Dear Dr. Li,

Thank you very much for responding to the comments from two reviewers. I would appreciate it very much if you could upload the latest version of your manuscript to the BG website with the point-by-point responses. Once we receive your manuscript, I will ask the two reviewers to evaluate your modifications soon.

Again thank you for your nice efforts to improve your paper.

Kind regards,

Koji Suzuki
Associate Editor

Dear Authors,

Thank you very much for making a revised version of your manuscript. Although Prof. Campbell as a referee has recommended to publish your manuscript from BG, another anonymous refereee is not satisfied with your modifications, especially in the pH-related section (see below). Additionally, I also made some technical corrections below. So we would be very grateful if you could make a futher modified version of your manuscript with point-by-point responses to these comments.

Again thank you for your nice efforts to improve your manuscript.

Kind regards,

Koji Suzuki
Associate Editor

Comments from Referee #2:
The authors showed the negative effect of OA in winter to S. costatum by presenting the slower growth rate and stating lower photosynthetic rate under HC (OA condition). However, the P-C curve measurement was done with Tris-buffered media at different temperatures (pH tuned to 8.12 at room temperature). A Tris buffer pH 8.12 at room temperature means the pH at 5 °C to be around 8.6, which would cause pCO2 much lower in the Tris-buffered media than that in the culture media with the same total DIC. Researchers should be critical when interpreting such results.
On the net photosynthetic rate measurements, the P-I curves of winter conditions did not show difference between HC and LC (Figure 4), especially the data points falling in the range of 100 to 200 µmol photons m^-2 s^-1. This does not support a lower photosynthetic rate and its explanation for slower growth rate for S. costatum under winter OA condition (cultured under 150 µmol photons m^-2 s^-1).
As for the quantification of RbcL and PSII proteins, the elevated expression levels of photosynthetic proteins and a lower rate of photosynthesis (as the authors stated) under winter OA condition is confounding, and the authors did not provide sufficient convincing explanation.
On the net photosynthetic rate measurements, the P_I curves of winter conditions did not show difference between HC and LC (Figure 4), especially the data points falling in the range of 100 to 200 µmol photons m^-2 s^-1. This does not support a lower photosynthetic rate and its explanation for slower growth rate for S. costatum under winter OA condition.
As for the quantification of RbcL and PSII proteins, the elevated expression levels of photosynthetic proteins and a lower rate of photosynthesis (as the authors stated) under winter OA condition is confounding, and the authors did not provide sufficient convincing explanation.


Technical corrections suggested from Editor:
L20: low temperature and short daylength on the abundace of RbcL and key photosystem II (PSII) proteins (D1 and D2).
L97: Remove the underscore between costatum and was.
L102: Not milli-Q, but Milli-Q.
L110: insert a space between s^-1 and light.
L141: The abundance of Riblose-1,5-biphosphate carboxylase/oxygenase...
L341: rbcL should be italic.

Dear Drs. Li and Xu,

We are delighted to inform you that your manuscript has been accepted for publication in Biogeosciences. Thank you very much for considering the comments from the last referee, who was also satisfied with your modifications.

Again thank you for choosing the journal Biogeosciences.

Kind regards,

Koji Suzuki
Associate Editor