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1. Summary

In “Aurora vent field is a hotspot for microbial hydrogen oxidation in the Arctic Ocean”, the authors asked how the Aurora vent field’s (AVF) high H2 concentrations influence microbial communities, particularly their energy metabolism. First, they sampled areas around the Aurora Vent field chimney wall including hydrothermal deposits, chimney wall precipitates, and sediment cores. Using 16s rRNA sequencing and metagenomics, they characterized the diversity of organisms, predicted their metabolic capabilities, and did a phylogenetic analysis of the metagenomes and certain hydrogenases. They found that the microbiome is largely dominated by bacterial phyla, including Acquificota and Campylobacterota, with archaeal phyla represented among most samples. Many organisms present had flexible metabolic capabilities with no obligate autotrophs identified. They also identify and genomically characterize novel microbes within the Mariprofundus genus and Aquificota phylum. The presence of novel hydrogenases also suggests that these microbes are potential H2 oxidizers.

2. General Assessment

This study provides a significant contribution to the field of microbial ecology of deep-sea hydrothermal vents. The AVF has a higher hydrogen concentration than anywhere else on earth. Therefore, the AVF presents a unique environment in which to study the environment's impacts on microbial energy metabolism and the basis of the food chain, as the oxidation of hydrogen can be used as a sole source of energy for autotrophs or as a supplemental energy source for heterotrophs. The findings of this paper contribute to better predictions of microbial community responses to environmental change, modeling microbial food webs, and our understanding of the basis of life without sunlight through chemosynthesis. The characterization given the constraints of the study design and marine systems with few laboratory toolkits are impressive due to the >1500 assembled metagenomes (MAGS) and the comprehensive characterization of the taxonomy and energy metabolism. Additionally, the genomic characterization of two novel microbes that have H2 oxidation potential is an exciting finding. Though not discussed in the paper, this research also has exciting implications for astrobiology and imagining life on other planets.

The introduction is well-written and gives a descriptive summary of relevant background. The methods are logical and easy to follow, and the data, both main text and supplemental, are easily accessible. The spatially broad sampling regime and diversity of sampling locations was a strength of the study.

It is a descriptive study and does not have the solid backing of experimental evidence. However, this type of data is hard to achieve in marine systems with uncharacterized bacteria due to the inaccessible nature of the environment, a lack of model systems, and a lack of culturability among organisms. They did not use statistical analysis to identify differentially abundant taxa in samples or compare across sample types which would have strengthened conclusions. With 70 samples taken, the sample size is high enough that statistical testing would have been appropriate.

3. Major Concerns

Since over 70 samples were collected, the study would have benefited from statistical testing suitable to the data type like a PERMANOVA that could have tested whether microbial communities differed between sites. If the MAGs for the differentially abundant bacteria were identified as well, metabolism genes between sites could be compared. Statistical testing between hydrogenase gene counts between different sites would answer the question of whether hydrogenases are more present at AVF sites. This analysis may address how the conditions around the AVF influence microbial communities and could even change conclusions. Without this kind of testing, it is hard to conclude whether observed differences are due to true biological patterns or sampling variation.

4. Minor Concerns / Line Edits

“Aurora Vent Field” is capitalized throughout the paper but left in lowercase in the title.

Fig 2: 16s sequencing phyla with 2% prevalence or more are shown, but the text discusses organisms with 1% or more prevalence.

L60-63: This sentence is very long and could be split into two sentences.

L97-L99: “other nearby black smoker vents within the field yielded a single vent fluid sample with a similarly high temperature and H2 abundance 27, suggesting that such extreme H2 contents in AVF source fluids are stable, at least over several years.”

- Is finding one fluid sample with high temperature and high H2 suggestive of stability? I would change wording to “…suggesting that such extreme H2 contents in AVF source fluids could be stable over several years.”

L173: “…taxonomic distribution between AVF sediment and background sediment samples.”

- We found the name “background samples” was not descriptive enough. Background implies that it is a baseline or control. Instead, the authors could say “distant sediment samples” or “near-AVF sediment” vs “far-AVF sediment samples”.

L196: The potential roles of the archaeal phyla are not discussed at length, likely because they are at a lower abundance than bacteria phyla. The potential roles of archaeal phyla should be discussed more.

L200: The significance of the Sox cassette is not discussed, and the acronym is not defined.

L222: What is the less conserved role of cyc2? Only mention this secondary role if it is described in the text.

L234: “Consistent with elevated reduced iron availability near venting structures…”

- We found “elevated reduced iron” a confusing phrase to read. I suggest changing it to “elevated Fe(II) availability” or “greater reduced iron availability”.

L268: How are hydrogenase maturation genes relevant? What do they do? Explain the system for a broader audience.

L275: “Notably, MAGs encoding fast-acting, low affinity hydrogenases (groups 1b and 1d)…”

• From this line onwards, various types of hydrogenases are frequently discussed. A table or glossary detailing the types and functions of them would be very helpful to the reader.

L382-L384: What is meant by rapid shifts in substrate availability? The time scale is not clear here.

Competing interests

The authors declare that they have no competing interests.

Use of Artificial Intelligence (AI)

The authors declare that they did not use generative AI to come up with new ideas for their review.