Anoxia-adapted cyanobacteria in a marine blue hole

Vertical transmission of marine particles brings ocean surface cyanobacteria into the deep ocean, where heterotrophic cyanobacterial lineages probably evolve to adapt to new environments even in oxygen-depleted zones. At present, active cyanobacteria have rarely been reported in dark and anoxic water columns in the deep sea. In this study, we recovered three metagenome-assembled genomes of cyanobacteria from the Yongle blue hole located in the South China Sea, two of which were actively transcribed in a dark, anoxic environment at 250 m depth, through integrated metagenomic and metatranscriptomic analyses of water samples from 21 stratified depths collected using <i>in situ</i> microbial fixation and filtration. These anoxia-adapted cyanobacteria were phylogenetically approximate to the sponge cyanobacterial symbionts, while the genomic features showed similarities with both free-living and sponge symbiotic counterparts. They exhibit genomic features shared with symbiotic lineages, including loss of substrate utilization, biosynthesis pathways, DNA repair, and circadian regulation. Conversely, they retain selected metabolic characteristics of free-living lineages, including phenylalanine biosynthesis and phosphoserine metabolism. Additionally, the discovery of taurine transport proteins in the genomes suggests the potential for organic sulfur uptake from the environment. Altogether, these findings reveal a distinct genomic configuration in cyanobacteria inhabiting a permanently dark and anoxic marine system, characterized by the retention of oxygen-dependent metabolic potential alongside sustained transcriptional suppression under <i>in situ</i> conditions. This study provides new insights into the ecological persistence and evolutionary adaptation of cyanobacteria under long-term oxygen limitation.IMPORTANCEWe report metabolically active cyanobacteria thriving in darkness and oxygen deprivation at 250 m depth in the ocean. Genomics results show these microbes share evolutionary roots with sponge cyanobacterial symbionts but developed unique adaptations for anoxic and sulfidic environments. Strikingly, they retain photosynthesis genes as genomic remnants (with no detected transcription) while losing genes critical for environmental stress responses, including DNA repair, osmotic regulation, and circadian control, suggesting a potential evolutionary connection to symbiotic relatives. Crucially, they maintain metabolic autonomy via phenylalanine biosynthesis and light-independent serine biosynthesis, exhibiting traits absent in most symbionts. This demonstrates how cyanobacteria adapt to anoxic environments through targeted genome reduction, revealing novel survival strategies in oxygen-depleted oceans and providing a research case for microbial resilience during marine deoxygenation.