Living on the edge: Host and habitat-associated symbiodiniaceae dynamics in corals of the World's hottest sea (the Persian Gulf).

Coral persistence in the environmentally extreme Persian Gulf is strongly influenced by associations with Symbiodiniaceae, yet the relative importance of host identity, geography, depth, and season in structuring these symbioses remains incompletely understood. This study characterized Symbiodiniaceae communities associated with four coral species (Porites harrisoni, Platygyra daedalea, Acropora downingi, and Acropora microphthalma) from Kish Island (Iran) and Sheraoh Island (Qatar) during winter and summer 2023. Community diversity was examined across locations, depths, and seasons using Oxford Nanopore sequencing of the internal transcribed spacer 2 (ITS2) region. This study combines a multi-factorial sampling design spanning coral host species, geographic sites, depths, and seasons with Oxford Nanopore long-read ITS2 sequencing to characterize Symbiodiniaceae communities in the Persian Gulf. ITS2 sequences successfully assigned to known Symbiodiniaceae types were recovered from 42 coral colonies, yielding 13 dominant sequence types belonging primarily to C- and D-type lineages. Community composition differed significantly among coral species and locations, whereas seasonal and depth effects were detected only within some host species. Acropora microphthalma showed no statistically significant seasonal variation in ITS2 composition and remained D-type-dominated in both seasons, a pattern consistent with relative stability of the dominant Symbiodiniaceae community. A. downingi showed a non-significant trend toward greater C-type representation in summer. Porites harrisoni and Platygyra daedalea displayed spatial variation, with D-type symbionts dominating communities in Kish but C-type profiles prevailing in Sheraoh colonies (location and depth are confounded for P. harrisoni). These results document diverse host- and habitat-associated Symbiodiniaceae community patterns in corals inhabiting extreme environmental conditions. The findings indicate that Symbiodiniaceae diversity in the Persian Gulf cannot be explained by a single thermotolerant lineage. Host identity and local conditions particularly geographic site differences were important structuring factors in this dataset, though depth effects could not be disentangled from location for P. harrisoni. These findings highlight diverse host- and habitat-associated Symbiodiniaceae communities in one of the world's most environmentally extreme reef ecosystems.