Review of “Influence of Ni-related enzymes on the Ni cycle in the Southern Ocean:insights from isotopes and metagenomics” by Lemaitre et al., submitted to EGUsphere, Feb. 2025

In this ms. the authors present a suite of new Ni concentration and isotope data from the dissolved pool of upper water column samples from the Southern Ocean. Metagenomics of these samples is also presented, and an attempt is made to determine the biologic controls on Ni concentration and isotope distributions. The ms. is very well-written and illustrated (thank you!).

This ms. reflects a bold attempt at making progress on an important but very tricky problem: how to disentangle controls on a complex biochemical system. And in the end, their conclusions are unsurprisingly weak - essentially what we already suspected, that the “Ni biogeochemical divide likely reflects the combined activity of multiple microorganisms and enzymatic processes.”

The authors do present some take-home messages that might guide future work (e.g., separation of eukaryotes) and, as an explorative piece of science I give the authors kudos: the results just didn’t work out as hoped. I do think there are some “negative results” that could be better thought-through that are likely important (like the hound that didn’t bark…). Perhaps the authors could explore these implications further? For example, the lack of nitrogen fixers is interesting; why not apply the metagenomics to nitrate/nitrite, or some well-understood nutrient, first? Wouldn’t this inform us on the microbial consortium better than jumping straight to Ni? When not in a HNLC environment (e.g., the polynya in full bloom), the Ni does fractionate significantly.  If biologic fractionation of Ni predicated on limitation, would you expect to see any metagenomics expressed in this HNCL region? Moreover, if the fractionation of Ni is closed-system Rayleigh-type (from figure 6), how much depletion of the reservoir is represented? It looks like Ni never goes lower than 2 nM, even at station 11 (or anywhere according to figure 5).  But the upwelled water looks to be all similar at 7 nM and 1.3 permille. More insight in how Ni fractionation could be so different in their sites (figure 6) in conditions that seem geochemically similar (figure 2) would be interesting. Here, negative metagenomic results are very impactful, and would seems to point to a component of dissolved Ni in the water that is complexed (unavailable).  In light of this, I think the authors really need to address the Archer et al., 2020 and John et al., 2022, 2024 hypotheses described at the start of the ms.

The authors should remove station 11 at the outset and then from all statistical analyses, as they make it abundantly clear that it was not typical for the Southern Ocean. There is a lot of leading the reader down a dead-end trail with the discussion including and not including station 11. Station 11 should be treated as a separate experiment to answer broader questions (e.g,. is a polynya diatom bloom different than “normal” ocean diatom blooms? Why would the microbes, or Ni, be different in a polynya? Certainly there is nothing that would make station 11 stand out when looking at the Ni and NO3- data presented.) Similarly, there is a lot of method and results text describing the metagenomics (someone with expertise should review this). I understand it is a lot of work, but the description is excessive in comparison to the conclusions drawn from these data. Particularly when excluding the station 11 data. All the various statistical analyses could not conclusively point to a mechanism for the latitudinal variance: why prolong this discussion? And if the results are distinctly negative, this is important, but could be presented quite readily I would think. I think that they do infer negative results, but these points were lost to me in the noise of the statistical hoops trying to get a positive result. (I admit the analyses and statistics done are impressive – I had to look up what a redundancy analysis is – but they seem gratuitous.)

In summary: the authors collected a large and interesting data set, with some unfortunate oversights. They did a great job in presenting the work done, although a lot could be trimmed without losing any of the conclusions. I offer some thoughts above that the authors may consider in revision, but generally I see no logical flaws in their work and recommend this for publication. [with just one silly grip: I  read many studies claiming to be “the first time” doing something. While maybe justified, the statement is ostentatious.]

Dear Handling Editor, 

Please find my comments below. I apologise fro my delay in reviewing. 

Best,