Summer temperature maxima already challenge thermal capacities of coral in the Southern Persian/Arabian Gulf

The future of coral reefs remains uncertain as intensifying ocean warming continues to drive coral decline and demise worldwide. Corals occurring in extreme environments such as the Persian/Arabian Gulf (PAG), the world’s warmest sea, represent a promising resource to study the genetic architecture of thermal tolerance. However, their thermal capacities remain unresolved. We applied short-term heat stress assays using the Coral Bleaching Automated Stress System (CBASS) to determine signatures of thermal tolerance from three common species (Porites harrisoni, Platygyra daedalea, and Cyphastrea microphthalma) across three sites (two in the extreme southern PAG and a more benign site in the neighboring Gulf of Oman) prior to summer heat-loading. We measured the highest absolute thermal tolerance thresholds ever recorded in a coral population for P. harrisoni at the warmest, southern PAG site. However, these same corals exhibited the lowest relative thermal thresholds in comparison to their mean summer maxima, and we found evidence for an onset of thermal stress already at current summer temperatures. Interestingly, corals exhibiting a higher onset temperature displayed a lower thermal limit, providing insight into putative trade-offs associated with different thermal capacities. Notably, heat stress tolerance differences aligned to specific Cladocopium thermophilum and Symbiodinium microadriaticum genotypes, reinforcing the importance of the algal assemblage for coral resilience, and also aligned with bleaching climate metrics such as degree heating weeks (DHW) and sea surface temperature anomalies (SSTA). Our study shows that although southern PAG corals exhibit the highest absolute thermal tolerance threshold measured thus far, they already experience baseline stress under the contemporary climate, suggesting little capacity to tolerate further warming.