Molecular mechanisms of thermal tolerance regulated by phycosphere bacteria in isolated Symbiodiniaceae

Thermal stress disrupts the coral-Symbiodiniaceae-microbes holobiont, causing coral bleaching. As a key component of this system, the specific molecular mechanisms by which phycosphere bacteria regulate Symbiodiniaceae thermal tolerance remain largely unclear. This study used an isolated Cladocopium C1 strain and three associated phycosphere bacterial strains ( Bacillus sp., strain a; Exiguobacterium sp., strain b; Erythrobacter sp., strain E). Through co-culture experiments at 32°C, combined with physiological measurements, bacterial amplicon sequencing, and Symbiodiniaceae transcriptome sequencing, we elucidated the molecular mechanisms by which beneficial phycosphere bacteria enhance the thermal tolerance of Symbiodiniaceae. Results showed that the addition of strains b and E significantly increased Symbiodiniaceae cell density, reduced intracellular reactive oxygen species (ROS), intracellular dimethylsulfoniopropionate (DMSP) and phycosphere nitric oxide (NO) levels, thereby alleviating oxidative stress in Symbiodiniaceae. Amplicon sequencing revealed an increased abundance of antioxidant functional bacterial genera in the growth-promoting groups, and functional predictions indicated a decrease in nitrogen metabolism-related functions of the phycosphere bacteria. Transcriptomic analysis further revealed that in the growth-promoting groups, genes related to DNA repair, cell cycle, and FoxO signaling pathway were up-regulated in Symbiodiniaceae, while pathways associated with photosynthesis and high energy consumption were down-regulated. Thus, beneficial phycosphere bacteria can enhance the thermal tolerance of Symbiodiniaceae by remodeling the functions of phycosphere bacterial communities, alleviating oxidative damage, enhancing genomic stability, and optimizing host resource allocation. This study provides a theoretical foundation for understanding coral thermal tolerance through the lens of algae‑bacteria interactions and for developing microbiome‑assisted coral reef restoration strategies.