Dear Dr. Angel,

Thank you for submitting your manuscript “The origin and role of biological rock crusts in rocky desert weathering” to Biogeosciences. Two expert reviewers and I have assessed your submission and feel that it has potential for publication as it addresses important questions on the effect of microorganisms on weathering in arid and semi-arid environments. However, the reviewers also identified numerous ways to improve the manuscript.
At this stage of the review process, I am providing the post-discussion editor decision which would normally be followed by revision. However, you have already revised your paper according to the reviewers’ comments which puts you ahead of this step. I have evaluated your revised manuscript and provided additional comments to be addressed below. The paper is well-written and I enjoyed reading it. The figures are well formatted and thoughtful for the information presented; the microbial data figures are especially nice.
At this point, I ask that you please revise your paper based on my comments and resubmit for further review. In addition, please submit a version of your paper so all changes marked as track changes.
Please feel free to contact me with any questions.

Sincerely,
Dr. Denise M. Akob

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Editor’s Comments

- The dust and rock samples were collected a full year apart. How can you be sure that the communities are representative of the conditions at both time points? Meaning, how do you know that the differences between dust, soil and rock are not due to temporal changes? Acknowledging this experimental fact would be valuable on Pg. 8.
- Be consistent with spelling of hyperarid. In some cases you write hyper-arid and others hyperarid.

P. 1, L. 33: replace [ with a comma
P. 3, L. 16: How many replicate dust samples?
P. 3, l. 29-30: revise to “samples that had their BRC mechanically removed using a diamond saw (Dremel, Racine, WI, USA) to a depth of 5 cm,”
P. 4, l. 3-5: no spaces between delta and the isotope.
P. 4, l. 15-20: this isn’t clear. Can you provide more info on how the cores were covered with epoxy (Devcon) and aluminium foil? Its not clear how “only the upper base of the crusted and bare cylinders” were left uncovered. Also, correct the spelling of aluminum.
P. 5, L. 27: 16S rRNA is not italicized
P. 5, L. 20: change to Illumina MiSeq
P. 5, l. 34: please consider putting all of your code for sequence processing and R analysis in the supplemental material. This is not required but it’s a great way of making your data more accessible for the reader and for future metanalysis. If you don’t opt this time please consider in future papers.
Pg. 6, l. 44: change to drying experiment
Pg. 7, l. 3-4: revise to “inevitably lead to decreased weathering rates.”
Pg. 7, l. 4-5: This sentence is awkward, revise for clarity. Maybe, “Moreover, our results differ from thos performed on temperate sandy stones that showed no effect of BRC on decreasing water transport rates (Slavík et al., 2017).”
Pg. 7, l. 8: omit “a”
Pg. 7, l. 15: change wording –first there is were and was and it should be was. Second, I wouldn’t call diversity “poor”—low is a better term
Pg. 8, l. 9: this doesn’t make sense: “The BRC bacterial communities were previously described”. You mean other BRC communities have been described not the same ones you are working on. Revise to be more clear that you are referring to other data and how your work is different.
Pg. 8, l. 12: “the two BRCs we studied”
Pg. 8, l. 35-37: not clear what you mean by a discrepancy between structure and function. Function is really only potential function in this case since you are doing DNA-based work.

Dear Dr. Angel,

Thank you for revising the paper based on the reviewers and my suggestions. I am happy to accept your paper for publication in Biogeosciences!

Best,
Denise Akob