Seasonal and Depth-Driven Shifts in Coral-Associated Microbial Communities Under Extreme Environmental Conditions in the Persian Gulf

Coral-associated microbial communities are vital to coral resilience under environmental stress. We characterized the microbiomes of five scleractinian species-Porites harrisoni, Platygyra daedalea, Pavona decussata, Acropora downingi, and Acropora microphthalma-from northern (Kish, Iran) and southern (Sheraoh, Qatar) Persian Gulf reefs using full-length 16S rRNA sequencing. Sampling across seasons (winter and summer) and depths (shallow: 4-5 m; deep: 10-18 m) enabled comparison of diversity and reference-based taxonomic composition. A total of 3,984 unique taxonomic units were resolved using a full-length alignment-based approach (Emu). Microbial taxon richness was higher in deep reefs (Kish: 1,900; Sheraoh: 1,800) compared to shallow sites (~1,300), with overall seasonal declines observed. Microbiomes comprised bacteria (70.9%), dominated by Pseudomonadota (62.4%), and archaea (21.9%) including Halobacteriota, Thermoproteota, Thermoplasmatota, Methanobacteriota, and Candidatus Korarchaeota. Species-specific restructuring included declines in Pseudomonadota for P. harrisoni (from 85.05% to 70.1%) and P. daedalea (from 75.0% to 50.05%), whereas A. downingi exhibited a sharp increase (from 2.0% to 52.3%). Notably, A. microphthalma remained archaea-dominated (>88%) year-round, suggesting potential functional stability under extreme conditions. These results demonstrate that depth, season, and host species shape microbial assemblages and highlight archaeal lineages and hydrocarbon-degrading bacteria as potential indicators of coral adaptation in hypersaline, thermally variable, and oil-exposed Gulf reefs.