Genotypic and molecular characterization of a moderately thermophilic cyanobacterium, Gloeocapsa sp. strain BRSZ

A unicellular-colonial cyanobacterium, designated "BRSZ," was isolated from a neutral-alkaline hot spring in Thailand. Morphological characterization revealed distinctive features consistent with those of the genus Gloeocapsa. Physiological assessments demonstrated that BRSZ is a moderately thermophilic and halotolerant cyanobacterium with the potential for chemoheterotrophic growth in dark conditions. Molecular phylogenetic analysis based on 16S ribosomal RNA (rRNA) gene sequences placed BRSZ within a well-defined Gloeocapsa clade, a finding corroborated by 16S-23S internal transcribed spacer (ITS) rRNA secondary structure analyses. Genome comparisons, including average nucleotide identity (ANI), genome-to-genome distance (GGD), and digital DNA-DNA hybridization (dDDH), between strain BRSZ and closely related taxa showed an ANI value of 95.45 %, near the lower boundary of the species delineation threshold (95-96 %). A GGD of 0.0374 (>0.0258) and dDDH of 69 % (<70 %) further supported genomic differentiation. Genome-based analysis revealed a mycosporine-like amino acid biosynthetic gene cluster likely involved in sunscreen compound production. Cultivation-based production of a UV-absorbing compound confirmed the functional relevance of this gene cluster. These findings expand the described diversity within the Gloeocapsa complex and enhance our understanding of the taxonomy of this group. In addition, they underscored the importance of hot spring environments as sources of novel extremophiles.