the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Oct 2026
Active Calcifying Fluid pH Upregulation Facilitates Resilience of the Coral Tubastraea aurea to Ocean Acidification
Abstract. Ocean acidification poses a critical threat to marine calcifiers, yet the physiological mechanisms allowing certain species to thrive in high-CO₂ environments remain poorly understood. Kueishan Island (Taiwan) provides a unique natural laboratory to address this gap. While nearby non-vent reefs support diverse coral communities, the strongly acidified hydrothermal vent fields at Kueishan Island are dominated by the azooxanthellate coral Tubastraea aurea. Here, we tested whether this persistence is driven by regulation of the calcifying microenvironment. Using skeletal boron isotope (δ¹¹B) composition and scanning electron microscopy (SEM), we reconstructed calcifying-fluid pH (pHCF) and assessed skeletal microstructure in specimens from the vent site (HV, pH ~6.6) and a reference site (FU, pH ~8.0). Despite a ~1.4-unit difference in ambient seawater pH (pHSW), the estimated pHCF was indistinguishable between sites (HV 8.45 ± 0.02, FU 8.38 ± 0.01), resulting in a markedly larger ΔpH (pHCF − pHSW) at the vents. While SEM analysis revealed no gross dissolution at the resolution examined, vent colonies were smaller than reference counterparts, suggesting possible growth constraints under extreme conditions (exploratory given limited colony replication). These findings indicate that T. aurea maintains elevated pHCF in exceptionally low-pH seawater, highlighting a physiological capacity that could contribute to skeletal maintenance under future acidified oceans.
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Status: open (until 08 Dec 2026)
- RC1: 'Comment on egusphere-2026-5987', Joseph Stewart, 29 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5987', Anonymous Referee #2, 10 Nov 2026
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This manuscript investigates the potential physiological mechanisms underlying the persistence of the azooxanthellate coral Tubastraea aurea in an acidified hydrothermal vent environment at Kueishan Island, Taiwan. The study addresses an interesting and ecologically relevant question regarding coral resilience under ocean acidification. The comparison between vent-associated and reference populations, together with skeletal microstructural observations and colony morphometric analyses, provides useful insights into the possible responses of this coral species to extreme environmental conditions.
I particularly appreciate the authors' efforts to combine physiological and morphological observations and to acknowledge the limitations associated with the relatively small sample size and the complex environmental conditions of hydrothermal vent systems. The discussion of potential energetic trade-offs between maintaining calcifying-fluid homeostasis and colony growth is also interesting and provides a useful direction for future research.
Overall, the manuscript is well structured, and the study provides potentially valuable information on the ecological and physiological responses of an azooxanthellate coral to extreme environmental conditions. I have a few comments, mainly concerning the interpretation of morphological observations, sampling limitations, and clarification of some methodological details. Addressing these points would further improve the manuscript.
The study estimates pHCF from skeletal δ¹¹B; it does not directly measure calcifying-fluid pH, ion transport, proton removal, calcification rate, or energetic costs. The Discussion also raises possible effects of boric-acid diffusion, seawater exchange, kinetic fractionation, and distinct mineralization compartments. These caveats complicate attribution of the inferred pHCF specifically to active physiological regulation. Please describe the results as consistent with elevated calcifying-site pH and revise stronger claims such as “mechanistic evidence,” “demonstrate a strong capacity,” and “active pH regulation.” The title, Abstract, and Conclusion should not imply that the study establishes resilience to ocean acidification or proves that pH regulation facilitates persistence.
Seawater was measured on only three occasions over 16 days, and retrieval time was not recorded precisely. These measurements may not represent the conditions during skeletal formation. Please specify the pH scale, probe calibration procedure, and how pH uncertainty was handled. Alkalinity, DIC, pCO₂, and saturation state are not reported for the sampled water, limiting interpretation of carbonate chemistry. Explain how the pH measurements relate to the period represented by each coral sample, and present them as limited observations if longer-term exposure cannot be established. The vent environment should be treated as a complex site contrast, not an isolated ocean-acidification treatment.
The study compares one vent population with one reference population. The sites differ in depth and may differ in other environmental factors, and possible clonality among clustered HV colonies cannot be ruled out. With three colonies per site, and only two per site for the morphology subset, the data cannot separate pH effects from site or population effects. Please frame the findings as a case study of the sampled populations and avoid broad species-level or future community-change claims.
With three colonies per site, the permutation tests have limited p-value resolution; the reported values near 0.10 appear to reflect this limitation. Yet the bootstrap confidence intervals in Supplementary Table S2 exclude zero for δ¹¹B, pHCF, and ΔpH. With so few observations, these intervals need further justification and should not be treated as strong evidence of a site difference. Please provide colony-level data, distinguish analytical uncertainty from between-colony variation, and explain the confidence-interval procedure. Reconsider “indistinguishable” in light of the reported effect estimate and interval. The ΔpH contrast is largely driven by the ambient pH difference between sites and should not be treated as independent evidence of a physiological response.
The passage around line 279 appears to mischaracterize the Gagnon model by saying that isotopically heavy boric acid diffuses into the calcifying fluid and artificially elevates its δ¹¹B, followed by an explanation based on precipitation diluting a diffusion pump with bulk seawater. Please revise this description to reflect the model’s proposed effect of the balance between boric-acid diffusion and seawater exchange. As currently written, the mechanism used to dismiss this alternative explanation is not adequately supported. The authors should clearly distinguish what the model predicts from what is inferred from the present data.
The Methods state that the remaining skeletons were immersed in NaOH “to buffer the skeletal pH and improve structural stability.” A carbonate skeleton does not have a persistent pH in the same sense as living calcifying fluid, and NaOH is not explained as a structural consolidant here. Please state the specific purpose of this treatment, its concentration, and how it is expected to affect the skeletal surface or microstructure. If it was intended to neutralize residual acidity from an earlier step, identify that step. The authors should also explain whether this preparation could affect the SEM observations.
Figures 5-6 combine studies that differ in species, skeletal domains, methods, and environmental conditions. These comparisons can provide context but cannot establish that T. aurea has a greater resilience strategy or a particular comparative physiological position. Keep them descriptive. Likewise, energetic trade-offs, prey limitation, microbial subsidies, and future community shifts are not tested here; label them as hypotheses. Please streamline repeated cautions throughout the Results and Discussion while retaining the necessary limitations in the interpretation and a clear limitations paragraph near the end.
Line 50: The point that Tubastraea is a natural component of Taiwanese reefs is useful. Consider moving the fuller contrast with invasive populations elsewhere to the Conclusion, where the region-specific implications for conservation and management can be stated.
Line 63: Consider shortening the caveat about hydrothermal vents as OA analogues in the Introduction and discussing the broader limits to extrapolation in the Discussion. The Introduction can briefly identify the study’s scope and then state its objectives.
Line 83: “Hydrothermal effluent” may be less clear than “hydrothermal fluid” or “hydrothermal plume,” depending on the feature intended.
Line 87: Introduce the family Dendrophylliidae in the Introduction when Tubastraea is first described.
Line 88: Define “Yellow Vent” at first mention. A site map showing Kueishan Island, HV, FU, site coordinates, and the distance between HV and FU would help readers interpret the comparison.
Line 125: Acronyms such as IES and PP appear not to be used again. Consider spelling out the terms and omitting unused abbreviations.
Line 151: “Klochko et al. (Klochko et al., 2006)” repeats the citation.
Lines 167-173: Clarify how the 8-10 polyps per colony were selected, for example, randomly or from specified colony regions, because morphology may vary within a colony.
Line 173: Change “interpreted with cautious” to “interpreted with caution.”
Line 193: The salinity values are somewhat below typical open-ocean salinity. Please provide context if salinity is relevant to site interpretation; the values alone do not establish a strong freshwater influence.
Figure 3: Explain what the red error bar denotes. Also state the pH scale used for the seawater measurements
Lines 237-244: Colony length is reported as 13.0 ± 2.83 cm for FU and 3.5 ± 0.71 cm for HV in the main text, but as millimetres in Supplementary Table S4. Please correct the units and define the measurement in the Methods.
Lines 242-244: The main text refers to “coenosteum length,” whereas Supplementary Table S4 uses “coenosarc.” These are different structures; verify the measurement and use the correct term consistently.
Lines 355-357: The text cites Supplementary Table S3 for colony morphometrics, but these measurements appear in Table S4; Table S3 contains SEM-based measurements. Please correct the citation.
Line 370: Please verify that Jeng et al. (2004) supports the claim that vent T. aurea depends “primarily on marine snow.” If direct evidence, such as stable-isotope or gut-content data, is unavailable, qualify or remove the claim.
I suggest adding a consolidated paragraph on the limitation of the study before the conclusion.
Supplementary material: Use one consistent table-labeling style, such as “Table S1” throughout. Please also check the apparently unrelated bibliographic entry on tropical rainforests and clarify the data/materials availability statement, including the claim that no new materials were generated despite collection and analysis of coral skeletons.
Citation: https://doi.org/10.5194/egusphere-2026-5987-RC2
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- 1
The authors present skeletal boron isotope measurements and SEM imagery from Tubastraea aurea colonies collected across an extreme seawater pH gradient associated with hydrothermal venting near Kueishan Island, Taiwan. Boron isotope data are used to reconstruct calcification-fluid pH, allowing the authors to assess the extent to which these corals regulate their internal carbonate chemistry under highly corrosive environmental conditions. The results suggest that Tubastraea maintains elevated calcification-fluid pH despite exposure to low ambient seawater pH, highlighting a potential physiological mechanism underlying the resilience of this species to ocean acidification.
The motivation for the study is well founded. Understanding how marine calcifiers, and corals in particular, respond physiologically to declining seawater pH remains a major challenge in predicting the ecological consequences of ocean acidification. The study site provides a valuable natural laboratory for investigating these responses under environmentally relevant conditions, and the manuscript is generally well written and straightforward to follow. The boron isotope dataset is technically sound, and the integration of geochemical and microstructural observations provides an interesting perspective on the response of this species to extreme environmental conditions.
This is the second time I have reviewed this article and I am pleased to see that many of my previous comments have been addressed. I enjoyed reading the manuscript again and believe the data are worthy of publication in Biogeogeosciences with a few modifications:
Although Tubastraea is not invasive at the study site. I would include a line in the Abstract about its invasive nature at other sites (e.g. Caribbean or Brazil). This coral is a real problem in many Atlantic regions and understanding its resilience to OA beyond that of other corals has huge implications for coral ecology where this coral has invaded. Although mentioned later in the paper, it should be addressed again in the conclusions, and adding this line here in the abstract will make it clear why study of this particular taxon is needed and its resilience is far from good news globally.
Line 31: need to cite Al-Horani et al., 2003 with regards to pH upregulation in corals http://dx.doi.org/10.1016/S0022-0981(02)00578-6
Line 50: I agree with this point that in reefs of Taiwan Tubastrea are an important part of a natural healthy reef. Good to mention it here, but the line starting “Therefore, the invasive behaviour documented..” might be better placed in the conclusions section at the end as an important concluding remark about how the implications of these results differ depending on if you are managing coral conservation in the Atlantic or Pacific.
Line 63: “We acknowledge that shallow-water vents are not perfect analogues..” this line does the study a disservice and is not needed until the latter part of the discussion where the extent to which these results are scalable to the Tubastrea worldwide is discussed. I would leave a critique of the results to the end of the paper, here at the end of the intro just tell the reader what will be shown.
Line 83: hydrothermal “effluent” feels like a strange phrasing - more commonly used for a pollutant. Perhaps hydrothermal plume or vent fluid would be clearer and more standard terminology.
Line 87: introduce family Dendrophylliidae in the introduction when first describing Tubastrea
Line 88: What is a Yellow Vent mouth? Is the vent system call the Yellow vents? This is the first mention of Yellow. A map showing the location of Kueishan Island and where the low pH and Fulong Bay background sites were situated would be valuable for the reader.
Line 125: there are acronyms and shorthands defined here that are not used again. e.g. IES and PP. Just give the term in full and leave out the abbreviation.
Line 151: Klochko et al. (Klochko et al., 2006). Reference written twice here.
Line 156: I question this pre-processing before SEM analysis. “To buffer the skeletal pH and improve structural stability, the samples were immersed in a NaOH solution for 24 hours, subsequently rinsed with Milli-Q water, and air-dried.”. A carbonate skeleton does not have a persistent, measurable “skeletal pH” in the same sense as a living organism's calcifying fluid. If the intention is to neutralise residual acidity from a previous treatment, that would require a specific explanation of the preceding preparation steps. Calcium carbonate skeletons are not ordinarily stabilised by soaking them in NaOH. NaOH is not a conventional consolidant for aragonite or calcite. It does not provide an obvious mechanism for strengthening the mineral framework. If the skeletons are fragile, structural preservation is usually achieved by careful handling, appropriate drying, and, where justified, a suitable embedding or consolidation method. There may be a legitimate preparation procedure behind this description, but the authors need to explain what the NaOH is actually intended to do.
Line 173: interpreted with cautious should be with caution.
Line 193: These are very low salinities at both sites and suggest a strong freshwater influence (e.g. a semi-enclosed basin of some kind). this isn’t a problem, but oddities about the site are important when considering how these results extrapolate worldwide. Again, sample location map would be valuable.
Figure 2: the Cr Cn naming scheme for samples is not used here as it is in the text. I would add “(Cr)” and “(Cn)” to the relevant part of the legend so that it is clear. Also need to state somewhere what scale pH was measured on. Total scale I assume?
Figure 3: what does the red error bar denote?
Line 238: Despite my previous comments to exercise caution when interpreting these results at the global scale, statements like results are “interpreted as exploratory” are probably now overused in the text. I would give the caveats of sampling size like this once at the end of the paper, but in the results just describe what was found.
Line 279: Reference here to the Gagnon model requires editing. The passage says that isotopically heavy boric acid diffuses into the calcifying fluid and artificially elevates its δ11B. That is not an appropriate description of the net effect on skeletal δ11B in the Gagnon model. The model predicts that increasing DB/kz can drive skeletal δ11B lower, even with constant internal pH. The precipitation-and-dilution explanation given here is not the model's proposed mechanism for the isotope effect. The DIC dependence of δ11B is a change in the relative importance of diffusion versus seawater exchange. The statement that elevated DIC increases precipitation, which then dilutes the diffusion pump with bulk seawater isn’t quite right. The authors are right though, the DIC effect explored by Gagnon should act functionally in the same direction as pH (low pH, carbon rich waters giving low δ11B). this is clearly not what is driving the higher δ11B values at the vent site.
Line 299: again, “we conservatively interpret these findings”. Save all of these caveats for one paragraph at the end of the discussion.
Line 302: also Standish et al., 2024 https://www.nature.com/articles/s41598-024-61772-2
Figure 5 is a nice review of coral δ11B but there are other azooxanthellate corals. it could also include Gagnon et al., 2021 Balanophyllia data. There are also more Madrepora, Lophelia, and Enallopsammia data in Martin et al., 2016 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL066494 . Also consider using the same symbol for each taxon (i.e. combine data from papers together) so that there are fewer symbols and colours needed. Consider making the red data from this study larger so that they stand out. Also include a 1 to 1 line (pHsw = pHcf) to demonstrate how much higher the deep sea corals are in terms of pH upregulation.
Joe Stewart