Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for O2, pH and light

Abstract

During experimental light-dark cycles, O9 in the tissue of the colonial scleractinian corals Favia sp. and Acropora sp reached >250 % of air saturation after a few minutes in light. Immediately after darkenmg, 0; was depleted rapidly, and within 5 mm the 0; concentration at the tissue surface reached < 2 % of air saturation. The pH of the tissue changed within 10 min from about 8.5 in the light to 7.3 m the dark. Oxygen and pH profiles revealed a diffusive boundary layer of flow-dependent thickness, which limited coral respiration in the dark. The light field at the tissue surface (measured as scalar irradiance, Eo) differed strongly with respect to light intensity and spectral composition from the incident collimated light (measured as downwelling irradiance, Ed) Scalar irradiance reached up to 180 % of Ed at the coral tissue surface for wavelengths subject to less absorption by the coral tissue (600 to 650 run and >680 nm). The scalar irradiance spectra exhibited bands of chlorophyll a (chl a ) (675 run), chl c (630 to 640 nm) and pendinin (540 nm) absorption and a broad absorption band due to chlorop h y l l ~ and carotenoids between 400 and 550 nm. The shape of both action spectra and photosynthesis vs irradiance (Pvs I) curves depended on the choice of the light intensity parameter. Calculations of mi-ha1 slopes and onset of light saturation, Ik, showed that P vs Eo curves exhibit a lower initial slope and a higher 4 than corresponding Pvs Ed curves. Coral respiration in light was calculated as the difference between the measured gross and net photosynthesis, and was found to be >6 times higher at a saturating irradiance of 350 pEm m 2 s 1 than the dark respiration measured under identical hydrodynamic conditions (flow rate of 5 to 6 cm ssl).