Physiological Response of Shallow-Water Hard Coral Acropora digitifera to Heat Stress via Fatty Acid Composition

Success of the healthy reef is associated to the mutualistic relationship between the hard corals and symbiotic algae but an increase of seawater temperature has caused thermally induced bleaching. A wide array of biological parameters are used to reflect the phenomenon. This study exposed a shallow-water hard coral, Acropora digitifera to a series of elevated temperature over time while the interaction between Symbiodiniacea (SD) density, antioxidants, fatty acid (FA) composition and putative coral health indicators were evaluated. Increasing temperature caused gradual loss in SD density, consequently regulated antioxidant activities and significant change in FA composition. There is lack of evidence that A. digitifera experienced oxidative stress, nonetheless, a significant drop of MUFA and PUFA during the thermally induced experiment demonstrated that corals utilize their unsaturated FA as a final barrier or as a repair system against oxidative damage and it might also responded quickly by removing its symbiotic algae. Besides, the gradual decrease of SD significantly correlated with the putative coral health indicators (i.e. n-3 LC:n-6 LC, EPA:ARA, DHA:ARA ratio) and this might contribute to the potential role of these FAs as bio-indicators for assessing the physiological status of corals towards elevated temperature. This study also revealed the regulation of SD with FA characteristics and the practicality of FAs in chemo-ecological mechanism in hard corals as stress-relief, especially when heterotrophic feeding is limited. Future studies on FA profiles towards antagonistic or synergistic effects will offer a better understanding of the nature of this relationship under harsh climate.