Photoprotective Role of Endolithic Algae Colonized in Coral Skeleton for the Host Photosynthesis

Abstract

Reef-building corals have established an obligate symbiosis with dinoflagellates that is advantageous to survive in oligotrophic coral reef environment. Although corals can thrive in low nutritional conditions by utilizing photosynthetic products assimilated by their symbionts, the coral-alga symbiotic relationship is susceptible to environmental stress and its disruption leads to the coral bleaching phenomenon, a major impact of global warming. Because corals are important primary producers in tropical ocean, solutions are urgently needed to protect corals from the impacts of global climate changes. Here we report that endolithic algae colonizing the skeleton of corals may help to protect coral photosynthesis from high-light stress. Using the PAM chlorophyll a fluorescence technique, we compared the photosynthetic activity of endolithic alga-infected reef-building coral Acropora digitifera to that of a non-infected group. Short-term lab experiments (7 h) showed that the infected group maintained a higher maximal quantum yield of PSII (Fv/Fm) compared to the non-infected group under photoinhibitory stress conditions at normal growth temperature. Similar results were obtained during the course of a long-term monitoring (6 months from summer to winter) during which the infected and non-infected coral groups were exposed to ambient light conditions. Results from this study suggest that colonization of endolithic algae within the coral skeleton provides beneficial effects on coral photosynthesis in terms of high-light tolerance.