The reef building coral Stylophora pistillata uses stored carbohydrates to maintain ATP levels under thermal stress

Coral reefs are on the brink of collapse from global warming and associated coral bleaching. Coral bleaching is the loss of algal symbionts from the coral tissue. The reduction in photosynthates produced by the symbionts makes the survival of the coral dependent on heterotrophy and stored resources, which are catabolized into available energy, i.e., Adenosine Triphosphate (ATP). The present study examined how an increase in water temperature affects energetic reserves and available ATP in the Red Sea coral Stylophora pistillata. Following a 9-d hold at 1, 3, 5 °C above ambient summer temperature (~ 26 °C), ATP levels in the coral tissue remained constant. Similarly, no significant differences in the stored energy (proteins, carbohydrates, and lipids) of the holobiont were measured. However, half of the coral nubbins in the + 7 °C treatment experienced tissue dissociation, while the remaining nubbins bleached with a 34% decline in stored energy and a decline in respiration and photosynthesis rates by 69 and 72%, respectively. The + 7 °C treated coral nubbins had 75% lower carbohydrates compared to nubbins at ambient conditions and the lowest carbohydrates to lipid and protein ratio. This study demonstrates that exceeding the high bleaching threshold of S. pistillata in the Gulf of Aqaba is associated with a catabolic response to maintain ATP levels and highlights the energetic cost of thermal stress. Understanding anabolic and catabolic processes in corals under environmental stress is key to understanding their capacity to survive future thermal stress scenarios.