The Isotopic Composition of Respired Carbon Dioxide in Scleractinian Corals: Implications for Cycling of Organic Carbon in Corals

The origin of δ ¹³ C variations within the skeletons of zooxanthellate scleractinian corals is still a matter of considerable debate. In particular, the role respired CO ₂ plays in controlling the eventual δ ¹³ C of the skeleton remains unclear. In this study, the temporal variability of the δ ¹³ C of respired CO ₂ produced by Montastraea faveolata has been measured at approximately monthly intervals over a 1-year period. In these experiments, three corals maintained on a platform at 8 m depth near Molasses Reef in the Florida Keys were incubated in closed chambers for 24-h periods and samples of the incubation water analyzed for the δ ¹³ C of the dissolved inorganic carbon (ΣCO ₂ ) at ∼3-h intervals. Throughout the incubation, the concentration of O ₂ was measured continuously within the chamber. Our results show that during daylight, the δ ¹³ C of the ΣCO ₂ in the incubation water becomes enriched in ¹³ C as a result of fractionation during the fixation of C by photosynthesis, whereas at night the δ ¹³ C of the ΣCO ₂ becomes more negative. The δ ¹³ C of the respiratory CO ₂ ranges from −9‰ in the late spring to values as low as −17‰ in the autumn. The lighter values are significantly more negative than those reported by previous workers for coral tissue and zooxanthellae. An explanation for this discrepancy may be that the corals respire a significant proportion of isotopically negative substances, such as lipids, which are known to have values up to 10‰ lighter compared to the bulk δ ¹³ C of the tissue. The clear seasonal cycle in the δ ¹³ C of the respiratory CO ₂ suggests that there is also seasonal variability in either the δ ¹³ C of the coral tissue or the type and/or amount of organic material being respired. A similar temporal pattern and magnitude of change was observed in the δ ¹³ C of the coral tissue samples collected from a nearby reef at monthly intervals between 1995 and 1997. These patterns are similar in timing to the δ ¹³ C measured in the coral skeletons. We have also calculated an annual mean value for the fractionation factor between dissolved CO ₂ ⁻ in the external environment and photosynthate fixed by the zooxanthellae of 1.0121 (±0.003). This value is inversely correlated with the ratio of photosynthesis to respiration (P/R) of the entire organism and shows the highest values during the summer months.