Dear authors,
The topic of your manuscript has been evaluated as of great scientific interest and the overall presentation of high quality. However, one reviewer has raised some significant issues regarding the context in which results are presented (i.e. the connection between iron and colony formation), in particular, the fact that colonies and single cells were not quantitatively determined in any of the samples (including field samples) except for one culture experiment with strain 1871. I would, therefore urge you to constructively consider the comments of reviewer 1 (whereby, I believe that somewhat changing the title of the manuscript would go a long way). I would further refer the authors to the work of Luxem KE, Ellwood MJ, Strzepek RF (2017, Intraspecific variability in Phaeocystis antarctica's response to iron and light stress. PLoS ONE 12(7): e0179751. https://doi.org/10.1371/journal.pone.0179751) which has not been mentioned in this manuscript. Finally, I trust that the comments from reviewer 2 will be included as indicated by your answer.
Sincerely,
Christine Klaas

Dear Authors,
I am pleased to accept your manuscript for publication in Biogeosciences. Below please find an additional few minor comments for the final version of the manuscript.
Sincerely,
Christine Klaas

Lines 101-103: "Recent in vitro iron addition experiments provide evidence that iron nutrition influences P. antarctica growth in this region, with increasing P. antarctica biomass after iron addition (Bertrand et al., 2007; Feng et al., 2010)."

Lines 125-126: "continuous light. Each strain was acclimated to growth on one of six growth condition concentrations":
The meaning of this sentence is not clear: was each strain acclimated to one Fe concentration before starting the experiments (which one?), or were both strains acclimated to each one of the six growth conditions before sampling? How long (number of generations) was the acclimation phase?

Line 242: "colonies since sexual reproduction is thought to require revisiting the flagellate life cycle stage.": Is sexual reproduction what the authors meant? Please provide a reference for this statement.


Lines 304 -306: "6d and 6f, Supplementary Table 1). While during both experiments, cells were growing exponentially at the time of harvest, cell densities of strain 1374 were as much as 7.6 fold higher than in the experiment with strain 1871"

Line 307: "and as a result the denser 1374 strain might have also experienced iron stress"

Lines 313-314: "colonial P. antarctica spring bloom in 1998 were consistent with this interpretation, with ferredoxin concentrations below the detection limit and flavodoxin present (Maucher and DiTullio, 2003)."

Line 328: "and the proteome across the gradient in iron concentrations, we hypothesize that this diversity of iron stress".

Line 380: "organism, or with the influence of any other environmental stimuli in the genus Phaeocystis"


Section 3.5, p. 20: For genetic diversity and mixing of P. antarctica populations see: Gäbler-Schwarz and L. K. Medlin and F. Leese (2015). A puzzle with many pieces: the genetic structure and diversity of Phaeocystis antarctica Karsten (Prymnesiophyta). European Journal of Phycology, 50 (1): 112-124. https://doi.org/10.1080/09670262.2014.998295


Lines 548-549: " mentioned earlier, division and growth of P. antarctica colonies is believed to require transitioning back through the flagellate life cycle stage": please provide a reference for this statement.