MICROENVIRONMENT AND PHOTOSYNTHESIS OF ZOOXANTHELLAE IN SCLERACTINIAN CORALS STUDIED WITH MICROSENSORS FOR O-2, PH AND LIGHT

During experimental Light-dark cycles, O-2 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 darkening, O-2 was depleted rapidly, and within 5 min the O-2 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 in 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, E(0)) differed strongly with respect to light intensity and spectral composition from the incident collimated light (measured as downwelling irradiance, E(d)) Scalar irradiance reached up to 180% of E(d) at the coral tissue surface for wavelengths subject to less absorption by the coral tissue (600 to 650 nm and (680 nm). The scalar irradiance spectra exhibited bands of chlorophyll a (chi a) (675 nm), chi c (630 to 640 nm) and peridinin (540 nm) absorption and a broad absorption band due to chlorophylls and carotenoids between 400 and 550 nm. The shape of both action spectra and photosynthesis vs irradiance (P vs I) curves depended on the choice of the light intensity parameter. Calculations of initial slopes and onset of Light saturation, I-k, showed that P vs E(0) curves exhibit a lower initial slope and a higher I-k than corresponding P vs E(d) 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 mu Ein m(-2) s(-1) than the dark respiration measured under identical hydrodynamic conditions (flow rate of 5 to 6 cm s(-1)).