General comments:

In this manuscript, Slomp et al. initially described the continental margin sediments (>100 m deep) of hypo- to anoxic nitrogenous basins as a niche for the development of cable bacteria (CB). Then, they estimated the abundance and diversity within the Cadidatus Electrothrix lineage to provide an overview of the recently described diversity of this group, and to suggest a new genus adapted to these environmental conditions. The manuscript is fairly well structured, and its objectives are clear. The methodological approaches are comprehensive and aim to characterise the geochemical conditions of the bottom waters, the sedimentary compartment (pore-water and solid phases) and the microbial community concerned. The data sets are freely accessible on Zenodo. Despite the wide range and quality of the data, it is a snapshot in time, which limits the scope of the discussion on the factors controlling the CB dynamics in this potential niche. Overall, I think this is a robust and very interesting research paper which corresponds to the scope of Biogeosciences.

Specific comments:

Line 104: you present bibliographical data on Thioploca in the Soledad basin, but nothing on the other groups presented in the introduction (Beggiatoaceae and Thiomargarita). Is there any information available on this subject in these basins? Similarly, in line 108, you present (admittedly old) data on the meiofauna and macrofauna of this same basin, but you say nothing about the other basins? Have no data been published?

Lines 133-135: only one O₂ profile was done in San Clemente? How were the pH and H₂S measurements set up to achieve complete microprofiling to a depth of over 3 cm in less than 15 minutes? I assume that the waiting and measurement times are particularly short. Is this relevant for this type of measurement?

Line 160: one point I don't understand about the analysis of dissolved metals: why didn't you use the pore water Mn data from Bruggmann et al. (2023), as you did for Fe, in order to have the same vertical resolution for both? The vertical resolution in Bruggmann et al. (2023) is low and I find it relevant that you have carried out higher resolution analyses for Mn. But in that case, why didn't you do the same for Fe to get geochemistry within the same core?

Section 4.1: the densities observed in these hypo-anoxic basins are like those observed in situ on estuarine intertidal mudflats (oxygenated environment + sulphides) where CB could be particularly active (Daviray et al., 2024), or in the rhizosphere of aquatic plants (Scholz et al., 2019, 2021).

The presence of CB (DNA data) but the absence of activity also raises a hypothesis that is not discussed here: could it be the result of CB-enriched sediment transport into the basins? Do you have any information on the marine currents affecting these locations? In this case, it could be better to talk about a ‘potential’ niche.

Section 4.2: The discussion of the sources of H₂S and its temporal dynamics is stimulating. However, it remains hypothetical and suffers from a lack of (temporal) data, in my opinion. Have you tested correlations between the various parameters (i.e., C_org, total S, Fe sulphides, etc.) to perhaps highlight this dynamic and support a periodic (seasonal?) increase in sulphate reduction?

Lines 434-447: in my opinion, this paragraph lacks any link with the biogeochemical data to explain the diversity observed.

Line 441: what electron acceptor do the Bacteroidota use?

Line 453: it's a shame that we don't have this data, as it would have helped to underpin the discussion on interspecific competition.

Line 461: any suggestions on these factors (bioturbation or others generating sediment heterogeneity, Fe curtain, etc.)?

Lines 475-476: out of curiosity, do you have any idea what these benefits might be? The same for the selective pressures mentioned line 489. The section 4.4 is very interesting and frustrating: we want to know more!

Section 4.5: this section could perhaps be further summarised and incorporated into the second paragraph of the conclusion.

Figures:

Figure 1: the blue colour contrast is poor. Would it be possible to improve it like in Bruggmann et al., 2023?

Figure 2: you write that triplicates were achieved for pH and H₂S µprofiles. Is the absence of standard deviation on these profiles justified by their high reproducibility? I suppose so for H₂S (because there isn't any), but what about pH?

Figures 2, 3 and 4: please, put the unit of the vertical axis in cm.

Review of Slomp et al. A niche for diverse cable bacteria in continental margin sediments overlain by oxygen-deficient waters.

The manuscript of Slomp et al. describes a study where activity and abundance of Cabel bacteria were addressed in sediments from 5 hypoxic basins along the coast of Mexico and California, together with a multitude of geochemical features (O₂, pH, Fe, Mn, Nox, Al ) and microbial populations.  In general cable bacteria abundance were low and activity were below detection limit.  Phylogenetic  analysis revealed that the Cable bacteria belonged to the Candidatus Electrotrix lineage and included specimens affiliated with Electrotrix gigas. In addition several sequences were affiliated with a sister clade to Electrotrix and it is suggested that these represent a novel genius.

  In general, the manuscript adds novel information about the biogeography and diversity of cable bacteria in marine environments and has therefore merits justifying publication.  I recommend however that the authors put somewhat more effort in discussing the vast amount of geochemical data and provide the  motivation behind the  analysis these features.  There are severall fractions of solidphase  and dissolved  compounds that is not (to my knowledge )really linked to cable bacteria (e.g. Al, Mn, NH4 ): Why is this relevant in the given context?; What was the hypothesis? In addition the motivation behind the analysis of the general microbial community should be stated more clear. What is the relevance of this analysis for the target features: cable bacteria?

In my view these data should be more integrated in the overall framework of the study:

Would it e.g. be possible to apply e.g. network analysis to identify specific cablebacteria – microbe associations?

Minor:

L 32: remove the Nielsen et al . 2010 reference.  This paper says nothing about cable bacteria. The authors suggest nanowires or conductive minerals as mediators of the electric currents that runs through the sediment (exactly like in the Revil et al. (2010) paper!) The discovery of cable bacteria  was published in 2012. i.e. two years after the Nielsen et al. paper.

L 34: Remove the reference Nielsen and Risgaard-Petersen 2015 paper: This is a review paper that amongst other review the Risgaard-Petersen et al. 2012 paper on how cable bacteria influence the biogeochemistry.

L 49.  The Damgaard et al 2014 paper describes the construction and application of the silver silver chloride electrode used for electric potential measurements.  The focus in this paper is not relationships between cable bacteria activity and the electric potential.  Such is more thoroughly described in Risgaard-Petersen  et al. (2012) and in Risgaard-Petersen et al. (2014). Since both papers precedes the Damgaard et al. paper I suggest that that the authors include those here instead of the Damgaard et al. paper.

L 133.  Please spell out “EP”:  Electric potential.

L150: What sediment volume  was  collected for FISH:  0.5 ml?? Please specify.

L 195. The method description for quantitative FISH analysis is a bit odd. What is the rational for not using direct methods like those described in e.g. Schauer et al. (2014)?.  Why use Nycodens extractions? There is to my experience a great risk of loosing cells with this method.  Do the authors have any data on the  cell recovery efficacy ?

It apereas that Sybr Green I  staining were used for Cable bacteria quantification : Why  this step when you have FISH stained filaments already?

References

Revil, A., Mendonca, C.A., Atekwana, E.A., Kulessa, B., Hubbard, S.S., Bohlen, K.J., 2010. Understanding biogeobatteries: Where geophysics meets microbiology. J. Geophys. Res. (Biogeosci.) 115, G00G02.

Risgaard-Petersen, N., Damgaard, L.R., Revil, A., Nielsen, L.P., 2014. Mapping electron sources and sinks in a marine biogeobattery. J. Geophys. Res. (Biogeosci.) 119 1475–1486.

Risgaard-Petersen , N., Revil, A., Meister, P., Nielsen, L.P., 2012. Sulfur, iron-, and calcium cycling  associated with natural electric currents running through marine sediment. Geochimica Et Cosmochimica Acta 92, 1-13.

Schauer, R., Risgaard-Petersen, N., Kjeldsen, K.U., Bjerg, J.J.T., Jørgensen, B.B., Schramm, A., Nielsen, L.P., 2014. Succession of cable bacteria and electric currents in marine sediment. Isme Journal 8, 1314-1322.