Heterogeneous effects of heat and cold stress on carbon metabolism in coral-associated microorganisms

The high productivity of coral reef ecosystems depends on homeostatic regulation and efficient nutrient cycling, particularly carbon, mediated by symbiotic organisms. Both zooxanthellae and microbes are essential for coral carbon metabolism, yet their responses to temperature stress remain poorly understood. Here, we compared carbon metabolic profiles of thermally susceptible <em>Acropora pruinosa</em> and resilient <em>Porites lutea</em> under heat (30℃), ambient seawater temperature (24℃), and cold (16℃) conditions using integrated physiological and omics analyses. In <em>A.</em><em> pruinosa</em>, heat stress reduced pigmentation, zooxanthellae density, and photosynthetic efficiency, with even stronger impairments under cold stress. Both temperature extremes also reshaped microbial composition, diversity, network complexity, and influenced carbon-related functional genes, particularly under cold stress. In contrast, <em>P. lutea</em> exhibited only partial inhibition of zooxanthellae and microbial carbon metabolism under both heat and cold stress, reflecting greater resilience. Moreover, under stress—especially cold—zooxanthellae played a dominant role in sustaining carbon balance, supporting a model in which “zooxanthellae act as regulators and microbes as adaptors” in maintaining carbon metabolic homeostasis. This study demonstrates species-specific sensitivity of corals to temperature stress and shows that both low and high temperatures impose distinct, heterogeneous impacts on the carbon metabolism and sequestration efficiency of coral-associated microorganisms.