Biogeographical Patterns of Coral-Associated Symbiodiniaceae Across Thermal Regimes Including Trace Molecular Detection of Cladocopium thermophilum (C3-Gulf) in the Seychelles

Reef-building corals are under serious threat from ocean warming, which directly impacts their symbiotic partnership with dinoflagellates from the family Symbiodiniaceae; a partnership integral for the survival of coral reefs. Symbiodiniaceae types vary in their levels of tolerance to heat and light, with some reported to enhance a coral's ability to withstand thermal stress. This study investigated the composition and diversity of Symbiodiniaceae across different thermal regimes and latitudinal gradients covering three bioregions: Bahrain (Arabian Gulf?AG), the Seychelles (Western Indian Ocean?WIO), and Indonesia (Central Indo-Pacific?CIP). Coral fragments from 11 coral host species (n?=?183 colonies collected; n?=?157 retained following sequencing quality control) were sampled across six reef sites spanning these regions. Results acquired through next generation sequencing (NGS) targeting the ITS2 rDNA region identified three main genera: Cladocopium (dominant across all sites), Durusdinium, and Symbiodinium. Observed biogeographical patterns suggested lower Symbiodiniaceae diversity and richness in the higher latitude and more thermally extreme reefs of Bahrain compared to Indonesia and the Seychelles. Shifts in Symbiodiniaceae community composition from thermally sensitive ITS2 types in reefs under comparatively lower thermal stress (Degree Heating Weeks (DHW)?≤?4°C-weeks) toward more thermotolerant ITS2 types in reefs exposed to higher thermal stress (DHW?>?4°C-weeks) were observed. Our results provide the first ITS2-based molecular characterization of Symbiodiniaceae communities in Bahrain and document the widespread occurrence of the thermotolerant C3-Gulf (Cladocopium thermophilum) type across sampled Bahraini coral hosts. In addition, the C3-Gulf ITS2 type was detected at trace relative abundance (0.1%) within a coral sample from the Seychelles extending previous observations beyond AG, although the ecological significance of this low-abundance detection remains uncertain. Overall, the findings provide baseline insights into coral-Symbiodiniaceae associations across contrasting reef systems and highlight how symbiont community composition may vary under differing environmental, thermal, and biogeographic conditions.