CORALS
CORAL NITROGEN ISOTOPES AS A SYMBIOSIS PROXY
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 zooxanthellae, 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 coral Oculina arbuscula under 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. Read more about it here!
ISOTOPIC CONSTRAINTS ON THE FATE OF ANTHROPOGENIC NITROGEN IN THE NORTHERN GULF OF MEXICO
Human activities, such as agricultural practices and fossil fuel combustion, have significantly altered the global nitrogen (N) cycle. The current rate of anthropogenic N production exceeds 200 million tons per year, matching the rate of natural N fixation globally. Excess N in coastal waters can lead to eutrophication, hypoxic zones, fish kills, and habitat loss. The northern Gulf of Mexico (GoM) has been profoundly impacted by excess anthropogenic N inputs from the Mississippi-Atchafalaya River Basin (MARB), resulting in coastal hypoxia and harmful algal blooms in recent decades. However, the fate of anthropogenic N in the northern GoM is uncertain. It is unclear how much of the riverine N is transferred to the open waters of the GoM and how it affects the ecosystem and productivity there. Given that the riverine N input from the MARB has a higher 15N/14N ratio compared to the open ocean N source in the GoM, natural-abundance N isotope analyses provide a valuable tool for assessing the impact of riverine N on the open GoM. In this study, we generated a 40-year N isotope record (δ15N) using a Siderastrea siderea coral colony obtained from the Dry Tortugas National Park, Florida, USA and examined modern seawater nitrate isotopes (δ15N and δ18O) in the northern GoM collected, collected during five cruises. From 1972 to 2012, despite the increase in the coastal hypoxic zone area and harmful algal blooms in the GoM, there was only a minor increase (<0.3‰) in the annual mean coral skeletal δ15N values. Additionally, nitrate δ15N exhibits a minimum at ~200 m in the northern GoM, consistent with the patterns observed in the open waters of the GoM. This minimum has been previously interpreted as stemming from N2 fixation in the (sub)tropical Atlantic and subsequent water transport. Both the historical coral δ15N record and modern nitrate isotope data imply that that the MARB N inputs may be largely removed in coastal areas by sedimentary denitrification/anaerobic ammonium oxidation and organic N burial, limiting their impact on the open ocean.
