Sandococcus viridis gen. et sp. nov . (Chroococcaceae, Cyanobacteria) a novel coccoid cyanobacterium isolated from tropical marine environments

Mat-dwelling coccoid cyanobacteria are diverse and ecologically significant members of marine microbial communities. Despite their global presence, much of their diversity remains undocumented, particularly in tropical coastal environments. In this study, a coccoid cyanobacterial strain, KBR, was isolated from a microbial mat growing on concrete jetty surfaces on Pulau Perhentian Besar, an island in Terengganu, East Malaysia. The strain exhibited morphological features characteristic of the genus Chroococcus, forming spherical to hemispherical cells occurring singly or in dyads. However, 16S rRNA phylogenetic analysis revealed that strain KBR forms a distinct clade within the family Chroococcaceae. It was closely related to Chroococcus turgidus but separate from the core Chroococcus sensu stricto clade, i.e. genera Neochroococcus and Inacoccus. The 16S rRNA gene sequence of strain KBR shared less than 95% identity with type strains of other Chroococcaceae genera. In addition, strain KBR was morphologically distinct, displaying larger cell diameters (20.1–35.8 μm), olive-green pigmentation, weakly granulated cytoplasm and cell division in a single plane. Secondary structure analysis of the 16S-23S ITS region in the D1–D1′ helix further supported its taxonomic uniqueness. Based on a polyphasic approach, we propose that strain KBR represents a novel genus and species, Sandococcus viridis gen. et sp. nov. The establishment of this new taxon contributes to ongoing revisions in Chroococcaceae systematics and sheds light on the hidden cyanobacterial diversity of tropical marine ecosystems.