In situ diel cycles of photosynthesis and calcification in hermatypic corals

Diel cycles of photosynthesis, respiration, and calcification were studied in situ on two hermatypic corals, Acropora eurystoma and Favia favus, using a submerged respirometer deployed at 5-m depth in the Gulf of Eilat, Israel. A fast-repetition-rate fluorometer (FRRF) was placed in front of one coral in each incubation. Net photosynthesis (PN) and calcification were positively correlated with photosynthetically available radiation (PAR). A hysteresis phenomenon occurred in both photosynthesis and calcification relative to PAR: rates were higher in the afternoon than in the morning. This hysteresis is reversed (counterclockwise) compared to that observed in other photosynthetic organisms (plants and algae); however, the quantum yield (ΔF': Fm9) and the rate of photosynthetic electron transport per PSII reaction center (P _f) showed a regular (clockwise) hysteresis. The photosynthetic compensation point (I_c) was significantly higher in the morning than in the afternoon, 900 and, 300 μmol photons m^-2 s^-1, respectively. We suggest that PN, reversed hysteresis, and lower morning compensation points are caused by higher respiration in the morning than in the afternoon. This may be a result of higher predation rates by the corals during the night, when plankton is more abundant, followed by higher respiration during the digestion of the prey. Calcification counterclockwise hysteresis was observed during high-PAR periods (<400 and 200 μmol photons m^-2 s^-1 for A. eurystoma and F. favus, respectively). We suggest that high morning respiration may induce a decrease in internal tissue pH, causing lower calcification rates.