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

Authors

Peter Koenraad Swart, University of Miami
Alina Szmant, University of North Carolina - WilmingtonFollow
James W. Porter, University of Georgia
Richard E. Dodge, Nova Southeastern UniversityFollow
Jennifer I. Tougas, University of Georgia
John R. Southam, University of Miami

Document Type

Article

Publication Title

Geochimica et Cosmochimica Acta

ISSN

0016-7037

Publication Date

3-15-2005

Keywords

Coral reefs

Abstract

The origin of δ¹³C 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 δ¹³C of the skeleton remains unclear. In this study, the temporal variability of the δ¹³C 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 δ¹³C of the dissolved inorganic carbon (ΣCO₂) at ∼3-h intervals. Throughout the incubation, the concentration of O₂ was measured continuously within the chamber. Our results show that during daylight, the δ¹³C of the ΣCO₂ in the incubation water becomes enriched in ¹³C as a result of fractionation during the fixation of C by photosynthesis, whereas at night the δ¹³C of the ΣCO₂ becomes more negative. The δ¹³C of the respiratory CO₂ 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 δ¹³C of the tissue. The clear seasonal cycle in the δ¹³C of the respiratory CO₂ suggests that there is also seasonal variability in either the δ¹³C 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 δ¹³C 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 δ¹³C 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.

DOI

10.1016/j.gca.2004.09.004

Volume

69

Issue

6

First Page

1495

Last Page

1509

Comments

©2005 Elsevier Ltd.

Additional Comments

NSF grant #: OCE-9217993

NSUWorks Citation

Swart, Peter K., Alina Szmant, James W. Porter, Richard E. Dodge, Jennifer I. Tougas, and John R. Southam. "The isotopic composition of respired carbon dioxide in scleractinian corals: implications for cycling of organic carbon in corals." Geochimica et cosmochimica acta 69, no. 6 (2005): 1495-1509.