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

During experimental Light-dark cycles, O-2 in the tissue of the coloni al scleractinian corals Favia sp. and Acropora sp. reached >250% of ai r 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-de pendent thickness, which limited coral respiration in the dark. The Li ght field at the tissue surface (measured as scalar irradiance, E(0)) differed strongly with respect to light intensity and spectral composi tion from the incident collimated light (measured as downwelling irrad iance, E(d)) Scalar irradiance reached up to 180% of E(d) at the coral tissue surface for wavelengths subject to less absorption by the cora l tissue (600 to 650 nm and (680 nm). The scalar irradiance spectra ex hibited 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 depen ded on the choice of the light intensity parameter. Calculations of in itial 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 correspond ing 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 hydrody namic conditions (flow rate of 5 to 6 cm s(-1)).