Coral-associated microbiome dynamics under thermal and pollution stress.

Globally, coral reefs are undergoing rapid degradation due to climate change. Microbiomes associated with coral are integral to host metabolism and play critical roles in coral resilience. Determining the changes in compositions and functions of these coral commensal microbes is essential for forecasting coral responses to environmental stress and guiding conservation. We investigated the structure and function of Symbiodiniaceae and bacteria from 587 coral samples (5 orders, 62 genera, and 166 species) spanning a 15° latitudinal range in the South China Sea by combining environmental factor measurements with ITS2 and 16S rRNA gene amplicon sequencing analyses. The abundance of dominant Symbiodiniaceae and bacteria varied with latitude, primarily driven by sea surface temperature. A higher proportion of heat-tolerant Symbiodiniaceae (Durusdinium, C15, and C3u) and copiotrophic bacteria (e.g., Endozoicomonas and Terasakiellaceae) was observed in low-latitude corals. Increased expression of bacterial genes was associated with triglyceride and glycogen degradation, and there was a decreased expression of genes involved in their biosynthesis. These findings suggest that corals cope with heat stress by reshaping symbiont composition and abundance, thereby enhancing thermal tolerance and optimizing energy metabolism. Based on the results, we propose region-specific conservation strategies, including the introduction of heat-tolerant symbionts to low-latitude corals, reducing nutrient pollution for high-latitude corals, and emphasizing reduction in global emissions as the ultimate solution to thermal stress.