Preventing Bleaching in Tropical Corals by Using Thermally Resilient Symbiont Zooxanthellae: All Hands-On Deck!

The current rapid climate warming is expected to cause an ocean temperature increase of 3°C–5°C by 2100, leading to deoxygenated and acidified tropical seas. Without mitigation measures, the total loss of tropical corals is inevitable. Already, one-third of tropical reefs are considered permanently lost. Coral bleaching initiated by the loss of symbionts, the photosynthetic zooxanthellae, is the main process whereby corals respond to thermal stress, followed by recovery. However, increased thermal stress and frequency of bleaching have caused widespread coral recovery failure. Zooxantheallae of the genus Symbiodinium are considered the thermally vulnerable part of the coral symbiosis. In recent decades, warming has displaced genotypes of lower thermal resilience to subtropical latitudes; few genotypes of higher temperature tolerance remain abundant in tropical seas, but these will not withstand warming predictions either. Interestingly, high temperatures in the Red Sea have selected for exceptionally heat-resistant coral genotypes and for the highest known thermal resilience in endemic zooxanthellae at the same time. Actions to overcome the coral bleaching crisis have been proposed by combining coral ecophysiology and mass culturing of thermally resilient Red Sea symbionts for naturalisation to the global tropical ocean, including restoration of collapsed reefs using corals with thermally resilient symbiont genotypes.