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The resilience of coral reefs in oligotrophic, (sub)tropical oceans is largely due to the symbiotic relationship between scleractinian corals and Symbiodiniaceae algae, which enables efficient internal nutrient recycling. Investigating the history of this coral symbiosis can provide insights into its role in sustaining the health of both present and future coral reefs. The isotopic composition of organic nitrogen (15N/14N or δ15N) bound within coral skeletons has been utilized to trace the existence of symbiosis in fossil corals, suggesting that coral symbiosis dates back to at least 210 million years ago. The basis of this proxy is that symbiotic corals are expected to exhibit lower δ15N compared to their non-symbiotic (aposymbiotic) counterparts within the same environments, owing to internal nitrogen recycling between the coral host and algal symbiont, and reduced leakage of low-δ15N ammonium into seawater. However, this hypothesis has not been adequately tested in contemporary settings. In a laboratory experiment, we examined the δ15N differences between the symbiotic and aposymbiotic branches within the same genetic backgrounds of the facultatively symbiotic coralOculina arbusculaunder well-fed conditions. Across five different genotypes in two separate experiments, symbiotic branches consistently showed lower δ15N than their aposymbiotic counterparts. These findings corroborate the use of δ15N as a proxy for identifying coral symbiosis in the past, particularly when multiple species of corals coexisted in the same environments.
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This content will become publicly available on March 26, 2026
Nitrogen isotope ratios across the Bermuda coral reef: implications for coral nitrogen sources and the coral-bound nitrogen isotope proxy
The nitrogen (N) isotopic composition of coral tissue provides insight into N sources and cycling on reefs, and coral skeleton-bound organic matter (CS-δ15N) can extend these insights into the past. Across the Bermuda platform, we measured the δ15N of four coral species and their potential N sources, as well as an asymbiotic filter feeder as a comparative heterotroph and benthic macroalgae as a comparative autotroph. Organisms and organic N pools from the coral reefs exhibit a δ15N increase toward the Bermuda coast, likely due to anthropogenic N inputs. At all sites, the δ15N of bulk coral tissue is consistent with corals feeding dominantly on zooplankton-sized organic matter and some smaller suspended particulate N. The corals lack the trophic δ15N elevation that characterizes serpulids; this is consistent with internal recycling and retention of low-δ15N metabolic N by symbiont-bearing corals. The data are inconsistent with corals’ reliance on the dissolved inorganic N used by macroalgae at the same sites. Among coral species, two species with smaller polyps (1-2 mm) have ~1‰ lower bulk tissue δ15N than two counterparts with larger polyps (5-10 mm), perhaps due to differences in food source. Taxon-specific δ15N differences are also observed between coral tissue and skeleton-bound N, with larger differences in the two small-polyp species. In net, however, CS-δ15N mean values and spatial gradients were similar in the four species studied.
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- PAR ID:
- 10646792
- Editor(s):
- Hajime, Kayanne
- Publisher / Repository:
- Frontiers
- Date Published:
- Journal Name:
- Frontiers in Marine Science
- Volume:
- 12
- ISSN:
- 2296-7745
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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