Climate change drives mass bleaching events through the disruption of the symbiotic relationship between corals and their dinoflagellate symbionts (family Symbiodiniaceae). A pathway to enhanced coral thermotolerance lies in the association with thermally resilient symbionts. However, the mechanisms governing the establishment and persistence of this symbiosis remain poorly understood. Additionally, competition among Symbiodiniaceae induces species-specific metabolic shifts under hot-stress in the environment. But within the coral host, there is a research gap regarding the competitive strategies of symbionts. This study investigates the dynamics of the thermally sensitive Cladocopium goreaui (SymC) and the thermally tolerant Durusdinium trenchii (SymD) as model symbionts in the coral species Acropora kenti and Acropora millepora during early colonization, focusing on the effects of thermal stress and interspecific competition, by using methods including pulse‑amplitude‑modulated (PAM) fluorometry, fluorescent in situ hybridization (FISH), and flow cytometry.

Over an eight‑week experiment at the Australian Institute of Marine Science, we found that both infection success and survivorship under ambient conditions were markedly higher than under hot-stress, with A. kenti consistently exhibiting higher infection and survival than A. millepora across the observation period. Infections were first detected in A. kenti under both ambient and hot-stress conditions in the SymD and Mix (SymC:SymD = 1:1) treatments in week 3. And from week 3 onward, survival in the Mix treatment under hot-stress exceeded that in SymD but lower than that in SymC in both host species. Discrete-time binomial generalized linear mixed model (GLMM) further indicated that, relative to SymC, the SymD- and Mix-related changes in weekly mortality probability were weaker in A. millepora than in A. kenti. The next steps involve analyzing the imaging-PAM data and conducting FISH-flow cytometry to determine how in hospite symbiont cell growth relates to their ability to form symbiosis, by comparing cell cycle progression with infection rates.

This work informs the ecological factors that influence symbiont growth and stability in corals, which is important for supporting conservation interventions against the backdrop of the escalating climate crisis.