We split colonies collected in Kāneʻohe Bay into three parts for preconditioning before exposing them to acute heat stress (32ºC; Fig. 1A). After 5 days of the heat stress, non-preconditioned (NPC) corals were visually more bleached than PC corals (Fig. 1B). To quantify this difference, we used the naturally occurring fluorophores in the host tissue (GFP) and algal photosynthetic apparatus (chlorophyll a), and live imaged each fragment repetitively at the beginning of the experiment and at days 3 and 6 of the heat stress. The bleaching rate was expressed as the gradual loss of symbiont-derived signal to host-derived signal over time (Fig.1C). The use of live laser-scanning confocal microscopy has been previously established to evaluate coral bleaching and it showed that the host-derived signal relates to the tissue thickness, and algal-derived signal mirrors both Symbiodinium cell density and chlorophyll a content (Huffmyer, O’Neill, & Lemus, n.d.).