Then, six PC and 6 NPC corals were exposed to acute heat stress (32ºC) and simultaneously treated with 1μM venetoclax (ven+) or with a solvent control (DMSO; ven-). NPC corals bleached in both groups while PC corals visibly bleached only when treated with venetoclax (Fig S4). Time-series confocal microscopy and subsequent linear regression analyses confirmed that even though all conditioning-treatment combinations led to statistically important loss of symbiont signal compared to host-derived signal over time (p<0.001 for each), there was a significant difference in the bleaching rate between PC and NPC heat-stressed corals (p(PC ven- ­– NPC ven-) < 0.001) that was lost after the addition of venetoclax (p(PC ven+ – NPC ven+) = 0.9871) (Figure 4). In other words, after preconditioning, the corals bleached at a significantly slower rate compared to their non-preconditioned counterparts, but this beneficial phenotype was lost after the addition of venetoclax, a specific Bcl-2 inhibitor.