Coral reef ecosystems are highly vulnerable to climate change, as rising sea surface temperatures and prolonged marine heatwaves can lead to coral bleaching, and disrupting the essential symbiosis between scleractinian corals hosts and their algal partners. Understanding the resilience and adaptive potential of symbiont communities is vital for predicting coral survival under changing climate scenarios. In this study, we applied high-throughput next-generation amplicon sequencing on the ITS2 rDNA marker to characterize the Symbiodiniaceae diversity in 64 scleractinian coral colonies across 14 reef sites in the Perhentian and Redang Islands, Malaysia.  ​Our results show that the genus Cladocopium (average 82%) dominates these reefs, but we also identified a significant presence of the thermotolerant genus Durusdinium (18%). Notably, we observed that several coral hosts, including Pocillopora, Galaxea, and Diploastrea, associate with heat-tolerant Durusdinium types, which can potentially increase the bleaching threshold of the holobiont by 1-2 degrees. This association is particularly significant given the mechanism of "symbiont shuffling," whereby corals can shift their symbiont communities—replacing stress-susceptible types with background or rare, heat-tolerant symbionts—to facilitate survival following stress-induced bleaching. Our network analysis further suggests that Durusdinium variants are highly competitive and play a central role in regulating the symbiosis. By assessing the Symbiodiniaceae diversity in a relatively healthy tropical reef system, this study underscores the role of symbiont composition in maintaining coral resilience and identifying potential "winners" in an era of global climate change.
 

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