Dear Zhuang and Co-workers,

thank you for submitting your answers to the referees. One of your referee suggests minor, the second major revisions. Most criticism is based on lacking details, which I presume you are able to provide.

Please also remove figures that have already been published in previous publications. I understand your study is different from the Liu et al. 2017, Marine Environmental Research 129, p. 229-235 as you report novel data on trace gas production during the experiment. But the referee is right that the journal owns the copyrights for the published figures.

With kind regards
Tina Treude

Dear Dr. Zhuang and Co-Workers,

let me start by apologizing for the delay. I decided to review your revised manuscript by myself, since the former referees where not available anymore, but then I got interrupted by a three-week travel through conferences in Europe.

I read through the referee's comments, your responses, and the revised manuscript and most of the scientific recommendations have been implemented. There are some points left that I will further explain below. There are a couple, but they should be easy for you to implement.

The main issue that remains is the poor scientific English of the manuscript. I am not a native English speaker myself, but I fear your manuscript needs major copy-editing. Biogeosciences does copy-editing but only to a certain degree. Do you have a native English speaker in your institution who could revise the manuscript for you to improve its readability? I will try my best to give you some advice in my detailed comments below, but it is a major undertaking and beyond my duties.

With kind regards
Tina Treude

1. General Comments

- The manuscript has too many abbreviations, which makes it hard to read. I strongly recommend reducing the abbreviations to a minimum. For example: write 'ocean acidification' not 'OA', write 'high pCO2' and 'low pCO2' not 'HC' and 'LC', write 'iodine' not 'I'. There are many more unnecessary abbreviations (DCB, MPN, ASW etc.). Please do not abbreviate if a term is used less than 5 times or if the term consists of only two words.

- Species names are written out (Phaeodactylum tricornuntum) the first time they are mentioned, after that the genus name is abbreviated (P. tricornuntum)

- Figure and table captions must be self-explanatory. Please provide definition of all abbreviations used in the figure and table captions and figures and tables.

2. Scientific Comments

L33: Add species names in brackets after "phytoplankton species"

L102: This paragraph should end with a hypothesis. "respond to OA" is too vague.

L104: "General experimental device" What do you mean here? The setup? Device sounds strange.

L105: Start this paragraph with the main purpose of the experiment

L105: Does "floating platform" mean in the water? Offshore? Provide some details.

L114: Define low and high pCO2 levels.

L119-122: I am not sure I am able to follow what is meant by coastal environment (algae) and just environment (trace gases). Is there a difference between the environments? How large was the filter (pore size and type) to remove the algae and bacteria? Which trace gases do you mean? How did you measure them?

L140-141: "no meaningful numbers" is not a scientific term. Please provide a minimum threshold, such as "less than xxx cells per Liter".

L166-174: As requested by the referees you added the enumeration of the DMSP-consuming bacteria. However, unless I missed it, the data are nowhere presented or discussed in the text and Fig. S1 is not cited. Please add accordingly. Please integrate Fig. S1 into the main manuscript. It should not be a supplementary, since the data are essential to this study.

L184: Provide a method to "well control" temperature and salinity.

L201: I could not find a method for "microscopic inspections". Please add.

L300-301: Please shortly elaborate what the major differences between the cited literature are, because you can't expect the reader to first read all the literature to understand your argument.

L301-302: Which mesocosm experiment? All of them? 40.2% reduction of what? I also think the reviewers asked you to round before the digit (40%).

L313: I think the reviewers asked you to round before the digit (28%).

L316-317: I still do not fully understand the part about the trace gases (see my earlier comment). Maybe it just needs clarification.

L315-317: Is this really a main conclusion or just a technical detail? I don't understand why it is mentioned in this chapter.

L321: Details needed for "a range of biological parameters"

L323-324: "biological parameters" is too vague and needs detail. What do you mean by "biological control"? Specify


3. Scientific English and structure

L86: change "that they may account" to "accounting"

L93: Through = in

L93: Write: "in addition, another mesocosm..."

L96-102: This paragraph is poorly written and hard to understand. Please revise thoroughly and let it be edited by a native English speaker.

L97 (and many other places throughout the manuscript): Please do not use "Meanwhile,". It is a temporal term and is consistently misused in this manuscript.

Line 98: I do not understand why you write "were required" (past tense)

L99-100: "a mesocosm experiment was conducted" Are you talking about your study? Unclear.

L100: Write "The aim of the present study..."

L101: Delete "further", delete "s" in "productions"

L119-122: Change to: "Seawater taken from the coastal environment was first filtered to remove algae and their attached bacteria before usage in mesocosms. Trace gases produced..."

L134-141: This entire paragraph should be moved up to the beginning of the "Algal strains" chapter, because this step happens prior to the mesocosm experiments.

L143: delete "s" in hydrocarbons", delete "generally", replace "obtained" by "taken", add "the" after "from"

L151: "h" = "hrs"

L167: Which experiment do you refer to in this sentence? Please add information (in which sample did you count the DCBs?).

L169: Write "into 6 mL"

L170: Delete "off"

L181-204: This section is missing a discussion. You are just presenting results, but this is a combined "Results and Discussion" Chapter. Please add a discussion of the data and make literature comparisons where appropriate.

L183: What does "well combined" mean? Not proper English.

L184: Move ",respetively" after "29"

L193-194: add ", respectively," after "41 µmol L-1" and "38 µmol L-1".

L206-215: The paragraphs 206-211 and 211-215 are a bit repetitive. They should be better combined.

L223: delete "obviously"

L228 & 230: "In this study" and "throughout this study" is repetitive.

L238: Not sure if "once-daily" is a proper English term

L239-249: Needs English copy-editing.

L255: delete "the"

L273: delete "have"

L274: Separate into two sentences: "...bromocarbons (Karlsson et al. 2008). Significant correlations..."

L309-310: Change to: "...photochemical reaction could be responsible for the reduction of CH3I concentrations in the high pCO2 treatment."

L315-316: Change to: "...was filtered prior to the experiment, algae from the coastal...were not present...".

L318: Which phenomenon?

L321-322: "as they were affected" is not proper English

L322: Start new sentence with "Elevated pCO2..."

L478, 496, 503, 513: Replace "changes" by "development"

L517: Change to: "Dissolved inorganic carbon (DIC), ??? (pHT), pCO2 and nutrient concentrations in the mesocosm experiments. "-" depicts that values were below detection limit." Define ???

L520 and 528: What does "Relationship" mean? What are the numbers in the table? Ratios? Then they should have no unit.

Fig. S1 caption: Would need to be changed to "J. Yu, unpubl. data". But will be unnecessary to cite since the data will be integrated into the main manuscript and hence become published data.

Dear Dr. Zhang et al.

Thank you for submitting your revised manuscript to Biogeosciences. I detected a major improvement of the readability of the manuscript and consider it acceptable subject to technical corrections. Please find my details below.

With kind regards
Tina Treude

Technical Corrections:

- Of Fig. 2 only A and B are cited. Please add a citation for 2-C.

- Of Fig. 3 only D is cited. Please add a citation for 3-A, 3-B, and 3-C

- The sentence "Affected by the filtration procedure, three bromocarbons compounds measured in this study were not correlated with P. tricornuntum and T. weissflogii, and Chl a." (Line 322-323) is not clear to me. What is meant by "Affected by the filtration procedure"? Please provide a broader context.