Interspecific differences in the response of coral trophic status to the decrease in symbiotic zooxanthellae in summer

Abstract

Coral bleaching events are increasing in frequency and severity worldwide. From the perspective of energy supply, a decrease in symbiotic zooxanthellae density (ZD) during bleaching leads to a reduction in the amount of energy provided by photosynthesis to the coral host. However, there are still many unknowns about how corals maintain the stability of their energy supply when zooxanthellae decrease. In this study, 89 coral samples, including the relatively stress-tolerant massive Favia palauensis and Porites lutea and the relatively vulnerable branching Acropora millepora and Pocillopora damicornis, were collected from the Xisha Islands in the South China Sea in spring and summer 2020. The physiological parameters ZD, chl a, tissue biomass and lipid content were measured. The δ13C compositions of zooxanthellae and hosts were analyzed to explore the changes in trophic status in summer. Results show that the ZDs of A. millepora, P. damicornis, F. palauensis and P. lutea significantly decreased by 32, 29, 22 and 22%, respectively, in summer. P. lutea and F. palauensis maintained metabolic energy requirements depending largely on heterotrophy. A. millepora and P. damicornis were more dependent on consuming their own lipids. Our study shows that there are intergeneric differences in the energy maintenance mechanisms used to cope with decreases in ZD. We suggest that the higher heterotrophic ability of F. palauensis and P. lutea renders their trophic status more plastic than that of A. millepora and P. damicornis. This characteristic may potentially affect their bleaching resilience.