Symbiotic Symbiodiniaceae mediate coral-associated bacterial communities along a natural thermal gradient.

The coral-associated microbiome plays a vital role in the holobiont, enabling coral adaptation to diverse environments by modulating its composition and mediating interactions among its constituents. However, the responses of coral microbiomes, particularly the interactions between Symbiodiniaceae and bacteria, to environmental changes remain unclear. To fill this knowledge gap, we examined Pocillopora acuta, an environmentally sensitive coral species, collected from three sites along the southeastern coast of Hainan which exhibit moderate environmental differences. We measured the physiological characteristics of Symbiodiniaceae and conducted amplicon sequencing to analyze the structure of Symbiodiniaceae and bacterial communities. Our results revealed that P. acuta in southeastern Hainan maintains stable symbiosis with Symbiodiniaceae sub-clades such as C1, C42.1, C3, D1, D4, and D6, as evidenced by ΔF/Fm' values ranging from 0.45 for P. acuta dominated by Durusdinium (PaD) to 0.6 for counterparts dominated by Cladocopium (PaC). However, the composition of Symbiodiniaceae varied among the three sites, primarily due to differences in the abundance of dominant sub-clades. These variations may reflect adaptations to distinct environmental conditions, which in turn significantly influence the associated bacterial communities. Notably, our results suggest that Symbiodiniaceae may exert a greater regulatory role on the coral-associated bacterial community than environmental differences. Specific bacteria, such as Endozoicomonas and Synechococcus_CC9902, exhibit strong correlations with particular Symbiodiniaceae genera or sub-clades, indicating that the dominant Symbiodiniaceae shape bacterial community dynamics. Despite the observed variations, we identified modular co-occurrence patterns in bacterial networks, with PaC exhibiting a more complex and stable structure. Overall, these results highlight the critical role of various Symbiodiniaceae genera in influencing bacterial community dynamics, emphasizing their importance in maintaining coral health and resilience in the face of changing environmental conditions.