for an unidentified species of the genus [73]. However, all these species showed different morphology in terms of mucilage (warts only in young stages of A. stagnina) and colour (from colourless to yellowish, yellow-orange and brown in A. belizensis and A. stagnina, only lamellar but wartless in Asterocapsa sp.), the position of the cells in the colonies (biscuit-like, concentrated in the centre in A. stagnina) and much larger cells and colonies in all these species with slightly granulated cell contents and sometimes visible chromatoplasm (Table 2). In addition, the four species from North (Central) and South America differed markedly from our extremophilic material due to their development in conventional freshwaters. The only species for which an extremophilic character could be surmised was Asterocapsa rupivolcanica H. X. Xiao [68]. Although volcanic life was not explicitly stated in the species description and in the locality description, the epithet rupivolcanica of this species, described from steep cliffs in China, indicates a possible development in volcanic terrain, which can be regarded as a certain similarity to our material. However, the red to orange-golden coloured mucilage of this species and the larger cell and colonial dimensions (4.5–5.5 × 6.5–8 µm and 20–70 µm, respectively), combined with an almost homogeneous cell content with small granules of the global to nephroid and even triangular cells, clearly distinguish it from our material, which had a transparent, colourless mucilage and much smaller cells with some large cell inclusions.
The importance of ecological characteristics and mode of life was emphasised in the instructions for species identification in the first volume on Cyanoprokaryta of the Central European freshwater flora [56]. There it was underlined that in the case of an ecological discrepancy, the name from the manual must not be assigned to the species to be determined. In view of this statement and in addition to the observed morphological peculiarities, we found a reason to describe a new species, Asterocapsa thermalis sp.n., which is mainly characterised by its extremophilic lifestyle in a unique habitat, but also by a number of cyto-morphological and reproductive features, which clearly distinguish it from previously described species. Furthermore, to date, no Asterocapsa strain has been studied in culture [73], no molecular data have been obtained on the genus and its species, and all descriptions are based only on features observed by conventional light microscopy (e.g., [42,43,44,45,56,60,73]).
In recent years, the geyser system where the new species was found has undergone many transformations that have led to unfavourable changes in the algal habitat and the search for the species after 2005 was not successful. Due to the short period of time since the species was discovered, we do not declare it extinct here, but classify it as Critically Endangered and propose its inclusion in the Red List of Bulgarian microalgae. We recommend further targeted search for this rare and interesting alga in other thermal systems in Bulgaria and abroad with the idea of cultivating it and conducting molecular genetic studies as suggested in modern polyphasic studies on cyanoprocaryotes [79,80,81].

Author Contributions

Conceptualisation, M.S.-G.; methodology, M.S.-G., G.G., B.U.; investigation, M.S.-G.; resources, M.S.-G., G.G., B.U.; writing—original draft preparation, M.S.-G.; writing—review and editing, B.U.; visualisation, M.S.-G.; supervision, G.G.; project administration, B.U.; funding acquisition, B.U. All authors have read and agreed to the published version of the manuscript.

Funding

This study is financed by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project No BG-RRP-2.004-0008, part 3.1.11 Algology. The APC was funded by the same project.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

The authors wish to remember and acknowledge the late Ivan Ivov Stoynev (21 April 1988–29 July 2024) for his logistical support during the first visit by MSG and GG of the Geyser Sapareva Banya on 21 May 2003.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Map of Europe with Bulgaria in dark green; (b) map of Bulgaria with location of the unique European continental geyser in the town Sapareva Banya (green dot) (modified after [49]).
Figure 1. (a) Map of Europe with Bulgaria in dark green; (b) map of Bulgaria with location of the unique European continental geyser in the town Sapareva Banya (green dot) (modified after [49]).
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Figure 2. Geyser Sapareva Banya on11May 2006: (a) general view of the pulsating geyser; (b) sampling from the lower fountain laver; (c) general view with the undergeyser construction; (d) close view of the undergeyser room with a pipe system. Geyser on 21 July 2008: (e,f) geyser reconstructed, with added circular stone fundament, new spherical stone for steam release and “ancient” statue in its upper part; (g) general view with the undergeyser room; (h) the visitor “theatre”. Geyser on 6 August 2024: (i) general view of the upper part of the geyser; (j) view with the undergeyser room.
Figure 2. Geyser Sapareva Banya on11May 2006: (a) general view of the pulsating geyser; (b) sampling from the lower fountain laver; (c) general view with the undergeyser construction; (d) close view of the undergeyser room with a pipe system. Geyser on 21 July 2008: (e,f) geyser reconstructed, with added circular stone fundament, new spherical stone for steam release and “ancient” statue in its upper part; (g) general view with the undergeyser room; (h) the visitor “theatre”. Geyser on 6 August 2024: (i) general view of the upper part of the geyser; (j) view with the undergeyser room.
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Microbiolres 16 00204 g003
Figure 3. Stone fountain in the geyser thermal system, from the bottom of which the underwater green algal mats were collected: (a,b) view of the fountain and bottom mats on 11 May 2006; (c) view of the fountain on 21 July 2008.
Figure 3. Stone fountain in the geyser thermal system, from the bottom of which the underwater green algal mats were collected: (a,b) view of the fountain and bottom mats on 11 May 2006; (c) view of the fountain on 21 July 2008.
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Figure 4. Microphotos of selected thermal algae from the fountain system of the Geyser Sapareva Banya: (a) Leptolyngbya thermalis—coiled and straight filaments from the algal mats in the main geyser fountain; (b) single cell of Asterocapsa thermalis sp. n. (arrow) with warted mucilage between the Leptolyngbya filaments in a sample from the main geyser fountain; (c) agglomerations of single cells and few-celled colonies of Asterocapsa thermalis among Leptolyngbya filaments from the bottom of the fountain feed by the geyser waters; (d–p) Asterocapsa thermalis (single cells, colonies, agglomerations) from the main geyser fountain observed directly under light microscope (d–i,k) and after staining with Gentian Violet (j,l,m–p). Different levels of the same colony are shown as observed on different light microscope focus (g,h). Scale bar 10 µm (a) is the same for all microphotos.
Figure 4. Microphotos of selected thermal algae from the fountain system of the Geyser Sapareva Banya: (a) Leptolyngbya thermalis—coiled and straight filaments from the algal mats in the main geyser fountain; (b) single cell of Asterocapsa thermalis sp. n. (arrow) with warted mucilage between the Leptolyngbya filaments in a sample from the main geyser fountain; (c) agglomerations of single cells and few-celled colonies of Asterocapsa thermalis among Leptolyngbya filaments from the bottom of the fountain feed by the geyser waters; (d–p) Asterocapsa thermalis (single cells, colonies, agglomerations) from the main geyser fountain observed directly under light microscope (d–i,k) and after staining with Gentian Violet (j,l,m–p). Different levels of the same colony are shown as observed on different light microscope focus (g,h). Scale bar 10 µm (a) is the same for all microphotos.
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Figure 5. Microphotos of Asterocapsa thermalis sp. n. from the main fountain of the Geyser Sapareva Banya under immersion after staining with Gentian Violet: single cells with warty delimited mucilage (a,d,e) and young few-celled colonies, formed after binary division, with visible individual and common colonial mucilage with short, wart-like projections (b,c,f–h). Scale bar 5 µm (a) is the same for all microphotos.
Figure 5. Microphotos of Asterocapsa thermalis sp. n. from the main fountain of the Geyser Sapareva Banya under immersion after staining with Gentian Violet: single cells with warty delimited mucilage (a,d,e) and young few-celled colonies, formed after binary division, with visible individual and common colonial mucilage with short, wart-like projections (b,c,f–h). Scale bar 5 µm (a) is the same for all microphotos.
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Table 1. Sampling data and number of samples taken from the bottom mats of the main fountain of the Sapareva Banya geyser (MFSP) and the closely connected fountain with flowing water (FFW).
Table 1. Sampling data and number of samples taken from the bottom mats of the main fountain of the Sapareva Banya geyser (MFSP) and the closely connected fountain with flowing water (FFW).
DateMFSPFFW
11 May 200642
21 July 200842
6 August 202442
Table 2. Comparison of the cyto-morphological, reproductive and ecological characters of Asterocapsa thermalis sp. n. from the Geyser Sapareva Banya with the type species [59] and described aquatic species of the same genus [42,72]. The terminology used strictly follows the English texts of the relevant authors.
Table 2. Comparison of the cyto-morphological, reproductive and ecological characters of Asterocapsa thermalis sp. n. from the Geyser Sapareva Banya with the type species [59] and described aquatic species of the same genus [42,72]. The terminology used strictly follows the English texts of the relevant authors.
SpeciesCell Shape, Colour, Dimensions and Arrangement, Colony DimensionsCell
Content		Mucilage
Envelopes		ReproductionHabitat
and Continent		Reference
Asterocapsa
thermalis sp. n.		Cells oval; blue-green;
2–2.5 (3)x4 µm; regularly and densely arranged in the colonies; colonies small, up to 15–25 µm, often forming larger aggregations		With large cyanophycin (?) granulesColourless and transparent in all stages; lamellate, with regular, short rounded projections (warts)Binary
consecutive
division		Benthic in Leptolyngbya thermalis mats in
a geyser thermal system;
Europe		This paper
Asterocapsa gloeocystiformis Chu
(type species)		Cells spherical to oblong;
olive-green, bright blue-green or
brownish green;
12–16 × 7 µm;
mature colonies spherical; colonies
70–250 µm 		Homogenous or finely granulated (?pseudovacules)Colourless, brownish or reddish; very thick, firm, lamellated or not; irregular short or long, minute or stout warts on the surface Binary division in two or three
planes, fragmentation of colonies, aplanospores		Aerophytic on wet rocks;
Asia		[60]
Asterocapsa submersa Azevedo, Sant’Anna, Senna, Komárek et Komárková Cells spherical to subsphaerical; dark blue-green;
6.4–9.1 µm
in diameter (colonies up to 54 µm); aggregated in the centre of the colony		HomogenousColourless to pink and golden brown; firm; slightly lamellated; smooth or slightly granular
at the surface		Binary fusion in different planes, fragmentation of the colonies or their disintegration in solitary
cells		Benthic in Phormidium mats in periodically flooded channels;
South America		[42]
Asterocapasa belizensis Komárek et Komárková-LegnerováCells oval to irregularly oval; olive green or bright blue-green; 7–12 µm long or in diameter; spherical colonies 70–130 (280) µm
in diameter; cells irregularly but +/− evenly arranged in colonies, in small groups 		Slightly granular content; sometimes visible chromatoplasmColourless to yellowish, yellow-orange and brown; usually ±lamellatedDisintegration of colonies or through escaping of small cell groups through an opening in the slime marginBenthic, not common in cya-noprokayote
mats in low conductivity marshes and rare in limestone-based marshes;
North America 		[73]
A. stagnina Komárek et Komárková-LegnerováCells spherical, hemispherical to irregularly spherical; pale greyish blue; 3.6–6.2 µm in diameter; sphaeroid colonies, 30–70 (rarely up to 160) µm; biscuit-like arrangement of cells, concentrated in the centre of the colonySlightly granulated; sometimes visible chromatoplasmIntensily yellow brownish; lamellated; warts only in young stagesIrregular cell division, mainly libe-ration of small cells and sub-co-lonies after
splitting of the firm envelopes 		Benthic, rare and solitary dispersed in cyanoprokaryote mats in low to medium conductivity marshes;
North America		[73]
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Stoyneva-Gärtner, M.; Gärtner, G.; Uzunov, B. Asterocapsa thermalis sp. nov. from the Unique European Continental Geyser in Sapareva Banya (Bulgaria). Microbiol. Res. 2025, 16, 204. https://doi.org/10.3390/microbiolres16090204

AMA Style

Stoyneva-Gärtner M, Gärtner G, Uzunov B. Asterocapsa thermalis sp. nov. from the Unique European Continental Geyser in Sapareva Banya (Bulgaria). Microbiology Research. 2025; 16(9):204. https://doi.org/10.3390/microbiolres16090204

Chicago/Turabian Style

Stoyneva-Gärtner, Maya, Georg Gärtner, and Blagoy Uzunov. 2025. "Asterocapsa thermalis sp. nov. from the Unique European Continental Geyser in Sapareva Banya (Bulgaria)" Microbiology Research 16, no. 9: 204. https://doi.org/10.3390/microbiolres16090204

APA Style

Stoyneva-Gärtner, M., Gärtner, G., & Uzunov, B. (2025). Asterocapsa thermalis sp. nov. from the Unique European Continental Geyser in Sapareva Banya (Bulgaria). Microbiology Research, 16(9), 204. https://doi.org/10.3390/microbiolres16090204

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