First assessment of the diversity and composition of coral-associated diatoms in the Saudi Arabian Gulf

Epizoic diatoms are an essential but often overlooked component of microbenthic biodiversity in coral reefs, as they reflect both the specific characteristics of the host and local environmental conditions. This study provides the first comprehensive characterisation of diatom assemblages associated with corals in the Arabian Gulf, one of the marine environments most subject to thermal and salinity extremes. Diatom communities associated with four coral genera ( Cyphastrea , Porites , Platygyra , and Favites ) collected from four coastal sites in Saudi Arabia were analysed using Scanning Electron Microscopy. In total, 32 diatom taxa belonging to 11 genera were identified, with Amphora (33.2%) , Cocconeis (20.0%), and Mastogloia (11.0%) as the dominant components. The host identity was found to be the main factor influencing community composition, while total abundance was influenced by both the host and the sampling location. PERMANOVA confirmed a significant effect of coral host identity on diatom community composition (p = 0.005). Favites sp. hosted the richest and most diverse assemblages, properties that promote stable attachment and biofilm development in diatom colonization. The diatoms were not distributed evenly but clustered into small patches often associated with mucus produced by corals, suggesting that mucus dynamics regulate colonization and renewal processes on a microenvironmental scale. Despite the extreme environmental conditions, the assemblages observed were diverse, balanced, and ecologically stable. These results provide a baseline for future studies on coral-microalgae interactions and highlight the potential of epizoic diatom communities as sensitive bioindicators of coral reef health in extreme environments.