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SUMMARY This work, performed on the scleractinian coral Stylophora pistillata, aims at providing new information on the `light-enhanced calcification' process. In a first step, in controlled conditions of culture and constant light supply, we studied the diurnal cycle of calcification. We determined that calcification rates are constant during the day and the night with a 2.6-fold difference between day and night calcification rates. We also showed that the photosynthetic rate is constant throughout the day when a constant light intensity is applied. Our results on free-running experiments in prolonged conditions of light or dark suggest that calcification is not regulated by an endogenous circadian rhythm. In a second step, using a kinetic isotopic approach with 45Ca, we characterized the transition stages between day and night and vice versa. Under our experimental conditions, the lag-phase necessary to switch from the light to the dark calcification rate is the same as the lag-phase necessary to switch from the dark to the light calcification rate. We discuss our results in the context of two hypotheses of the light-enhanced calcification process: (1) the role of photosynthesis on the pH in the coelenteron and (2) the role of photosynthesis in supplying precursors of the organic matrix.

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