Rising seawater temperature is one of the greatest threats to the persistence of coral reefs. While great efforts have been made to understand the metabolic costs of thermal acclimation, the exact roles of many secondary metabolites involved in the immediate response exhibited by soft corals remain largely unknown. Herein, an untargeted metabolomics approach using ultra-performance liquid chromatography coupled to high-resolution mass spectrometry (UPLC-MS) was employed to investigate thermal stress-induced modifications to the de novo synthesis of secondary metabolites in two soft coral species, Sarcophyton ehrenbergi and S. glaucum. Exposure to elevated temperature resulted in symbiont photoinhibition primarily via either damage to photosystem II (PSII) or the loss of algal symbionts during coral bleaching. This was suggested by a decrease in pulse amplitude modulated (PAM) measurements of corals incubated at different temperatures. Thermal stress was also found to impair the production of diterpenoid secondary metabolites in soft corals. Principally, reduction in the levels of a number of diterpenes, viz. sarcophytoxide and deoxysarcophytoxide, in heat stressed S. ehrenbergi and S. glaucum was observed indicative that thermal acclimation is energetically costly and will necessitate downstream changes in secondary metabolic pathways. Our data suggest that, while the host controls the production of ecologically important terpenes, when energetic contribution from the algal symbiont is reduced or absent as a result of a bleaching event, energy reserves may be insufficient to maintain the production of such energetically cost chemicals. This study provides for the first time a holistic assessment of secondary metabolite changes imposed in soft corals during exposure and acclimation to elevated temperatures.
Keywords: Metabolomics; PAM measurements; S. glaucum; Sarcophyton ehrenbergi; Thermal stress.