Corals reefs worldwide are facing many challenges due to global warming. Of these challenges, increasing sea temperatures represent a major threat. An increase of 1 °C above summer mean levels is greatly associated with coral bleaching, and massive coral bleaching is observed when accumulation of thermal-stress reaches 4 °C-weeks according to Degree Heating Weeks (DHW) measurements. The coral holobiont is an assemblage of many organisms including Symbiodiniaceae; a family of photosynthetic microalgae that form an endosymbiotic relationship with corals. Studies suggest that Symbiodiniaceae may drive the performance of the host, therefore, resilience of the coral host against thermal stress may be largely influenced by Symbiodiniaceae. The aim of this study was to compare the physiological performance under acute heat and light-stress conditions of several Symbiodiniaceae strains (clade A and B) isolated from different geographical locations with distinct thermal profiles (Red Sea, Hawaii, and North Carolina). Oxygen production, respiration rate, photosynthetic efficiency, and production of ROS were measured under conditions of acute heat and light-stress. The Red Sea strains (RS-B*, RS-B, and RS-A) exhibited a higher photosynthetic efficiency with increasing temperatures than the Hawaii and North Carolina strains (SSBO1, SSAO1, respectively). After heat-stress of 34 °C, RS-A was found to have the best thermotolerance with regard to ROS production. Oxygen production and respiration rate data showed high biological variation between culture replicates which prevented inter-strain comparisons and limited observation of consistent trends. The observed variability was largely due to the differential age of the cultures used, and the inability of the cell counting method to differentiate between live and dead cells. The results of this study indicate that Symbiodiniaceae strains originating from warmer geographic locations exhibit an overall better performance under acute heat-stress conditions. Variability in the physiological response of three samples from the same species (Breviolum minutum) exemplifies the large diversity in the family Symbiodiniaceae, and indicates the need to support genomic identification of Symbiodiniaceae isolates with physiological studies. Efforts to predict the future of coral reefs under current threats of climate change will only be productive if we have a comprehensive understanding of the complex interactions between corals and Symbiodiniaceae.