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Cyanidium chilense (Cyanidiophyceae, Rhodophyta) from tuff rocks of the archeological site of Cuma, Italy

Phlegrean Fields is a large volcanic area situated southwest of Naples (Italy), including both cave and thermoacidic habitats. These extreme environments host the genus Cyanidium; the species C. chilense represents a common phototrophic microorganism living in anthropogenic caves. With a view to provide a comprehensive characterization for a correct taxonomic classification, morpho‐ultrastructural investigations of C. chilense from Sybil's cave (Phlegren Fields) was herein carried out and compared with the thermoacidophilic C. caldarium. The biofilm was also analyzed to define the role of C. chilense in the establishment of a biofilm within cave environments. Despite the peculiar ecological and molecular divergences, C. chilense and C. caldarium shared all the main diacritic features, suggesting morphological convergence within the genus; cytological identity was found among C. chilense strains geographically distant and adapted to different substrates, such as the porous yellow tuff of Sybil cave, and calcyte, magnesite and basaltic rocks from other caves. C. chilense is generally dominant in all biofilms, developing monospecific islets, developing both superficially or between fungal hyphae and coccoid cyanobacteria. Extracellular polymeric substances (EPS) were recorded in C. chilense biofilms from Sybil cave, confirming the role of EPS in facilitating cells adhesion to the surface, creating a cohesive network of interconnecting biofilm cells.