Thermophilic_CYANOBACTERIA
Thermal springs harbor diverse cyanobacteria with unique adaptations to extreme environments. Despite growing interest, genomic resources for thermophilic cyanobacteria remain limited, hindering our understanding of their ecological and metabolic potential. This study aims to characterize the genomic features of thermophilic cyanobacteria, with a focus on adaptations to high-temperature environments and their potential for producing secondary metabolites. We analyzed six genomes, including three newly described species and the first Icelandic Calothrix thermalis genome, and compared them with reference genomes from thermophilic and mesophilic cyanobacteria. Functional gene analysis, genome size comparisons, and biosynthetic gene cluster (BGC) mining were performed to assess metabolic and ecological adaptations. Genome sizes varied substantially, with Hillbrichtia pamiria showing a notably reduced genome, whereas C. thermalis and Thermoleptolyngbya hindakii were similar to or larger than related mesophiles. Secondary metabolite BGCs were generally reduced in thermophiles, especially in H. pamiria, although larger genomes retained diverse BGCs, including cyanotoxins, NRPS, RiPPs, and terpenes. Core functions, including the Calvin cycle and nitrogen fixation, were conserved, while lineage-specific variation occurred in TCA cycle, sulfur assimilation, and denitrification genes. Amphirytos necridicus harbors features suggesting adaptation to low-energy environments, with alternative TCA cycle enzymes likely supporting efficient energy conservation.   These findings expand the genomic and functional landscape of thermophilic cyanobacteria, revealing lineage-specific metabolic adaptations and potential biotechnological applications. Thermal springs are underscored as reservoirs of novel cyanobacterial diversity, highlighting the need for additional high-quality genomic data to better understand thermophilic adaptations.