Stage-Dependent Succession of Bacterial Communities in the South China Sea Stony Coral Goniopora sp. During Bleaching

Although coral bleaching–associated microbial changes have been widely studied, bacterial succession during bleaching, particularly in partly bleached corals, remains poorly understood. Here, we investigated bacterial community dynamics in healthy, partly bleached, and bleached Goniopora sp. collected from the Sanya Coral Reef Conservation District, South China Sea. A total of 599,003 valid sequences were obtained and clustered into 5094 operational taxonomic units (OTUs). These OTUs spanned 45 bacterial phyla and were identified by 16S rRNA gene sequencing, revealing a highly diverse bacterial community associated with Goniopora sp. Alpha diversity differed significantly among health statuses, with partly bleached Goniopora sp. (PBG) exhibiting the highest bacterial diversity (Shannon index: 6.25 ± 0.11), followed by bleached Goniopora sp. (BG) (5.49 ± 0.18) and healthy Goniopora sp. (HG) (3.04 ± 0.17). Beta diversity analyses showed clear separation of microbial community structures among HG, PBG, and BG. Successional analyses revealed a progressive decline in putatively beneficial bacterial taxa, including the phylum Pseudomonadota and the genus Cohaesibacter with increasing bleaching severity, whereas the relative abundance of opportunistic or stress-associated bacteria, such as Blastopirellula, Mycobacterium, and some unclassified taxa, increased. Notably, many bacterial taxa, including Acidobacteriota, Woeseia and Ruegeria, displayed non-linear abundance patterns, with pronounced shifts during the partly bleached stage. These findings highlight substantial microbial restructuring during coral bleaching and underscore the importance of the partly bleached status as a transitional phase in coral-associated bacterial succession.