Energetic cost of photoinhibition in corals

Photoinhibition may constitute an energetic cost for photosynthetic organisms through damage to the photosynthetic apparatus, or by increased metabolism due to damage avoidance or repair. For several species of scleractinian corals, fluorescence techniques have revealed a significant reduction in photochemical efficiency of symbiotic dinoflagellates within coral tissue in response to excess light absorption. To date, it has been unclear whether or not photoinhibition has a negative impact on energy budgets in corals. We simultaneously quantified the effect of exposure to excessive light on net rates of photosynthesis and on fluorescence-derived photochemistry. We acclimated colonies of the reef-building coral Turbinaria mesenterina to 3 different irradiance regimes in the laboratory. The corals were then exposed to light levels up to 10 times higher than their acclimation irradiance and assayed for rates of photosynthesis and photochemical yields. Results indicated that daily costs of photoinhibition are negligible. Reduced net rates of photosynthesis in the afternoon, compared to the morning, were predominantly due to enhanced afternoon rates of dark respiration. However, photoacclimation to high light levels reduces daily energy acquisition in the long term, primarily due to decreased chlorophyll concentrations. Therefore, although changes in the photosynthetic activity of symbiotic dinoflagellates over a diurnal irradiance cycle do not cause a measurable decline in net oxygen evolution for coral colonies, repeated exposure to excessive irradiance can reduce energy acquisition per unit surface area, and hence influence the upper limit of the depth distribution of scleractinian corals.