Bio-optical properties and radiative energy budgets in fed and unfed scleractinian corals (Pocillopora sp.) during thermal bleaching

Corals live in symbiosis with algal dinoflagellates, which can achieve outstanding photosynthetic energy efficiencies in hospite approaching theoretical limits. However, how such photosynthetic efficiency varies with environmental stress remains poorly known. Using fiber-optic and electrochemical microsensors in combination with variable chlorophyll fluorescence imaging, we investigated the combined effects of thermal stress and active feeding on the radiative energy budget and photosynthetic efficiency of the symbiotic coral Pocillopora sp. At ambient temperature (25°C), the percentage of absorbed light energy used for photosynthesis under low irradiance was higher for fed (~5-6%) compared to unfed corals (4%). Corals from both feeding treatments responded equally to stress from high light exposure (2400 µmol photons m-2 s-1), exhibiting a decrease in photosynthetic efficiency, down to 0.5-0.6%. Fed corals showed increased resilience to thermal-induced bleaching (loss of symbionts) compared to unfed corals. In addition, while unfed corals decreased their photosynthetic efficiency almost immediately when exposed to thermal stress, fed corals maintained a constant and high photosynthetic efficiency for 5 more days after onset of thermal stress. We conclude that active feeding is beneficial to corals by prolonging coral health and resilience during thermal stress as a result of an overall healthier symbiont population.