Responses of the Coral Symbiont Cladocopium goreaui to Extreme Temperature Stress in Relatively High-Latitude Reefs, South China Sea.

Global climate change has led to frequent extreme temperature events in oceans. Corals are susceptible to extreme high-temperature stress in summer and extreme low-temperature stress in winter in the relatively high-latitude reef areas of the South China Sea (SCS). The most abundant symbiotic coral Symbiodiniaceae in the higher-latitude reefs of the SCS is Cladocopium goreaui, predominantly associating with dominant coral hosts such as Acropora and Porites. However, to date, relatively few studies have focused on the response and mechanism of C. goreaui to the extreme high- and low-temperature stress. In this study, the responses and regulatory mechanisms of the dominant C. goreaui to extreme high- and low-temperature stress were investigated based on physiological indexes, transmission electron microscopy (TEM), and transcriptome analysis. The results showed that (1) under 34 °C heat stress, the disintegration of thylakoids triggered photosynthetic collapse in C. goreaui; survival is enabled through metabolic reprogramming that upregulates five protective pathways and redirects energy via pentose/glucuronate shunting to sustain ATP homeostasis, revealing a trade-off between damage containment and precision energy governance under thermal extremes. (2) Low temperature exposure induced suppression of maximum quantum yield (F/F), compounded by glutathione pathway inhibition, crippling ROS scavenging. The transcriptome results revealed that C. goreaui prioritizes gene fidelity maintenance under low temperature stress. These findings reveal that energy allocation trade-offs constitute the core strategy of C. goreaui temperature response: prioritizing energy maintenance under high-temperature stress, while safeguarding genetic fidelity at the expense of antioxidant defense under low-temperature stress.