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1. Coral growth persistence amidst bleaching events

   https://doi.org/10.1002/lol2.10340  

   Mantanona, Hanna C.; DeCarlo, Thomas M. (October 2023, Limnology and Oceanography Letters)

   Abstract As mass bleaching events decimate stony coral populations, production of calcium carbonate is diminished on reefs, dampening their capacity to keep pace with rising sea levels. However, perturbations to the calcification process of surviving wild corals during bleaching are poorly constrained, owing to the lack of suitable techniques to retroactively extract this information from coral skeletons at sufficient resolution. Here, we use novel Raman spectrometry techniques to test the biogeochemical response of long‐lived corals before, during, and after bleaching. Maintenance of high aragonite saturation state (Ω Ar ) in the coral calcifying fluid is key to driving rapid skeletal growth but would be expected to decrease when corals become energetically depleted without their symbionts. Contrary to this expectation, our results demonstrate that corals upregulate calcifying fluid Ω Ar during bleaching and for at least 2 yr after recovery. This indicates that the calcification process of coral‐bleaching survivors is unexpectedly resilient. 

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2. Coral growth along a natural gradient of seawater temperature, pH, and oxygen in a nearshore seagrass bed on Dongsha Atoll, Taiwan

   https://doi.org/10.1371/journal.pone.0312263  

   Pezner, Ariel K; Courtney, Travis A; Chou, Wen-Chen; Chu, Hui-Chuan; Frable, Benjamin W; Kekuewa, Samuel_A H; Soong, Keryea; Wei, Yi; Andersson, Andreas J (October 2024, PLOS ONE)

   Laruelle, Goulven G (Ed.)

   Coral reefs are facing threats from a variety of global change stressors, including ocean warming, acidification, and deoxygenation. It has been hypothesized that growing corals near primary producers such as macroalgae or seagrass may help to ameliorate acidification and deoxygenation stress, however few studies have explored this effect in situ. Here, we investigated differences in coral growth rates across a natural gradient in seawater temperature, pH, and dissolved oxygen (DO) variability in a nearshore seagrass bed on Dongsha Atoll, Taiwan, South China Sea. We observed strong spatial gradients in temperature (5°C), pH (0.29 pH units), and DO (129 μmol O₂kg^-1) across the 1-kilometer wide seagrass bed. Similarly, diel variability recorded by an autonomous sensor in the shallow seagrass measured diel ranges in temperature, pH, and DO of up to 2.6°C, 0.55, and 204 μmol O₂kg^-1, respectively. Skeletal cores collected from 15 massivePoritescorals growing in the seagrass bed at 4 sites revealed no significant differences in coral calcification rates between sites along the gradients. However, significant differences in skeletal extension rate and density suggest that the dynamic temperature, pH, and/or DO variability may have influenced these properties. The lack of differences in coral growth between sites may be because favorable calcification conditions during the day (high temperature, pH, and DO) were proportionally balanced by unfavorable conditions during the night (low temperature, pH, and DO). Alternatively, other factors were simply more important in controlling coral calcification and/or corals were acclimated to the prevailing conditions at each site. 

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3. Effects of year‐long exposure to elevated pCO₂ on the metabolism of back reef and fore reef communities

   https://doi.org/10.1002/lno.12504  

   Edmunds, Peter J; Doo, Steve S; Carpenter, Robert C (March 2024, Limnology and Oceanography)

   Abstract The implications of ocean acidification are acute for calcifying organisms, notably tropical reef corals, for which accretion generally is depressed and dissolution enhanced at reduced seawater pH. We describe year‐long experiments in which back reef and fore reef (17‐m depth) communities from Moorea, French Polynesia, were incubated outdoors under pCO₂regimes reflecting endpoints of representative concentration pathways (RCPs) expected by the end the century. Incubations were completed in three to four flumes (5.0 × 0.3 m, 500 L) in which seawater was refreshed and circulated at 0.1 m s⁻¹, and the response of the communities was evaluated monthly by measurements of net community calcification (NCC) and net community productivity (NCP). For both communities, NCC (but not NCP) was affected by treatments and time, with NCC declining with increasing pCO₂, and for the fore reef, becoming negative (i.e., dissolution was occurring) at the highest pCO₂(1067–1433μatm, RCP8.5). There was scant evidence of community adjustment to reduce the negative effects of ocean acidification, and inhibition of NCC intensified in the back reef as the abundance of massivePoritesspp. declined. These results highlight the risks of dissolution under ocean acidification for coral reefs and suggest these effects will be most acute in fore reef habitats. Without signs of amelioration of the negative effects of ocean acidification during year‐long experiments, it is reasonable to expect that the future of coral reefs in acidic seas can be predicted from their current known susceptibility to ocean acidification. 

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4. Crystallographic and chemical signatures in coral skeletal aragonite

   https://doi.org/10.1007/s00338-021-02198-4  

   Farfan, Gabriela A.; Apprill, Amy; Cohen, Anne; DeCarlo, Thomas M.; Post, Jeffrey E.; Waller, Rhian G.; Hansel, Colleen M. (February 2022, Coral Reefs)

   Abstract Corals nucleate and grow aragonite crystals, organizing them into intricate skeletal structures that ultimately build the world’s coral reefs. Crystallography and chemistry have profound influence on the material properties of these skeletal building blocks, yet gaps remain in our knowledge about coral aragonite on the atomic scale. Across a broad diversity of shallow-water and deep-sea scleractinian corals from vastly different environments, coral aragonites are remarkably similar to one another, confirming that corals exert control on the carbonate chemistry of the calcifying space relative to the surrounding seawater. Nuances in coral aragonite structures relate most closely to trace element chemistry and aragonite saturation state, suggesting the primary controls on aragonite structure are ionic strength and trace element chemistry, with growth rate playing a secondary role. We also show how coral aragonites are crystallographically indistinguishable from synthetic abiogenic aragonite analogs precipitated from seawater under conditions mimicking coral calcifying fluid. In contrast, coral aragonites are distinct from geologically formed aragonites, a synthetic aragonite precipitated from a freshwater solution, and mollusk aragonites. Crystallographic signatures have future applications in understanding the material properties of coral aragonite and predicting the persistence of coral reefs in a rapidly changing ocean. 

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5. Corals in ocean acidification and the role of calcium ion homeostasis to maintain calcification

   https://doi.org/10.1093/icesjms/fsaf050  

   Armstrong, David A; Bahr, Keisha D (April 2025, ICES Journal of Marine Science)

   Browman, Howard (Ed.)

   Abstract Coral calcification is essential to provide the structural foundation for coral reefs and is integral in supporting marine biodiversity reliant on reef ecosystems. The drivers for calcification in corals are undoubtedly highly complex and require several perspectives to identify vulnerabilities in the context of environmental change. Specifically, ocean acidification (OA) resulting from the rise of anthropogenic carbon dioxide (CO2) emissions poses a potential threat to the physiological mechanisms that drive calcification in corals. Therefore, this report goes beyond environmental seawater chemistry to examine the physiological mechanism of calcium ion homeostasis. Calcium's role in calcification physiology is well established, but how calcium homeostasis could shift under acidification has little been considered a significant driver in reduced calcification. Calcium is potentially the most actively transported substrate in coral calcification, though in high chemical abundance in seawater, corals are likely utilizing the most energy to concentrate calcium at the site of calcification. We argue for increased consideration of the calcium ion in the context of OA when identifying sensitivities. The concepts proposed here are justified through a combination of results from novel RAMAN spectroscopy and molecular work that provides insight into shifts in calcium homeostasis when exposed to acidification. We speculate that future work incorporating methodologies considering calcium dynamics in OA could benefit by narrowing in on what physiological mechanisms are potentially vulnerable. It is imperative that we identify what drives lower calcification in corals under OA to inform efficient directives in identifying species sensitivities to future climate change. 

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