Description of Two Novel Coccoid Cyanobacterial Species, Aliterella oplontina and Aliterella pliniana, from the Archeological Site of Oplontis (Naples, Italy)

The cyanobacterial genus Aliterella (Chroococcidiopsidales) comprises coccoid taxa with a broad geographic and ecological distribution, occurring in habitats ranging from polar environments to arid deserts and tropical regions. In this study, we describe two novel species isolated from phototrophic biofilms colonizing the columns of the portico of Villa Poppea at the archaeological site of Oplontis (Naples, Italy), representing the first record of the genus from the Mediterranean Basin. The strains were investigated using a polyphasic approach combining morphological observations, transmission electron microscopy (TEM), 16S rRNA gene and 16S–23S ITS sequencing, phylogenetic analyses, and comparative ITS secondary structure reconstruction. The results support the establishment of Aliterella oplontina sp. nov. and Aliterella pliniana sp. nov. Aliterella oplontina is characterized by irregular colonies emb edded in a thick mucilaginous sheath, small oval cells, and a phylogenetic position closely related to A. antarctica. Aliterella pliniana forms compact colonies composed of cylindrical cells and occupies a distinct evolutionary lineage closely associated with A. diffluens and A. bergstromii. Genetic distance analyses based on the 16S rRNA gene confirm the placement of both taxa within Aliterella while supporting their recognition as separate species. Comparative analyses of the D1D1′ and Box-B secondary structures of the ITS region provided additional diagnostic characters and revealed shared structural patterns among species groups within the genus. Aliterella oplontina possesses a unique combination of ITS features, whereas A. pliniana shares several structural affinities with A. diffluens and A. bergstromii. These findings significantly expand current knowledge of the diversity, phylogenetic relationships, and biogeographical distribution of Aliterella, while further demonstrating the value of an integrative polyphasic approach for cyanobacterial taxonomy.