Dear Drs. Maldonado and Segovia,

We would appreciate it very much if you could submit the latest version of your manuscript to us with poing-by-point responses to the reviewers' comments.

Once we get the document files, we will send these to the former referees soon.

Thank you again for your nice effort.

Kind regards,

Koji Suzuki
Associate Editor

Dear Dr. María Segovia,

We received two reviews on your revised manuscript. Although one expert considered that your paper has been improved significantly, another referee has still a number of concerns about your manuscript (see below). As an editorial issue, the latter reviewer noted the Figure 1a, 1b and 2 are totally same figures as reported by Segovia et al. (2017) published from Marine Ecology Progress Series. Please confirm the copyright of these figures with the publisher Inter-Research. If needed, please modify the figures to avoid any copyright issue. I also agree that these figures should be used as the supplemental information, because they have already been published.

We would be very grateful if you could make a further revised manuscript with point-by-point responses.

Thank you again for your nice effort to improve the manuscript.

Best regards,

Koji Suzuki
Associate Editor

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Referee #1

The authors have done an acceptable job in addressing my comments as well as those of other reviewers. Therefore, I would recommend publication.

Some minor comments.

Lines 62-65. “For instance, Fe occurs…more bioavailable.”: first, references are needed for this sentence; second, what described here actually cannot serve as examples for the dependence of trace metal chemistry on seawater pH.

Line 84. “, and that are actively…” should be “, and that they are actively…”

Line 85. the impact on, not in, the trace metal content…

Line 97. “E. huxleyi”…please also check the rest of the manuscript

Line 100. 1) Delete “organic”, as CaCO3 is not organic, 2) delete “carbon export”, and 3) Hutchings, 2011 is not in the reference list.

Line 104. What “realistic” refers to here?

Line 123. Replace “iron” with Fe, and also check the rest of the manuscript.

Line 127. Should be “resulting in…”

Line 135. Please provide the vendor of HCl in the first place.

Line 252. The effects of…on, not in, the plankton community

Line 393. E. huxleyi.

Lines 401-402. At low pH, CO3 concentration decreases, which should lead to a decrease in CuCO3.

Although the diatom bloom is not the primary focus of this study, I would suggest the authors briefly discuss how the change in CO2 may affect the biogenic fraction of particulate Co, Zn, and Cd at the beginning of the mesocosm experiment.


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Referee #2

“Particulate trace metal dynamics in response to increased CO2 and iron availability in a coastal mesocosm experiment”
Lorenzo et al.

- General comments
Authors have responded to most of my previous comments.
I have found significant improving of this manuscript, especially for presenting their results with adequate “Figures” which is more understandable for readers. However, I still found some issues for this manuscript which have to concern before publications. Please consider for following comments.

Since authors present their data with some “Figures”, it became more clearly that parts of presentation in this manuscript (MS) are overlapped to previous published paper, Segovia et al. (2017). For instance, Figure 1a, 1b and 2 are totally same figures as reported by Segovia et al. (2017). Is it really possible to publish only with the statement as “Figure reproduced with permission from Segovia et al. Mar. Ecol. Prog. Ser. 2017” in the caption? It might be better to place in “supplementary Figures”. I would like to ask about this aspect to Editor. Also, authors should make clear in MS that “what is original purpose of this MS” in introduction, “which is original findings from this MS” in “Abstract”. Original findings of this MS are not clear in the “Abstract”(see below).

Table 1 showed the result from statistically analyses of the particulate(P) trace metal (TM) data obtained from this study. This statistically analyses are one of the important points for discussion in the MS. However, the detail for the statistically analyses are not clear for reader. For instance, which “day’s” data did authors used for evaluating the difference among treatments for each P-TM (d12, d17, d21??)? Which day’s data did authors used for evaluating the time difference (is this means significant difference between d12 and d21?)? Please explain more clearly in the text (2.4 section) and in the caption of Table 1. Same for Table 3.

- Abstract

Line 30-38; From “Future predicted high…… future ocean dynamics.”. This sentence should be supported by results of this study which is described in the one sentence before. Author should state here “what is the original important result and major findings from this study”. Present sentence is not clear in this point. I think, main important results and findings from this study, which should be claimed in abstract, are described in line 376-404, and “conclusion”. Authors should not include speculation in the “Abstract”. The findings should strongly be supported by this study’s own results.

- Introduction

Line 74-86; I found that some parts of contents in this sentence are overlapped to previous sentence (line 57-71). To make more concise introduction, please compile and reconstruct these two sentences and avoid repetition.

Line 95-105; This sentence is very important for explaining the aim of this MS. In the present description, authors explain “importance of E. huxleyi bloom for biogeochemistry in the ocean”. This is the research motivation for study of Segovia et al. (2017), but this is not motivation for investigating particulate trace metals in this MS. Whereas, Authors only describe the aim of this study as “The aim of the present study was …… CO2 and Fe bioavailability.” in line 104-105. I think author should add more explanation about “why the measurement of particle trace metals and characterize the change of particle trace metals during E. huxleyi bloom are important?”.

Line 97; I could not find “Segovia et al., 2018” and “Lorenzo et al., 2018” in the reference list in end of MS. Please check all reference in the text and reference list in the MS.


- Material and method

Line 114, Authors described that CO2 concentration in the mesocosms were measured by NDIR analysis system. Whereas, in line 116-117, authors described that CO2 concentrations in the mesocosms were calculated from pH and alkalinity. Which is correct? According to Segovia et al.2017, “CO2 concentration in inlet flow” is only measured by NDIR.

Line 121, Why the DFB concentration was set to 70 nM? This value is important for this study because particle dissolution for some trace metal was strongly influenced by the DFB concentrations as authors described in discussion.

Line 128, “gentle vacuum pumping”. Is this pumping system clean for level of trace metal work? Detailed description is needed for explaining sampling system, cleaning procedure, because these are critical part of trace metal work.

Line 139, How did authors collect the seawater (by using pumping as described in line 128?). “0.2 uM” should be change to “0.2um”.

Line 147; “trace metal hydrochloric acid” should be “trace metal grade hydrochloric acid”.

Line 154; “0.2 uM” should be change to “0.2um”.

Line 156; Is this “centrifuge tubes” same tube as “2mL PP” tubes?? Reader will be confusing.

Line 225-249; Section 3.2, 3.3 and 3.4. Authors need to explain detail about statistically analysis for Table 1 and Table 3, see general comment.


- Discussion

Line 286 “well-known metal content in biogenic particles” and line 287 “normalized to Al (mol Me: mol Al in the Earth crust)”. Please indicate the number which was used for this calculation and indicate reference for the number.

Line 289; “0.0051 for mol Fe:molP”, “0.331 mol Fe : mol Al”. How did authors select this number? Which information did author refer? Maybe, “Ho et al., 2006” in Table 2 (A) for “0.0051” and Table 2 (B) for “0.331”. Table 2 (B) crustal ratios need reference (Taylor 1964?). Add ref. to “Table 2 (B)”.

Line 321; I could not find “Table 5” in MS.

Line 347; “Zn and Cu dissolution (ref.)”. Please add reference.

Line 353-355; “Rain events…… (Muller et al., 2005)”. Do authors have some information on rain event before or during the experiment? If they have, it is important additional information for this interpretation.

Line 355-358; “, and indigenous plankton……Knauer et al., 1998).”. This sentence is too speculative, and not main discussion for particulate Cu:P ratio. I feel author can delete this sentence.

Line 362, 363; I can not find “Figure 6” in the MS.

Line 376; “in that it was the only trace element whose particulate concentration significantly and uniquely affected…….”. I think this is not true. According to Table 1, other metals (Co, Zn, Cd Mn Mo) were also affected by CO2.

Line 384-387; Authors discussed about partitioning of DFe and PFe with using Figure S2. I think this results and discussion are one of very important finding of this MS. This result is base of predicting future ocean in the following discussion. I recommend that Figure S2 should be placed for the normal Figures (not in Supplementary).

Line 396; I could not find Table 5 in the MS.

Line 424; “due to increased Fe bioavailability”. How authors judged the increasing Fe bioavailability in the LC+DFB treatment? Is there any evidence which indicate that DFB-bounded Fe is available for E. huxleyi? Please discussed more carefully about the changing Fe bioavailability in “discussion”.

Dear Dr. Segovia,

We have just received a review of your further modified manuscript. The referee was very pleased to find the significant improvements of your manuscript, but still has some concerns. So please revise your manuscript further following the comments and suggestions from the reviewer. After this I am going to make a final decision on your paper.

Again thank you for your nice efforts to improved the manuscript.

Kind regards,

Koji Suzuki
Associate Editor

Dear Dr. Segovia,

It is a great pleasure now to accept your manuscript for publication in Biogeosciences. Thank you very much for responding to the reviewers' comments carefully. Before it is published from the journal, please consider my technical corrections below.

Again thank you for choosing the journal Biogeosciences.

Kind regards,

Koji Suzuki
Associate Editor

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L14–34: In Abstract, the authors often indicated the unit of mol L-1, but the values were not shown. So it appears that the unit is generally redundant and can be removed from the text.

L48: pH because the latter

L60 and hereafter: It appears there are a number of blanks in the text. For example, in L60, there is a space between “pH.” and “Fe”. Please remove such blanks from the text carefully.

L88 and hereafter: Please use a semicolon (;) between papers cited.

L92: E. hulxeyi should be italic.

L110: Please remove the comma between “al.” and (2017).

L111: Please indicate “partial pressure of CO2 (pCO2) and dissolved Fe (dFe)”, because these terms appeared here in the text for the first time.

L114: The specific CO2 concentration and the inlet flows

L119 and hereafter: Please use “and”, not “&”.

L119: Remove the space between “t” and “o”.

L124 and hereafter: Please use a comma between the author name(s) and year published in parentheses. For example, (Wells, 1999).

L141: Not MQ, but MQw (see L158, L163, etc.)

L145: Zamazow et al. (1998)

L146: Segovia et al. (2017)

L158: with MQ and 10%. Please see L141.

L160: 12 h

L179: MQw

L208: Tables S2 and S3

L223: Please do not italicize the “et al.”.

L224 and hereafter: Please remove the “-supplemental material” from the text.

L273: Not 2027, but 2017.

L295, L345: The authors generally used the capital “P” for significance level.

L326: (Figure 2, Table 2).