ABSTRACT

Temperature stress disrupts the coral–Symbiodiniaceae symbiosis and contributes to coral bleaching. Symbiodiniaceae, essential symbionts for reef-building corals, provide over 90 % of the coral holobionts photosynthetic energy. This study examined the thermal response strategies of two Symbiodiniaceae types,Cladocopium goreaui(C. goreaui) andDurusdinium trenchii(D. trenchii), at high (34℃) and low (16℃) temperatures.C. goreauiexhibited heat sensitivity, with reduced cell density, impaired photochemical efficiency of photosystem II (PSII) (Fv/Fm), pigment loss, and chloroplast degradation. In contrast,D. trenchiimaintained photosynthetic performance and upregulated proteins related to heat shock response, antioxidant defense, and membrane stability. Under cold stress, both species showed moderate physiological changes.C. goreauiactivated pathways associated with redox homeostasis and the TCA cycle, indicating enhanced cold tolerance.D. trenchiiprimarily responded by upregulating RNA splicing-related pathways, suggesting an adaptive post-transcriptional regulation mechanism. These results highlight distinct thermal response strategies and provide insight into species-specific resilience among Symbiodiniaceae under climate stress.