Delineating the emergence of thermally tolerant Symbiodiniaceae genotypes across the dominant coral species of a turbid reef: Adaptation and resilience strategy

Climate change-induced rise in sea surface temperatures has led to an increase in the frequency and severity of coral bleaching events, ultimately leading to the deterioration of coral reefs, globally. However, the reef-building corals have an inherent capacity to acclimatize to thermal stress on pre-exposure to high temperatures by altering their endosymbiotic Symbiodiniaceae community composition towards a thermal tolerant composition. This reorganisation may become an important tool in coral's resilience to rapid environmental change. Therefore, it is crucial to delineate the Symbiodiniaceae community in our reef to predict their resilience capacity. Our study aims to analyse the symbiont community associated with common reef corals in a heat-stressed, marginal turbid reef of Palk Bay, India. We employed next-generation sequencing-based high-resolution analyses of internal transcribed spacer two (ITS2) amplicons within the SymPortal framework to examine the diversity and organization of Symbiodiniaceae communities. The results revealed a dominance of heat-tolerant Durusdinium (D1-D4) and Cladocopium (C15) across coral species (Acropora cytherea, Acropora digitifera, Favites abdita, and Porites lobata) and reef environment (seawater, and reef sediment), with the presence of 18 ITS2 type profiles in our samples. To our knowledge, this is the first comprehensive study to delineate Symbiodiniaceae communities at fine scale resolution (ITS2 type profile) from scleractinian corals in India, establishing a baseline for future research in a marginal reef ecosystem. Several studies have highlighted the importance turbid reefs as prospective climate refugia under the future threat of climate change. Therefore, this study is crucial for researchers, scientists, stakeholders, and policymakers to identify reef sites and coral species with resilience capacity for devising future reef restoration and management strategies.