Fig. 5

A model explaining how “symbiont dominance” mediates corals’ response to thermal stress. The illustration summarizes the major findings in the present study, depicting how initial symbiont dominance in hospite, together with symbiont change in vivo, may affect P. damicornis physiology, health, and susceptibility to prolonged heat stress. a Under ambient temperature (control), the PdC exhibits higher calcification rate compared to PdD, due to higher rates of photosynthetic carbon fixation and translocation (indicated by large sun image and thick arrows) by Cladocopium as compared to Durusdinium. b Under thermal stress, the rates of photochemistry and carbon fixation are significantly compromised (indicated by small sun image and thin arrows) in Cladocopium, possibly due to PSII and chloroplast damage, and a significant loss of the Symbiodiniaceae despite a relatively faster increase in Durusdinium proportion in the symbiont community, ultimately leading to reduced calcification rates and coral bleaching in PdC. In contrast, photochemistry and calcification rates are only slightly reduced in the bleaching-tolerant PdD, possibly as a result of dominance of the thermally tolerant Durusdinium in the algal community. Coral growth is indicated with the skeleton size and epidermis thickness. Translocation of PFC is indicated with an arrow with the rates indicated by its thickness. Symbiont cells and chloroplasts are color coded to differentiate symbiont type and changes in photochemistry. Ect ectoderm, Mes mesoglea, End endoderm, Cal calcidodermis, Ske skeleton, PFC photosynthetically fixed carbon, D/C Durusdinium/Cladocopium, ROS reactive oxygen species