Diverse symbiont bleaching responses are evident from 2-degree heating week bleaching conditions as thermal stress intensifies in coral

Coral bleaching is the dysfunction of the coral–algal endosymbiosis and is characterised as a loss of Symbiodiniaceae cells from host tissues or the loss of photosynthetic pigments. This breakdown of symbiosis occurs as a result of elevated temperature beyond the organism’s thermal threshold. The thermal tipping points within the symbiosis have not yet been well resolved, and the mechanisms underlying the various cellular processes of the corals bleaching response remain unknown. This study characterised the cellular responses of the symbiont Cladocopium sp. (syn. clade C3) within the host coral Acropora aspera during exposure to thermal stress. Exposure to temperatures between 2 and 3°C below the bleaching threshold, equating to 2-degree heating weeks (DHWs), results in changes to the symbiont cell morphology and cell division rates. Once corals were exposed to 4 DHWs, over 90% of the symbiont cells showed signs of degradation. Although sub-bleaching thermal stress is not sufficient to trigger bleaching alerts at an ecological scale, this stressor substantially affects the coral symbiosis. It is therefore vital that we begin to quantify how sub-bleaching thermal stress affects the fitness of Symbiodiniacea populations, their coral hosts and subsequently reefs worldwide.