The upper thermal limit of reef-building corals is strongly regulated by dinoflagellate symbionts of the family Symbiodiniaceae. Experimental evolution of Symbiodiniaceae in vitro rapidly generates novel phenotypes that confer thermotolerance to young corals (e.g., larvae and recruits). Whether such benefits are conferred to adult corals has yet to be tested. Here we chemically removed symbionts of the coral Platygyra daedalea and inoculated the bleached corals with heat-evolved Cladocopium symbionts cultured at elevated temperatures for 10 years. When the symbiosis was functionally restored (symbiont densities and growth rates comparable to controls), we subjected the corals to a simulated thermal stress event. We measured symbiont densities, photosynthesis and respiration rates, coral calcification rates, and quantitative colour analysis as indicators of thermal bleaching, and tracked symbiont communities with DNA metabarcoding. Corals inoculated with novel symbionts were thermally tolerant relative to controls but displayed significant heterogeneity in phenotype. However, most corals harboured mixed communities at varying relative abundances of the inoculum, homologous ‘native’ symbionts, and additional Symbiodiniaceae genera. We are using these physiological responses to select samples for transcriptomic analysis and spatial metabolomics to resolve the functional roles of each symbiont community member in conferring thermal tolerance.