Multi-level osmoadaptation strategies of filamentous cyanobacteria within oxygenic photogranules under high salinity stress.

Filamentous cyanobacteria (FC) are key structural and functional components of oxygenic photogranules (OPGs) for saline wastewater treatment. However, how FC maintain cellular osmotic balance while supporting community stability under salinity stress remains poorly understood. Here, we integrated long-term reactor operation, physiological characterization, and multi-omics analyses to elucidate the multi-level osmoadaptation strategies of dominant FC within OPGs. Despite stepwise increases in salinity, OPGs maintained stable pollutant removal and showed enhanced FC growth, polysaccharide (PS) production, and photosynthetic activity. Genome-resolved multi-omics further revealed coordinated adaptive responses across community, interspecies, and cellular levels. At the community level, the dominant FC Nodosilinea showed increased expression of PS-assembly-related genes, while co-enriched FC Desertifilum exhibited complementary sugar nucleotide biosynthetic potential, suggesting a possible cooperative basis for enhanced PS production and extracellular osmoprotection. At the interspecies level, associated heterotrophic bacteria possessed carbohydrate turnover capacity and expressed genes involved in vitamin and cofactor biosynthesis, indicating potential metabolic cross-feeding that may support phototrophic growth and resource optimization within the consortium. At the cellular level, the dominant FC combined rapid K⁺ accumulation with sustained sucrose and trehalose biosynthesis, supported by active photosynthetic energy metabolism. These findings demonstrate that FC employ a hierarchically coupled adaptation strategy, integrating extracellular matrix protection, metabolic cooperation, and cellular osmoadaptation to coordinate individual stress tolerance with system-level stability. Overall, this study provides new insights into microbial resilience in phototrophic wastewater treatment systems and offers a conceptual framework for engineering robust OPG-based processes for saline wastewater treatment.