Evidence for microbial iron reduction in the methanic sediments of the oligotrophic southeastern Mediterranean continental shelf

Vigderovich, Hanni; Liang, Lewen; Herut, Barak; Wang, Fengping; Wurgaft, Eyal; Rubin-Blum, Maxim; Sivan, Orit

doi:https://doi.org/10.5194/bg-16-3165-2019

Articles | Volume 16, issue 16

https://doi.org/10.5194/bg-16-3165-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-16-3165-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 16

Research article

|

23 Aug 2019

Research article |

| 23 Aug 2019

Evidence for microbial iron reduction in the methanic sediments of the oligotrophic southeastern Mediterranean continental shelf

Hanni Vigderovich, Lewen Liang, Barak Herut, Fengping Wang, Eyal Wurgaft, Maxim Rubin-Blum, and Orit Sivan

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Final revised paper (published on 23 Aug 2019)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of bg-2019-21', Anonymous Referee #1, 09 Feb 2019
- AC1: 'Response to anonymous reviewer', Orit Sivan, 27 Feb 2019
RC2: 'review of manuscript bg-2019-21', Anonymous Referee #2, 13 Mar 2019
- AC2: 'Response to anonymous reviewer #2', Orit Sivan, 08 Apr 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (11 Apr 2019) by Jack Middelburg

AR by Orit Sivan on behalf of the Authors (13 May 2019) Manuscript

ED: Referee Nomination & Report Request started (15 May 2019) by Jack Middelburg

RR by Anonymous Referee #1 (22 May 2019)

RR by Anonymous Referee #2 (23 Jun 2019)

Suggestions for revision or reasons for rejection

The manuscript by Vigderovich represents the revised version of a manuscript that I have reviewed previously. Several of the issues pointed out in my previous review have been addressed by the authors. However, there are still some points and issues that need attention. It is striking that there are still many imprecise statements and numerous typos. I have corrected only a few (see specific comments below). In particular, the English still needs some substantial overhaul and careful polishing and check by a native speaker. Also some of the figures/plots (in particular Fig. 2 and caption) are insufficiently labeled and references cited in the text are missing in the list of references.

Besides these formal flaws the main points are: 1) The Discussion chapter still has many statements that are much too general and it is not clear how these relate to your data and to previous work. References are often missing to support the statements made; 2) Please expand and specify and discuss in detail your geochemical evidence showing the contributions of methane from deeper sources (the „gas front“) and that being produced in situ in the shallow sediments.

To conclude, I find this study and manuscript very interesting and definitely suitable for Biogesciences. However, it needs another intense round of clarification of the scientific discussion and formal polishing before I can recommend publication.

Specific comments

L. 28: microbially mediated

Ls. 33/34: I do not fully understand this last sentence of the abstract. What do you mean with „deeper microbial activity“ and „methanic iron reduction“? I also do not agree with the statement that (what I think you suggest) Fe reduction driven by methane oxidation is observed. Of course you observe Fe reduction within the methanic zone, which leads to liberation of Fe2+ into the pore water. Please, rephrase and specify accordingly.

Ls. 38, 40: add e.g. in brackets and also cite work by Lovley et al. here who have performed key studies on microbial Fe reduction.

L. 57: add e.g. in brackets; Niewöhner et al. (1998) is missing in the list of references.

L. 54: add Hoehler et al. (1994) here

Ls. 68 ff: Sentence is odd. Please rephrase. Furthermore, not all of the studies cited here, have performed modelling. Please, check carefully and correct accordingly.

L. 81: … „under“ natural conditions ….

L. 100: I would suggest to rephrase to: „Despite …. the link between the biogeochemical cycling of iron and methane in the methanic zone of marine sediments ….“

L. 103: … we report the observation of microbial iron reduction ….

L.105: What are „microbial profiles“? Please specify – profiles of what precisely?

L. 116: is composed of

L. 121: „contents“ inspite of levels

L. 123: I do not agree that TOC contents of more than 1 wt% - as given here – can be described as low or underlying oligotrophic areas. I find this rather high. Please, also specifiy whether you speak of trophic level of the surface waters (e.g. oligotrophic) or TOC contents of the sediments.

Ls. 125 ff. and 330 ff.: The statement and description of the gas front appear rather imprecise to me. What kind of gas front are you referring to? Why do you put it in brackets? Do you mean free gas? At which sediment depth was this gas front/free gas found. Furthermore, it is not clear to me whether the methane samples you describe as being of biogenic in origin come from this gas front depth or below the gas front or from shallower sediments overlying this gas front. If yes, what is then the link? Does the shallow methane analysed originate from the deeper sediments below the described gas front? Please specify.

L. 130: delete „seafloor“

L. 135: What do you mean with „methanogenesis characteristics“?

Ls. 136 ff. and 171 ff.: How was porosity determined? This is needed to calculate methane concentrations in pore-water.

L. 137: „anoxic“ instead of anaerobic

L. 153: apparant „from (instead of in)

L- 156: … to reach „a“ 1:1 …..

L. 176: with „a“ detection limit

Ls. 179/180: … the measured total sulfur concentrations in „pore water“ were ….

L. 182: delete „Several“

Ls. 189 ff.: How was the sediment dried? Was it freeze-dried. Please specify.

Ls. 195 ff.: Where/in which figures are these data - i.e. the results of sequential extraction – plotted. At least I could not find them in Fig. 2.

L. 196: The profile of pyrite was taken from Wurgaft et al. (2019).

Ls. 222 ff. This sentence needs tob e rephrased because the syntax is odd. How can you investgate the „sotrce“ of a process? This makes no sense.

Ls. 224: The next sentence also makes no sense. How can several profiles show complete depletion of total sulfur at one station?! (You say: The pore-water profiles ….)

L. 231 and throughout the manuscript: I do not like the term „traditional“ iron reduction zone. This is imprecise and makes no sense. Better speak of the „upper Fe reduction zone“ or „upper iron-rich zone“.

L. 234: „sediments“ instead of sediment cores.

L. 238 ff.: Are these methane concentrations in pore water? i.e. have the measured values been corrected for porosity? Please specify!

L. 249: resemble

L. 256: I would not speak of „iron mineral profiles“ but of „operationally defined iron mineral fractions“

L. 266: I thought the pyrite profile was from Wurgaft et al. (2019)?! If yes, please cite this study as the source of these data here.

L. 267 and throughout the manuscript: use „uppermost“ instead of „first“ etc.

Ls. 320 ff. and Fig. 2: The plots and profiles referred to here and depicted in Fig. 2 are insufficiently labeled. It is not clear to me at all what is shown in Fig. 2. Label the individual plots with a,b,c …. And refer to it in the figure caption and label the sequentially extracted Fe fraction as done in related publications.

L. 317 ff: This contradicts your statement in lines 223 ff. where you are mentioning „sources“. Please precisely present the objective of your study.

L. 321, last sentence, and Ls. 152 ff.: This belongs to the Materials and Methods chapter. Moreover, I would suggest to give a table in which the experimental set-up of the different slurry incubation experiments/vials is listed – otherwise it is very hard to follow.

Ls. 334 ff. and 495ff.: I have no idea which of your data show that and to what extent the „shallow sediment processes“ studied in this contribution are linked to the deeper gas reservoir. How do you know and show that part of the methane you have analysed in the shallow sediments originates from the gas front (whatever that is) and from methanogenesis in situ? Please expand and specify and discuss in detail your geochemical evidence showing the contribution of methane from deeper sources (the „gas front“) and that being produced in situ in the shallow sediments. I find it hard to believe that methanogenesis is really possible in these low-TOC (oligotrophic) deposits. If the sediments were really so low in reactive TOC as suggested by the title, I find it hard to believe that methanogenesis can happen in situ at all. I find it more plausible that the methane you have detected in the shallower sediments originates from deeper sources.

Ls. 344 ff.: See previous comment!

L. 349: Do you mean pore-water Fe here? If yes, please say so.
Ls. 348 – 351: These statements are much too general and it is not clear how this relates to your data.

Ls. 356 ff: Also this part is much too general and not at all supported by references. As has been shown by numerous studies, pore-water Fe is often below detection limit at the SMT due to pyrite formation (e.g. Riedinger et al., 2005, GCA, and 2017)

Ls. 358 ff.: The only study cited here is the one by Whiticar (1999). Please, give more recent ones as well. There is an enormous amount of studies and literature on this issues published in recent years.

Ls. 370/371: Please also add Oni et al. (2015) here.

Ls. 375/376: This part of the sentence is odd. How can organic matter be formed from upward migrating methane at the SMTZ?

L. 380: As already pointed out in a previous comment. I do not find TOC contents of more than 1 wt% particularly low.

L- 381: Again, how do you know how much of the methane is produced in situ and how much is coming from deeper sources? Please discuss.

Ls. 384 and 401: What kind of „biomass formation“ at the SMTZ are you referring to here? Again there are no references at all.

L. 386: To which „deep microbial community“ are you referring to here? Do you mean those at the depth of the „gas front“ or that in your shallower sediments? This is not clear here.

Ls. 399 ff.: Please also add Fischer et al. (2013), Nature Geoscience here.

Ls. 409 and 412: Please also cite Riedinger et al. (2005) and (2014) here. They were the first to highlight these environmental/depositional prerequisites.

Ls. 425 ff.: Discuss why the H2 levels are that particularly high at these study sites.

Ls. 470: Which evidence makes you „believe“ that – i.e. makes you make this assumption. Please discuss.

L. 625: has to be: Jørgensen, B. B., Böttcher, M. E., Lüschen, H.,
Check and correct the spelling of Jørgensen throughout the manuscript and the references.

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ED: Publish subject to minor revisions (review by editor) (24 Jun 2019) by Jack Middelburg

AR by Orit Sivan on behalf of the Authors (08 Jul 2019) Manuscript

ED: Publish subject to technical corrections (09 Jul 2019) by Jack Middelburg

AR by Orit Sivan on behalf of the Authors (14 Jul 2019) Manuscript

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Short summary

Microbial iron reduction participates in important biogeochemical cycles. In the last decade iron reduction has been observed in many aquatic sediments below its classical zone, in the methane production zone, suggesting a link between the two cycles. Here we present evidence for microbial iron reduction in the methanogenic depth of the oligotrophic SE Mediterranean continental shelf using mainly geochemical and microbial sedimentary profiles and suggest possible mechanisms for this process.