Microbiology & Virology

A Glimpse Into Darkness: Diversity Of Culturable Cyanobacteria, Green Algae And Fungi From Subaerial Cave Biofilms

By Keith Cowing

Status Report

Journal of Phycology

January 5, 2026

Filed under 16s rRNA, Biofilm, caves, Chalicogloea cavernicola, Chromochloris, cyanobacteria, genomics, Jenufa, Journal of Phycology, Oculatella subterranea, Phlebia, phytoparasite, Sample Return, saprophyte, Sporobolomyces, Stereum, subaerial biofilm, Timaviella karstica

A Glimpse Into Darkness: Diversity Of Culturable Cyanobacteria, Green Algae And Fungi From Subaerial Cave Biofilms — [LEFT] Photographs of cyanobacteria-dominated cave biofilms. (a) Nostoc sp. spheres. (b) Picosynechococcus sp. dominated blue-green biofilm on stones. (c) Aphanothece spp. dominated blackish biofilm. (d) Purple biofilm on stones dominated by Timaviella sp. and Oculatellales cyanobacteria. (e) Biofilm dominated by different Nostoc species. (f) Mixed biofilm. (g) Olive green patches of Stigonema sp. (h, i) Purple biofilms of Gloeobacter sp. (j) Biofilm dominated by Chalicogloea cavernicola associated with calcification. (k) Calcified trichomes of Scytonema julianum. (l) Calcified and dichotomously branched trichomes of Geitleria sp. (m) Chroococcalean cyanobacteria embedded in extracellular polymeric substances. (n) Cones made of a Nostocalean cyanobacterium. (o) Chroococcalean cyanobacteria embedded in extracellular polymeric substances with calcified trichomes of Geitleria sp. (p) Aphanothece spp. dominated blackish biofilm. (q) Mixed biofilm made of Gloeobacter sp. (purple), cf. Schizothrix sp. (green) and Stigonema sp. (whitish). (r) Purple biofilms of Gloeobacter sp. Scale bar 1 cm; (b, d, f) 10 cm. — Journal of Phycology [RIGHT] Micrographs of cyanobacteria-dominated cave biofilms. (a) Nostocalean cyanobacterium from cones. (b) Scytonema julianum with calcified sheaths. (c, d) Nostoc sp. (e) Stigonema sp. (f) Geitleria sp. with calcified Y-trichomes. (g, h) Gloeobacter sp. (i) Aphanothece sp. (j) Gloeothece sp. (k, l) Limnococcus sp. (m–r) various filamentous cyanobacteria. — Journal of Phycology

Caves and hypogean environments provide stable microclimates characterized by uniform temperatures, constant humidity, and low light levels.

In such sites, extremely low light irradiance can support the growth of subaerial biofilms (SABs) dominated by unique photosynthetic communities of cyanobacteria accompanied by chlorophytes, heterotrophic bacteria, and fungi.

This study aimed to apply a culture‐dependent approach to uncover the diversity of cyanobacteria, green algae, and fungi from SABs of a cave in Northern Spain. We isolated a total of 58 cyanobacteria grouped into 21 genera based on their 16S rRNA gene sequences; 24 green algae grouped into 10 genera based on their SSU rRNA gene sequences, and 41 fungi fell into 13 genera according to phylogenies based on the ITS rRNA region between the 16S and 23S rRNA genes (ITS rRNA region). The SABs were dominated by cyanobacteria, which also reflected by high diversity, including calcium‐carbonate sheath‐producing species such as Geitleria calcarea and Scytonema julianum.

Typical cave‐inhabiting species such as Chalicogloea cavernicola, Timaviella karstica or Oculatella subterranea were also isolated, alongside potentially new genera and species. Associated green algae were predominantly located closer to the cave entrance and included various lineages of the genera Jenufa and Chromochloris.

The fungal community directly derived from the SABs was dominated by decomposers, saprophytes, and phytoparasitic representatives such as Sporobolomyces, Stereum, and Phlebia, with a corresponding set of enzymes that were evaluated for all fungal isolates.

The results strongly support the hypothesis that specialized cyanobacterial communities are often located in caves as a result of niche specialization.

A glimpse into darkness: Diversity of culturable cyanobacteria, green algae and fungi from subaerial cave biofilms, Journal of Phycology via PubMed
A glimpse into darkness: Diversity of culturable cyanobacteria, green algae and fungi from subaerial cave biofilms, Journal of Phycology

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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