The isotopic composition of respired carbon dioxide in scleractinian corals: Implications for cycling of organic carbon in corals

The origin of VC variations within the skeletons of zooxanthellate scleractinian corals is still a matter of considerable debate. In particular, the role respired CO2 plays in controlling the eventual delta C-13 of the skeleton remains unclear. In this study, the temporal variability of the delta C-13 of respired CO2 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 delta C-13 of the dissolved inorganic carbon (Sigma CO2) at similar to 3-h intervals. Throughout the incubation, the concentration of 02 was measured continuously within the chamber. Our results show that during daylight, the delta C-13 of the Sigma CO2 in the incubation water becomes enriched in C-13 as a result of fractionation during the fixation of C by photosynthesis, whereas at night the delta C-13 of the Sigma CO2 becomes more negative. The delta C-13 of the respiratory CO, ranges from -9 parts per thousand in the late spring to values as low as -17 parts per thousand 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 delta C-13 of the tissue. The clear seasonal cycle in the delta C-13 of the respiratory CO2 suggests that there is also seasonal variability in either the delta C-13 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 delta C-13 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 delta C-13 measured in the coral skeletons. We have also calculated an annual mean value for the fractionation factor between dissolved CO2 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. Copyright (c) 2005 Elsevier Ltd.