Dear Authors,

I have had one reviewer, who previously reviewed this manuscript, review this version of your manuscript. I agree with the reviewers comments. Please incorporate the comments and provide a detailed list of changes made. The reviewers comments follow:

Reviewer:
I suggest that the manuscript should be reconsidered if certain revisions would be permuted. I think that a major part of the manuscript is worth publishing (also because some of the proposed modifcations of the first two reviews were incorporated), but I am not yet convinced if the single and not reproduced calcitic sample is valid to allow further interpretation.

Publications on Halimeda describe the influencing factors on biomineralization and bio-performance, such as ocean acidification (e.g. Robbins et al., 2009, BGD 6, 4895-4918) and nutrient availability. These factors have shown to influence growth and lead to structural changes of aragonite crystal shape, which I think could have an enormous impact on skeletal chemistry. If this would be the case than an optical screening (e.g. SEM) of the sampled aragonitic and/or calcitic Halimeda might be really beneficial to strengthen further interpretations. This also because particularly Ca isotope data on biogenic carbonates are still highly debated with interpretations often supported via a multi-proxy approach (e.g. Mg/Ca, Sr/Ca, light stable isotopes) or a complementary optical screening of sample material (e.g. SEM, element mapping; cf. analysis on single foraminifera tests).

Concerning a short paragraph on the biomineralization of Halimeda (either in the introduction or at the beginning of the discussion): I would suggest that a figure similar to the figures in Robbins et al., 2009 might be helpful. These authors also presented a figure about their experimental design and very beneficial SEM images. Moreover, Larkum et al. published a new paper on Halimeda growth in 2011 (PLoS ONE 6(7), e20841).

In comparison to the first version I think that the method sections has considerably been improved, with only minor modifications necessary. It is not mentioned if the calibration curve to determine the amount of aragonite vs calcite is based on a curve fit or a linear fit (Fig. 1).

I don’t know why the authors insist upon the vague interpretation of the single calcitic sample. This should be removed or treated more cautiously.

(1) Comparing the photographs of all cultured samples with Halimeda grown in their natural environment I would argue that all of the cultured specimens look malformed and shriveled, which would imply stressed growth conditions. Moreover the finding that skeletal mass fraction is reduced implies that biomineralization is likely hampered. I would argue that the process of biomineralization under these experimental and artificial conditions is not really understood. What about SEM screening or elemental fractionation (Mg/Ca, Sr/Ca) to get a glimpse of what happens under the new stressed conditions. As I have already mentioned before, the Ca isotope composition in biocalicifiers is still under debate, with Ca isotope fractionation in naturally grown carbonate-producing organisms not yet fully understood (cf. detailed study on planktic foraminifera by Kisakürek et al. 2011). In this study a biocalcifier that has not yet been systematically studied (how certain environmental parameters might affect skeletal chemistry) is directly transfered to a simulated experimental environment, which might introduce further uncertainties and bias interpretations.

(2) Concerning the XRD results: Calcite as a trigonal phase is well-detectable with XRD and generally exhibits better peaks than aragonite, which is orthorhombic. Thus even if only minor quantities are present, such as in the other seven samples, than a distinct peak should be present. One potential reason for a not well-developed peak might be that the crystals are not well developed and that the material is ‘X-ray amorphous’. A structural analysis of the newly-grown and sampled material would thus be very beneficial (e.g. SEM imaging). This would further clarify how the mineral assemblage of aragonite and calcite would look like in comparison to the mineral assemblages of wild-grown material.

Line 233-236 and line 253-268 within the result and discussion sections and Fig.5 are therefore debatable, particularly because of the exclusiveness of sample # 6.

Ca isotope fractionation is often discussed as kinetic fractionation, with precipitation rate as an important factor to be considered. Here, the authors however assess the potential differences in calcification rate only by physical appearance. They state that different calcification rates are obvious but do not accompany changes in δ44/40Ca, thus implying that calcification rate has no effect on Ca isotopic composition. In this respect, SEM images would be really supportive.

Finally, the vital effect is explained as a result of Rayleigh distillation or a reservoir effect. Here it could be debated if this explanation should be modeled or not.

I believe you have now adequately addressed the reviewers concerns. There are some very minor changes needed. Please make the minor corrections below and resubmit the manuscript for publication.

Abstract
Provide some data here – what is the fractionation of aragonite and calcite?
Avoid “isotopically heavier”; use higher δ44/40Ca values. Also, how much higher?

Lines 199 & 200 – “slightly enriched in calcite” – I don’t think you can use the word enriched in this context.

Line 207 - error bars is not the right way to phrase this, with and error of 15% would be better.

Acknowledgements
Reviewers have put significant work in your manuscript, you should acknowledge them here.

Please add a statement about the growth form of the lab grown specimens as compared to the wild specimens.