PONE-D-20-31393

Bacterial precursors and unsaturated long-chain fatty acids are biomarkers of North-Atlantic demosponges

PLOS ONE

Dear Dr. de Kluijver,

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'The fatty acid profile of one of the five sponge species that we analyzed has been used in an experiment with isotopically labelled food sources. The incorporation of label (13C) into fatty acids was used to quantify uptake by bacterial symbionts and sponge host. The manuscript is attached.'

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Additional Editor Comments:

Dear Dr de Kluijver

Thank you for submitting your manuscript entitled "Bacterial precursors and unsaturated long-chain fatty acids are biomarkers of North-Atlantic demosponges" to PLOS ONE. We have received two reviews for this manuscript. Both reviewers considered this a very well written interesting manuscript. I recommend the acceptation of this manuscript after minor revisions

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Reviewers' comments:

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Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

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Reviewer #1: Yes

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: N/A

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

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Reviewer #1: Reviewer’s comments

The manuscript of Dr. de Kluijver and co-authors contains results of a solid piece of work and it is clearly written. It is of great interest according to several points of view.

Firstly, the Authors investigated deep-sea sponges, collected in the North-Atlantic Ocean during different scientific expeditions, for phospholipid fatty acid (FA) compositions. Four sponges belonging to the genus Geodia, another one to the genus Stelletta, were studied. It should be noted that fatty acids of Geodia sponges are lowly documented to date.

Secondly, these sponge species being classified as high microbial abundance species, the Authors determined phospholipid FA composition for typically bacterial FAs and typically sponge FAs. Bacterial FAs (branched and monoenic short-chain FAs) constituted the majority of total FAs in all five deep-sea demosponge species. Interestingly, high concentrations of mid-chain branched FAs (MBFAs) were found in all five sponge species analyzed. A predominance of MBFAs is considered to be a typical feature of Demospongiae and it is also known that MBFAs are typically produced by bacteria.

Thirdly, biosynthetic pathways for some long chain FAs from their possible bacterial precursors are proposed. These pathways are supported by the small isotopic differences found in LCFAs using the δ13C values obtained by GC-C-IRMS.

Fourthly, the Authors conducted an interesting discussion according to a chemotaxonomic point of view about the possible origins of bacterial symbionts used as metabolic sources by sponges.

The use of DMDS adducts is very appropriate and clearly presented in the text and in figures.

In conclusion, the scientific investigation sounds fine and the strategic approach and fatty acid analyses are appropriated conducting to great results.

Thus, this paper should be recommended to be published with minor revision.

Minor corrections and improvements

Title

I would suggest to write “North-Atlantic deep-sea sponges“, although this is noted in the Short Title and Key words.

Methods

Sponge collection

This part is well detailed and interesting but a precious indication is missing. Indeed, it is important to give details about the voucher specimens and in which scientific institutions  they were deposited. This is usually required and it seems particularly important since the sponges studied are deep-sea ones. Furthermore, they mainly belong to the genus Geodia, which is relatively less documented.

FAME analysis

GC analysis. Split or splitless ?

Results

It seems surprising that FA compositions of total lipids and phospholipids were found similar. This point should be discussed. Thus, an interesting information is missing. The percentages of total lipids relative to dry matter are given but not the ones of phospholipids relative to total lipids. It seems necessary in order to a better comprehension of data showed in Table S1

Reviewer #2: This manuscript provides a detailed characterisation of fatty acid composition in a number of sponges. The manuscript is very well written, experimental methods are clear and so are the results. I find that the manuscript can be published almost as it is. I have just a few question, which may be considered.

p. 3 and elsewhere. The authors introduce and discuss FA from bacterial endosymbionts in relation to the extracted fatty acids and as precursors for long chain fatty acids. Sponges are known to retain particles less than 1 µm in size and may therefore feed on suspended bacterial cells, so what about a potential contribution of fatty acids obtained by the sponges from digestion of bacterial prey (maybe the discussion, Line 387-389 could be expanded)?

Line 420. I suggest it is specified that it is the LSFAs that may be used as phylogenetic markers, if this is the case? I was a bit confused first time I read this part.

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Reviewer #1: Yes: prof. Barnathan gilles

Reviewer #2: No

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Bacterial precursors and unsaturated long-chain fatty acids are biomarkers of North-Atlantic deep-sea demosponges

PONE-D-20-31393R1

Dear Dr. de Kluijver,

Thank you for addressing all the issues/suggestions raised by the reviewers.

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Kind regards and Merry Christmas

Clara F. Rodrigues

Academic Editor

PLOS ONE

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Reviewers' comments: