Dr Zhang and colleagues evaluate the interactive effect of changes on multiple environmental drivers (namely phosphorous concentration, pCO₂ and light intensity) on the elemental and macromolecule content of the model coccolithophore species Emiliania huxleyi. Authors identify that multiple stressors interact on the physiological response of E. huxleyi and that underlying mechanisms are complex. For example, authors identify that reduced phosphorus availability results in a reduction of both particulate organic nitrogen and protein contents under low light intensity, but not under high light intensity. Interestingly, reduced phosphorus concentration in combination with ocean acidification result in an increase in both POC and carbohydrate contents only under high light conditions. A large body of evidence indicate that coccolithophores are sensitive to projected changes in oceanic conditions driven by ongoing human-induced climate change, such as ocean acidification and changes in nutrient supply, mixed layer depth and light intensity. Given their abundance and fundamental role in the biological and carbonate counter pumps, changes in coccolithophore performance will most likely have impacts in the oceanic carbon cycle and marine ecosystems. Therefore, there is an urgent need of studies such as the one presented here to evaluate how multiple environmental drivers affect key marine organism in order to predict how ongoing environmental change will impact marine ecosystems. The paper is clearly written, the figures are appropriate and the findings interesting and useful for the scientific community. Therefore, I recommend acceptance of this manuscript after the comments listed below have been addressed.

Lines 84-85. The sentence “These ocean changes expose phytoplankton cells within the UML to multiple drivers,” is vague. Do authors mean that environmental changes in the UML will expose phytoplankton cells to physiological stress? Please clarify.

Line 109. Authors could dedicate a sentence or two to the importance/relevance of multi-stressor experiments highlighting that numerous environmental factors will simultaneously change in the future ocean and therefore, this kind of experiments are needed to evaluate the response of different organisms to ongoing environmental change.

Line 149. The specialized reader would appreciate an SEM picture of the coccosphere and or coccoliths either in the main text or as supplementary material.

Line 151. could authors mention the date when this strain was isolated?

Line 510 Coccolithophores play a complex role in the carbon cycle through production and export of organic carbon to depth but also through the carbonate counter pump (releasing CO2 during the calcification process). Authors should clarify this in the text.

Line 530. Could authors explain (or speculate) about the metabolic process behind the relationship between low light intensity and nitrate uptake reduction?

December 21, 2022

Reviewer’s comments for

Ms. Ref No.: egusphere-2022-947

Title: Responses of elemental content and macromolecule of the coccolithophore Emiliania huxleyi to reduced phosphorus availability and ocean acidification depend on light intensity

Authors: Yong Zhang et al.

Submitted to: Biogeosciences

General comments

              The authors show interesting experimental data on the responses of an important coccolithophore to changing three environmental drivers. Culture experiments are carefully designed and conducted. Results are complex and result descriptions can be more accomplished but acceptable. I found some concern in the present manuscript. The following comments should be considered before this paper being considered for publishing in Biogeosciences.

Major comments

1. I consider that a message from the present title (the importance of light condition on the coccolithophore responses) does not link with the conclusion of this paper (reallocation of material and energy to acclimate to climate change).
2. Results section. In cases of the difference between treatments being not statistically significant, one case is described as increase or decrease (e.g. line 331), another case is described as not significantly different (e.g. line 351). What is the threshold for authors to change the descriptions? I consider that this may lead readers to the direction of authors’ thought.
   
   Furthermore, the authors show statistical information on interactions in three-way ANOVAs in Table S1, but these information are lacking in the text.
3. Figures 3a and 3b, and L615-617. If the growth rate hypothesis is working here, all data lay on a line as shown in Figure 3C. Different P storage contents, as shown below, between LL and HL may cause the separation of the regression line among light conditions. Similar discussion may be required to interpret the results for POP contents (not organic actually I think).

Specific comments

4. To confirm carbonate chemistry in some case, salinity data are needed.
5. Is the low DIP 0.43 umol/L an upper end of DIP conc in coastal waters? I do not consider this as a general case in coastal waters.
6. Authors’ selection on total boron formulation also may be important.
7. I understand the procedures remove PIC, so the measurements represent POC. On the other hand, for nitrogen and phosphorus inorganic forms are not removed from the filter samples, so these are not PON and POP but total particulate N and P. This may be critical to understand the variations in cellular contents of N and P. In particular, P storages occur in inorganic forms, and environmental drivers can alter the P storage capacity of the cells.
8. I found no descriptions on protein-N:PON ratios.
9. Results for POC:PON ratios are shown in Table S1 and Figure S4, but why are these not described in the text?
10. The main cultures are conducted in 2 days, but authors conducted three pre-experimental cultures, resulting at least 8 generations acclimations. Therefore cells have enough time periods to change growth rate against the low phosphate conditions.
11. POC:PON is not discussed in Results and Discussion sections, so this discussion is sudden and not readily acceptable for readers.
12. L625-629. This discussion on carbon cycle is not match with this paragraph. A new paragraph should be made for this discussion.
13. Figures and Tables. For this kind of experiments, data should be shown as a mean and standard error.
14. Figures 1 and 2. I understand that statistical analyses are performed including two different light conditions (e.g. Figure 1a and b). I consider that two light conditions should be shown in a graph. Two columns can be shown for each treatment (e.g. HP+LC).
15. Figure S4. The label of treatments for right edge in each panel should be “LP+HC” but not “LP+LC”.
16. Figure S6. I do not understand the meanings for the authors showing these graphs.
17. Figure S7. No descriptions on methodology for the gene analyses are found in the text.