ABSTRACT                

Background Low salinity is a crucial environmental stressor that affects estuarine coral ecosystems considerably. However, few studies have focused on the effects of low-salinity conditions on coral-associated microorganisms and the adaptability of coral holobionts.

Methods We explored the community structure of coral symbiotic Symbiodiniaceae and associated bacteria in lowsalinity conditions using samples of six coral species from the Pearl River Estuary and analyzed the adaptability of coral holobionts in estuaries.

Results The symbiotic Symbiodiniaceae of all six studied coral species were dominated by Cladocopium, but, the Symbiodiniaceae subclades differed among these coral species. Some coral species (e.g., Acropora solitaryensis) had a high diversity of symbiotic Symbiodiniaceae but low Symbiodiniaceae density, with different adaptability to low-salinity stress in the Pearl River Estuary. Other coral species (e.g., Plesiastrea versipora) potentially increased their resistance by associating with specific Symbiodiniaceae subclades and with high Symbiodiniaceae density under low-salinity stress. The microbiome associated with the coral species were dominated by Proteobacteria, Chloroflexi, and Bacteroidetes; however, its diversity and composition varied among coral species. Some coral species (e.g., Acropora solitaryensis) had a high diversity of associated bacteria, with different adaptability owing to low-salinity stress. Other coral species (e.g., Plesiastrea versipora) potentially increased their resistance by having minority bacterial dominance under low-salinity stress.