Effect of light on skeletal δ13C and δ18O, and interaction with photosynthesis, respiration and calcification in two zooxanthellate scleractinian corals

The respective role of environmental and physiological controls on the isotopic composition of coral skeletons is a matter of debate. It has been shown that delta(13)C can be affected by light, seawater delta(13)C(DIC), nutrition, respiration. spawning, pH and temperature. We investigated the effect of light on photosynthesis, respiration. calcification, and stable isotope composition (delta(13)C and delta(18)O) of the skeleton in the zooxanthellate scleractinian corals Acropora sp. and Stylophora pistillata. Colonies were grown on glass slides under controlled conditions in the laboratory at low and high light (LL and HL: 132 and 258 mumol photons m(-2) s(-1)). The average net photosynthesis of Acropora sp. was significantly higher under HL than under LL. The difference was not statistically significant for S. pistillata. The respiration rate did not change significantly in both species under the two light conditions. The calcification rate of S. pistillata under HL was 17-fold higher than under LL and 2.5-fold higher for Acropora sp. The average skeletal delta(13)C and 5180 of Acropora sp. were significantly more negative under LL than under HL. For S. pistillata, skeletal delta(18)O was significantly more negative in the LL than in the HL condition. The delta(13)C value of the skeleton deposited under LL was also more negative than under HL, although the difference was not statistically significant. The skeletal delta(13)C was significantly correlated with the rate of calcification. both in LL and HL. No correlation was found between skeletal 513 C and the following other physiological parameters: net and gross photosynthesis (P-n and P-g), respiration (R), and the P-g/R ratio. The increase of skeletal delta(13)C With increasing light seems to support the model of Goreau (1977). Zooxanthellae mostly fix C-12 under HL, leading to an increased concentration of C-13 in the common carbon pool which supplies dissolved inorganic carbon (DIC) for calcification. Hence. the skeleton deposited is isotopically enriched in C-13. This general model needs revision to accommodate the recent finding that calcification and photosynthesis draw carbon from two reservoirs (seawater and metabolic DIC). and that respiratory CO2 is the major source of DIC for calcification. It is suggested that zooxanthellae mostly fix C-12[DIC] in LL; the organic matter respired, the CO2 released, and the CaCO3 deposited being therefore isotopically light. Under HL condition. zooxanthellar photosynthesis uses both [C-12]- and [C-13]DIC. The photosynthetic products catabolized by the coral, as well the respiratory CO2 and the CaCO3 precipitated are therefore heavier. (C) 2001 Elsevier Science B.V. All rights reserved.