the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2025
Volcanic ash leaching alters the trace metal distribution within the coral holobiont of Stylophora pistillata
Abstract. Explosive volcanic eruptions generate large amounts of volcanic ash that release essential and nonessential trace metals upon deposition in seawater, modifying its chemical composition. Tropical scleractinian corals, known for accumulating trace metals, are susceptible to these changes, making them valuable biomonitors for increased metal concentrations. In this study, we investigated how volcanic ash leaching influences trace metal partitioning within the hermatypic branching coral Stylophora pistillata through six-week coral culture experiments. Coral nubbins were reared under control and ash exposed conditions, with 2.5 g ash added three times a week (averaging 250 mg L-1 per week). We quantified trace metals (V, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) in the ash-seawater leachate, and in three distinct coral compartments (skeleton, tissue and symbionts). 24 hour ash leaching experiments at a ratio of 1:100 (g ash : mL seawater) demonstrated that ash from La Soufrière (St. Vincent) released trace metals in the order Mn, Zn, Co, Cu, Cd, Fe, and Ni into seawater, while Pb and V were scavenged. Trace metal concentrations in coral compartments correlated with seawater concentrations, with most significant changes observed in the skeletal metal content. Ash exposure enriched skeletal concentrations of V, Mn, Fe, Ni, and Zn while depleting Cu and Pb. Ash leaching also shifted the metal distribution in coral skeletons, affecting relationships between transition and alkaline earth/alkali metals. Apparent skeletal distribution coefficients (KEl) for labgrown corals showed most trace metals were less abundant in skeletons than seawater (KEl <1), except for Pb, Cd and Co (KEl >1). Metal concentrations varied between tissues and symbionts, with Mn and Fe significantly enriched in ash exposed tissues. Volcanic ash releases a range of trace metals, altering the coral metallome by affecting bioaccumulation and metal redistribution across coral compartments. These findings advance our understanding of coral trace metal dynamics at the organismal level.
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RC1: 'Comment on egusphere-2025-1713', Anonymous Referee #1, 30 Jun 2025
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AC1: 'Reply on RC1', Frank Förster, 16 Jul 2025
Dear Referee #1,
the authors thank you for the constructive feedback. Please find attached the "Response to the Reviewers", which provides point-to-point replies to all comments and concerns raised by the referees #1 and #2. Please be aware that the mentioned lines in the answers refer to the revised manuscript, which will be uploaded once the open discussion closes.
Best regards
Frank Förster
Corresponding Author
-
AC1: 'Reply on RC1', Frank Förster, 16 Jul 2025
-
RC2: 'Comment on egusphere-2025-1713', Anonymous Referee #2, 10 Jul 2025
The manuscript by Förster et al. details the results of a geochemical study into Stylophora pistillata cultured in tanks exposed to volcanic ash. The topic covered in this paper is important because it has implications for the health of corals and for their potential role as archives of past volcanic eruptions. The scope of the study is novel, in particular because it investigates the impact on the geochemistry of three different coral compartments from the same individuals - skeleton, tissue, and symbionts. Together this means the manuscript falls within the scope of Biogeosciences. The methods used are appropriate and, for the most part, adequately documented. The text and the figures are clear and concise, whilst the referencing is thorough. I enjoyed reading this manuscript, and I recommend its acceptance following some minor revisions listed below.
Minor Points:
- A little more emphasis on the big picture, and why this study is important, should be added to the abstract, and to a lesser extent the conclusion, to stress the significance of the study. I.e. ramifications for coral health, and presumably as an archive of past volcanic eruptions (in relation to the skeleton at least). This is currently alcking. Out of interest, do you envisage a scenario where the trace metal concentrations of coral skeletons could be used to estimate the volume of ash dropped on a reef, or would there be too many unknowns, such as oceanographic dynamics?
- Line 73, sentence starting with “Stewart et al. (2020) argues that…”: please rephrase, as currently unclear.
- Line 132, referring to accuracy of solution measurements. You state that recoveries ranged from 87% to 110%. So preconcentration procedures fractionate your solutions? Was this then corrected for when analysing your samples? If so, please detail this step. If not, then why not?
- In line 140 you state the culture conditions, then in line 416 you state that the impact of the ash dosing on light and pH was “negligible”. Can you expand on this, and state to what degree both of these parameters changed during the ash dosing, and or how long?
- Line 146, and the clause “natural containing metals with ash derived metals.”: please rephrase as this isn’t clear.
- Please comment on how your ash dosing (2.5 g, three times a week, into your 30 L culturing tanks, line 150) compares to natural analogues. What made you choose this mass and rate, and how common is this dose rate/amount is likely to be in nature? Would this constitute an extreme case of ash-fall, or more a more minor one’?
- Section 2.2.2: Please give details on how the major and trace element measurements of the tissue and symbionts were standardized, and the accuracy/precision of these measurements.
- Line 206: deleted the comma after “in this section” as it is unnecessary.
- Line 221: OES and ICP-MS are techniques, not instruments, so please rephase to: “…measured by ICP-OES…and QQQ-ICP-MS…”.
- Line 230: were any polishing agents used when grinding the skeleton samples for LA-ICP-MS?
- Line 257: please state what values were used for NIST612 when standardizing your data.
- Line 258: please state what Ca concentrations were used for internal normalization.
- Line 374, sentence starting with “Here, we present…”. Please rephrase as currently unclear. I think you may just need to say “…changes in the…”
- Line 411/Results section: you quite rightly focus on the geochemical differences between the skeleton/tissue/symbionts cultured in the two treatments, but to put these in context we are missing an assessment of the tank effect within treatment: are there any statistically significant differences between the three compartments grown in the replicate tanks? Similarly, were there any differences between nubbins grown within the same tank?
- Line 548: Chalk et al. 2021 (https://doi.org/10.1038/s41598-020-78778-1) also show this for S. siderea.
- Line 557: you say that each laser spot reflected a mixture of COC and fibrous aragonite. Is this assumed, or could you actually see the COCs to make certain of this? Please make this clear either way. It seems unlikely that you can be absolutely certain of equal proportions, so could some of your variation be caused by different COC:fibre ratios? It would be interesting to see if the ash-derived trace metals are preferentially incorporated into one or the other of these structural components.
Citation: https://doi.org/10.5194/egusphere-2025-1713-RC2 -
AC2: 'Reply on RC2', Frank Förster, 16 Jul 2025
Dear Referee #2,
the authors thank you for the constructive feedback. Please find attached the "Response to the Reviewers", which provides point-to-point replies to all comments and concerns raised by the referees #1 and #2. Please be aware that the mentioned lines in the answers refer to the revised manuscript, which will be uploaded once the open discussion closes.
Best regards
Frank Förster
Corresponding Author
Status: closed
-
RC1: 'Comment on egusphere-2025-1713', Anonymous Referee #1, 30 Jun 2025
The work by Föster et al. investigates the effects of volcanic ash exposure on corals across three biological compartments—tissue, skeleton, and symbiont. Studies that address all three compartments simultaneously are rare, and this manuscript sets a valuable precedent for improving our understanding of how changes in elemental composition affect coral physiology at different levels. The topic is novel and important: volcanic ash effects on corals remain understudied, and this work may also have broader implications for understanding how other disturbances introducing metals into marine environments impact coral health. The manuscript is generally well written, and the main ideas are easy to follow. However, I offer the following comments and suggestions to help improve the clarity and impact of the study.
General Comments
Given the size and mixing limitations of the experimental tanks, how relevant are the ash concentrations and dissolution rates studied here to natural reef environments? Most reefs are found at depths starting around 1.5 m and extending much deeper, which differs considerably from the experimental setup.
The discussion currently lacks a section addressing the possible mechanisms for trace element incorporation (e.g., V, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) into the coral skeleton. A consideration of these mechanisms would help interpret the measured concentrations and their biological significance.
Specific Comments
Line 59 – Could you clarify why the lack of correlation is interpreted as evidence that metal release occurs via dissolution of metal salts? Could alternative processes, such as scavenging or desorption, also be playing a role?
Line 70 – Please note that the cited source is not a peer-reviewed publication.
Line 73 – The sentence is difficult to follow; adding commas may help clarify the meaning.
Line 77 – The phrasing here could be misleading, potentially implying that corals photosynthesize. Please reword for clarity.
Line 97 – The term "trace metals" can be ambiguous in coral research, as trace metal concentrations in seawater and coral skeletons are not equivalent. It may be helpful to clarify this to avoid confusion.
Line 118 – What is the basis for the target concentrations selected for the experiment?
Line 150 – Same as above: what criteria were used to determine this specific concentration?
Line 152 – How was this statement assessed or quantified?
Line 193 – Was the observation based on visual inspection or another method?
Line 219 – There appears to be some redundancy between lines 219 and 220 (e.g., the lab is introduced twice).
Line 226 – How confident are you that all organic material was removed? In my experience, tissue remnants can be difficult to eliminate entirely, even after thorough cleaning. Also, were the same tips used for airbrushing and LA-ICP-MS analysis?
Line 271 – The choice of references is somewhat odd. Galochkina focuses on Sr-U and limitations of Sr/Ca proxies, while Hathorne is primarily a technical paper. These may not be the most appropriate citations for this context.
Line 275 – There seems to be some overlap with content in line 256.
Line 282 – As noted earlier, the term “trace elements” may be interpreted differently by the coral paleoclimate community. Clarification may be useful here.
Line 343 – The sentence suggests that the impacts of real-world events could be more severe, but it would be helpful to explain why the metal release levels differ between the experiment and natural scenarios.
Line 368 – The sentence is awkwardly worded, with an unclear subject and inappropriate comma usage. Please revise for clarity.
Line 460 – The mechanisms of metal incorporation into the skeleton may be more critical than their seawater concentrations. For example, Mg is more abundant in seawater than Sr, yet is depleted in coral skeletons, whereas Sr is enriched, largely due to selective incorporation. The manuscript does not sufficiently consider such mechanisms for the trace elements discussed.
Line 509 – This statement largely repeats earlier content, and again the supporting references do not appear to be the most appropriate.
Citation: https://doi.org/10.5194/egusphere-2025-1713-RC1 -
AC1: 'Reply on RC1', Frank Förster, 16 Jul 2025
Dear Referee #1,
the authors thank you for the constructive feedback. Please find attached the "Response to the Reviewers", which provides point-to-point replies to all comments and concerns raised by the referees #1 and #2. Please be aware that the mentioned lines in the answers refer to the revised manuscript, which will be uploaded once the open discussion closes.
Best regards
Frank Förster
Corresponding Author
-
AC1: 'Reply on RC1', Frank Förster, 16 Jul 2025
-
RC2: 'Comment on egusphere-2025-1713', Anonymous Referee #2, 10 Jul 2025
The manuscript by Förster et al. details the results of a geochemical study into Stylophora pistillata cultured in tanks exposed to volcanic ash. The topic covered in this paper is important because it has implications for the health of corals and for their potential role as archives of past volcanic eruptions. The scope of the study is novel, in particular because it investigates the impact on the geochemistry of three different coral compartments from the same individuals - skeleton, tissue, and symbionts. Together this means the manuscript falls within the scope of Biogeosciences. The methods used are appropriate and, for the most part, adequately documented. The text and the figures are clear and concise, whilst the referencing is thorough. I enjoyed reading this manuscript, and I recommend its acceptance following some minor revisions listed below.
Minor Points:
- A little more emphasis on the big picture, and why this study is important, should be added to the abstract, and to a lesser extent the conclusion, to stress the significance of the study. I.e. ramifications for coral health, and presumably as an archive of past volcanic eruptions (in relation to the skeleton at least). This is currently alcking. Out of interest, do you envisage a scenario where the trace metal concentrations of coral skeletons could be used to estimate the volume of ash dropped on a reef, or would there be too many unknowns, such as oceanographic dynamics?
- Line 73, sentence starting with “Stewart et al. (2020) argues that…”: please rephrase, as currently unclear.
- Line 132, referring to accuracy of solution measurements. You state that recoveries ranged from 87% to 110%. So preconcentration procedures fractionate your solutions? Was this then corrected for when analysing your samples? If so, please detail this step. If not, then why not?
- In line 140 you state the culture conditions, then in line 416 you state that the impact of the ash dosing on light and pH was “negligible”. Can you expand on this, and state to what degree both of these parameters changed during the ash dosing, and or how long?
- Line 146, and the clause “natural containing metals with ash derived metals.”: please rephrase as this isn’t clear.
- Please comment on how your ash dosing (2.5 g, three times a week, into your 30 L culturing tanks, line 150) compares to natural analogues. What made you choose this mass and rate, and how common is this dose rate/amount is likely to be in nature? Would this constitute an extreme case of ash-fall, or more a more minor one’?
- Section 2.2.2: Please give details on how the major and trace element measurements of the tissue and symbionts were standardized, and the accuracy/precision of these measurements.
- Line 206: deleted the comma after “in this section” as it is unnecessary.
- Line 221: OES and ICP-MS are techniques, not instruments, so please rephase to: “…measured by ICP-OES…and QQQ-ICP-MS…”.
- Line 230: were any polishing agents used when grinding the skeleton samples for LA-ICP-MS?
- Line 257: please state what values were used for NIST612 when standardizing your data.
- Line 258: please state what Ca concentrations were used for internal normalization.
- Line 374, sentence starting with “Here, we present…”. Please rephrase as currently unclear. I think you may just need to say “…changes in the…”
- Line 411/Results section: you quite rightly focus on the geochemical differences between the skeleton/tissue/symbionts cultured in the two treatments, but to put these in context we are missing an assessment of the tank effect within treatment: are there any statistically significant differences between the three compartments grown in the replicate tanks? Similarly, were there any differences between nubbins grown within the same tank?
- Line 548: Chalk et al. 2021 (https://doi.org/10.1038/s41598-020-78778-1) also show this for S. siderea.
- Line 557: you say that each laser spot reflected a mixture of COC and fibrous aragonite. Is this assumed, or could you actually see the COCs to make certain of this? Please make this clear either way. It seems unlikely that you can be absolutely certain of equal proportions, so could some of your variation be caused by different COC:fibre ratios? It would be interesting to see if the ash-derived trace metals are preferentially incorporated into one or the other of these structural components.
Citation: https://doi.org/10.5194/egusphere-2025-1713-RC2 -
AC2: 'Reply on RC2', Frank Förster, 16 Jul 2025
Dear Referee #2,
the authors thank you for the constructive feedback. Please find attached the "Response to the Reviewers", which provides point-to-point replies to all comments and concerns raised by the referees #1 and #2. Please be aware that the mentioned lines in the answers refer to the revised manuscript, which will be uploaded once the open discussion closes.
Best regards
Frank Förster
Corresponding Author
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- 1
The work by Föster et al. investigates the effects of volcanic ash exposure on corals across three biological compartments—tissue, skeleton, and symbiont. Studies that address all three compartments simultaneously are rare, and this manuscript sets a valuable precedent for improving our understanding of how changes in elemental composition affect coral physiology at different levels. The topic is novel and important: volcanic ash effects on corals remain understudied, and this work may also have broader implications for understanding how other disturbances introducing metals into marine environments impact coral health. The manuscript is generally well written, and the main ideas are easy to follow. However, I offer the following comments and suggestions to help improve the clarity and impact of the study.
General Comments
Given the size and mixing limitations of the experimental tanks, how relevant are the ash concentrations and dissolution rates studied here to natural reef environments? Most reefs are found at depths starting around 1.5 m and extending much deeper, which differs considerably from the experimental setup.
The discussion currently lacks a section addressing the possible mechanisms for trace element incorporation (e.g., V, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) into the coral skeleton. A consideration of these mechanisms would help interpret the measured concentrations and their biological significance.
Specific Comments
Line 59 – Could you clarify why the lack of correlation is interpreted as evidence that metal release occurs via dissolution of metal salts? Could alternative processes, such as scavenging or desorption, also be playing a role?
Line 70 – Please note that the cited source is not a peer-reviewed publication.
Line 73 – The sentence is difficult to follow; adding commas may help clarify the meaning.
Line 77 – The phrasing here could be misleading, potentially implying that corals photosynthesize. Please reword for clarity.
Line 97 – The term "trace metals" can be ambiguous in coral research, as trace metal concentrations in seawater and coral skeletons are not equivalent. It may be helpful to clarify this to avoid confusion.
Line 118 – What is the basis for the target concentrations selected for the experiment?
Line 150 – Same as above: what criteria were used to determine this specific concentration?
Line 152 – How was this statement assessed or quantified?
Line 193 – Was the observation based on visual inspection or another method?
Line 219 – There appears to be some redundancy between lines 219 and 220 (e.g., the lab is introduced twice).
Line 226 – How confident are you that all organic material was removed? In my experience, tissue remnants can be difficult to eliminate entirely, even after thorough cleaning. Also, were the same tips used for airbrushing and LA-ICP-MS analysis?
Line 271 – The choice of references is somewhat odd. Galochkina focuses on Sr-U and limitations of Sr/Ca proxies, while Hathorne is primarily a technical paper. These may not be the most appropriate citations for this context.
Line 275 – There seems to be some overlap with content in line 256.
Line 282 – As noted earlier, the term “trace elements” may be interpreted differently by the coral paleoclimate community. Clarification may be useful here.
Line 343 – The sentence suggests that the impacts of real-world events could be more severe, but it would be helpful to explain why the metal release levels differ between the experiment and natural scenarios.
Line 368 – The sentence is awkwardly worded, with an unclear subject and inappropriate comma usage. Please revise for clarity.
Line 460 – The mechanisms of metal incorporation into the skeleton may be more critical than their seawater concentrations. For example, Mg is more abundant in seawater than Sr, yet is depleted in coral skeletons, whereas Sr is enriched, largely due to selective incorporation. The manuscript does not sufficiently consider such mechanisms for the trace elements discussed.
Line 509 – This statement largely repeats earlier content, and again the supporting references do not appear to be the most appropriate.