Xin et al. measured the cell density of three marine phytoplankton species under different Ni concentrations to assess the influence of Ni on phytoplankton in the context of ocean alkalinity enhancement (OAE). They also measured the total dissolved Ni concentrations and calculated the free Ni concentrations in the growth media to examine the availability or toxicity of Ni. Overall, this study contributes to the current understanding of OAE-related Ni influence. The comments provided below are intended to assist the authors in refining their articles for publication.

Line 17: Please add a sentence to explain why you chose to study Ni and how it relates to OAE materials.

Line 17: As previously mentioned, Ni can act as a micronutrient at low concentrations. Please consider that dissolved Ni can have both positive and negative effects on phytoplankton growth. Change 'toxicity of nickel' to 'influence/effects of nickel'.

Line 8: Change “a range of Ni concentrations” to “9 Ni concentration gradients”.

Line 20 -24: Considering the results revealed some growth enhancement from Ni addition, the fertilization effects of Ni should be mentioned as well. Please write the specific inhibition effects and potential fertilization effects on phytoplankton, such as “XX% of growth rate inhibition” and “IC50 value was XX Ni concentrations”.

Line 22: The sentence “rapid response to exposure of Ni” is unclear.

Line 23: Add “diatom” before “Tholassiosira weissflogii”.

Line 58: The word “metal sensitivity” is a wide concept and considering only Ni was investigated in this study, I suggest changing it to “Ni sensitivity”.

Line 65: “EDTA”, please write out the full name the first time the abbreviation is mentioned.

Tabel 1. There were standard deviations in Fig.1 so please add the standard deviations in Table 1 for the measured Ni concentrations and potentially free Ni concentrations as well.

Line 111-114: Why did the NiCl2 stock solution (line 70) precipitate as nickel carbonate when its maximum solubility is 10.73 mol/L, while the stock solution was only 50 mmol/L? What caused this precipitation of NiCl2?

Fig 2,3,4: It’s hard to distinguish the cell density and growth rates in low Ni concentrations (0-5 umol/L). Please consider editing the figure to enhance the differentiation of data points in this low Ni concentration range. It appears that one data point may have been lost or obscured, possibly due to the similarity in results between the control (0 umol/L) and 0.01 umol/L conditions?

Discussion: I suggest giving each main discussion paragraph a subtitle. This will help to keep the audience engaged with the clear outlines.

Line 174: Please provide a reference for the sentence “Basic and ultrabasic rocks, which are widely recognized source minerals for OAE, would introduce high amounts of Ni into seawater during mineral dissolution”.

Line 179: (Hartmann et al., 2022) should be (Hartmann et al., 2023).

Line 185-209This paragraph draws comparisons between the responses of three different species based on the results, but it requires some further refinement:

Line 193: The influence of Ni at low concentrations (0-5 umol/L) is just as important at high concentrations since the natural Ni concentrations are around 10 nmol/L. It’s likely the added Ni from OAE will fall into the range of 0-5 umol/L, and E. huxleyi had enhanced growth rates in this range. Therefore, please discuss why E. huxleyi benefited from supplied Ni at low concentrations.

Line 195-204: If E. huxleyi and A. carterae produce more organic ligands to decrease the free Ni concentrations, we might observe a decline in free Ni levels at the end of the experiment. Conducting a significance test on the free Ni concentrations among the three species could help determine if the presence of additional ligands reduces Ni toxicity. However, variations in metal quotas among different species introduce additional uncertainty when testing this hypothesis. I recommend that the author revise this discussion.

Line 208: The discussion in this section lacks clarity. While the paragraph explains the potential detoxification mechanism in the diatom T. weissflogii, it fails to address why T. weissflogii exhibited higher sensitivity and lower tolerance to high Ni concentrations.

Line 212: The cited reference Guo et al., (2022) used different Ni concentrations (0-50 umol/L) and gradients, so please write the specific Ni concentrations compared here to avoid potentially misleading information. Change “in response to high Ni concentrations” to specific Ni concentrations.

Line 220: “…in our study led to orders of magnitude…” how many orders of magnitude?

Line 227: “… contained a low amount of Ni…” what’s the concentration of Ni used in Hutchins et al.

(2023)?

Line 231: “These studies showed a range of sensitivities to Ni among different groups.” Please change the word “groups” to a more specific description, like “plankton” etc.

Line 231: “LC 50” Please explain or at least write out the full name when the abbreviation first appears.

Line 234: I suggest using the IC50 instead of the “lethal concentration” if you have the information because in this study IC50 was calculated and discussed.

Line 235: Move “(Huang et al., 2016)” after the “1.7mmol/L”.

Line 229-239: Considering the information presented in Table 2 and the potential impact of nickel addition mentioned in line 176, it is advisable for the authors to delve deeper into the discussion of a nickel threshold in the context of OAE projects.

Line244: Please add a reference for the sentence “Nowadays Ni is a highly demanded metal resource for battery manufacture”.

Line 253: Limestone is not a metal-free resource mineral (Šiler, 2018). In fact, nearly all minerals used in OAE contain metals, including elements like Ca and Mg. Please change it into a more accurate description.

Line 253-257: The use of limestone differs significantly from that of olivine. Limestone cannot be employed directly as an OAE material, making the example provided somewhat misleading. A revision of this discussion is necessary to provide clarity and context for the subsequent discussion on energy consumption.

Please review and ensure the accuracy of the reference format, including the inclusion of all necessary information for each citation. Specifically, please make sure to use subscript "2" for "CO₂," superscript "2+" for "Mg²⁺," avoid capitalizing journal titles, provide DOI numbers, and use standard abbreviations for journal names.

References:

Šiler, P.: The possibilities of analysis of limestone chemical composition, Mater. Sci. Eng., 2018.

The manuscript by Xin et al. presents results on the response of important phytoplankton to increased Ni concentrations in the context of olivine-based ocean alkalinity enhancement. This study is novel, timely, and the methods, analyses, and interpretation seem to be scientifically sound. I recommend publication, but kindly ask the authors to consider the following questions and comments when revising their manuscript.

Line 39: “…co-benefit of mitigating ocean acidification…” – state that this would especially occur at OAE deployment sites, prior to seawater equilibration with CO₂.

Lines 41-42: Consider adding a brief discussion regarding the merits of using olivine over other proposed OAE minerals here.

Lines 55-56: Why did the authors select these three organisms for their study? Are they meant to be functional group representatives? If so, a brief note regarding the importance of the functional groups they represent would be useful.

Lines 70-80: This section would benefit from the inclusion of additional information. What were the experimental timeframes and why were they selected? Although timeframes are shown in Figs 2c, 3c, and 4c, discussing them here would be helpful.  The experimental timeframe for the A. carterae experiment appears to be two days longer than that of the E. huxleyi and T. weissflogii experiments – what was the reason for this? For how many generations was each species grown? The replication strategy is also not clear to me. Were samples collected from triplicate independent replicates at each sampling point? Finally, I would suggest editing this section’s heading – the heading implies that methods used for determining species responses will be presented, but it seems to focus more on the experimental design; section 2.3 seems to focus more on methods used to determine species response.

Lines 75-76: “…between 9:00 a.m. and 10:00 a.m.…” – to avoid the effect of the photocycle…

Lines 94-95: Consider rewording this sentence for clarity and to define IC50 (e.g. – “The inhibitory concentration [IC50], at which growth is inhibited by 50%, was…”).

Table 1: As the measured and free Ni²⁺ concentrations presented in Table 1 are from triplicate measurements, standard deviations be included here.

Figs. 2a-b, 3a-b, and 4a-b: The data points are a bit hard to read, especially at lower Ni concentrations.

Fig. 2c, 3c, 4c: Data points are missing error bars. Additionally, units should be provided for log-transformed cell densities.

Lines 139-140: “A similar trend…” – this sentence does not flow well.

Line 143-144: I find this interpretation a bit confusing: the authors note that E. huxleyi growth was inhibited after day 3 upon being exposed to Ni, pointing to Fig. 3c, but – in some treatments – log cell densities appear to be higher than the control after day 3. Clarifying which treatment level is being referred to would reduce ambiguity.

Line 153-154: Here, too, clarification about which treatment level is being referred to would be helpful.

Line 164: Please consider removing “Up to today,”. Consider the following as a potential alternative: “Trace metals are required by phytoplankton for numerous physiological processes and biochemical reactions; however, it is difficult to disentangle the distinct role of each element.”

Line 168: Wouldn’t “nutrient-enriched media” be a better choice here since f/2 media was used?

Lines 193-200: The authors should also discuss the observed increase in E. huxleyi  specific growth rates at low Ni²⁺ concentrations. What are the proposed hypotheses that explain this increase?

Line 226-227: If space permits, it would be useful to discuss the range of Ni concentrations found in olivine, especially within the context of the Ni concentrations selected for this study (Simkin and Smith [1970] could be useful for this).

Line 231: LC50 should be defined here.

Lines 252-253: Limestone and its derivatives are unlikely to be metal-free (e.g., Gabe and Rodella, 1999).

Line 256: Consider including a note about the slow dissolution times observed for olivine as well (e.g., Fuhr et al., 2022).

Lines 262-273: Based on their results, what do the authors suggest for future experiments?

References

Fuhr, M., Geilert, S., Schmidt, M., Liebetrau, V., Vogt, C., Ledwig, B., and Wallmann, K.: Kinetics of olivine weathering in seawater: an experimental study, Frontiers in Climate, 4, 2022.

Gabe, U. and Rodella, A. A.: Trace elements in Brazilian agricultural limestones and mineral fertilizers, Communications in Soil Science and Plant Analysis, 30, 605–620, 1999.

Simkin, T. and Smith, J. V.: Minor-element distribution in olivine, The Journal of Geology, 78, 304–325, 1970.