The authors address the effects of increasing CO₂ equilibrated ocean alkalinity on marine microbes in an oligotrophic system. This study is timely since there has been increasing insight that negative emissions will be required to avoid dramatic climate change. Therefore, studying the potential effects of these carbon removal strategies is essential to informed decisions about their efficacy and practicality.

The manuscript is well design and, overall, well written and provides an important step to the knowledge of the effects of ocean alkalinity enhancement. Therefore, I recommend this paper for publication in Biogeosciences, after minor revisions.

The manuscript by Marín-Samper et al. presents results on the response of planktonic communities to ocean alkalinity enhancement. The methods, analyses, and interpretation seem to be scientifically sound, and the study is both novel and timely – offering much-needed insight into the potential impacts of OAE on marine biota. I kindly ask the authors to consider the following questions and comments when revising their manuscript.

Lines 82-85: “Additionally, the type…” –This sentence is a bit convoluted; it also needs additional citations. For example, I don’t think including “biogenic” when citing Moras et al. 2022 is accurate as the referenced study examines runaway CaCO₃ precipitation when using proposed ocean-liming minerals (e.g., CaO and Ca[OH]₂) – and they hypothesize that precipitation likely occurred on the surface of the undissolved ocean-liming minerals used in their study; while this offers support to abiotic particles affecting OAE efficacy – at least when using the noted ocean-liming minerals – it seems the authors are also implying that the presence of biogenic CaCO₃ could induce similar precipitation. If so, additional citations should be included. Citations for the other potential influences on OAE efficiency noted in this sentence would also be helpful.

Line 120: What the authors mean by “air-equilibrated…stock solutions” is not entirely clear here (e.g., did they bubble their solutions?). Although further explanation is provided later in the manuscript, a brief explanation along the lines presented in Federer et al. (2022) would provide clarity early on. Additionally, the method of alkalinity addition being simulated in this study is not entirely clear: if the goal was to simulate an ocean-liming scenario (as stated elsewhere in the paper – e.g., line 370), why weren’t calcium concentrations also increased? Would these results also apply to aqueous hydroxide addition? For example, the use of NaOH – after its reaction with seawater – is effectively alkalinity enhancement through sodium carbonate addition. The authors also use “carbonate-based” (line 370) to describe the form of alkalinity addition being simulated – would these results apply to the use of other carbonate minerals (e.g., dolomite) as well?

Line 295: The spike in GP:CR in the Δ1200 treatment (Figure 2D) just before phase II is interesting – especially as differences were seen in the contribution of the micro size class and PER% during phase II for this treatment relative to other treatments (Figures 3 and 5). Do the authors have a hypothesis as to what may have caused the spike in this treatment?

Line 314 and throughout: Using “community composition” seems a bit misleading as we can’t deduce how relative species abundances or phytoplankton functional group (PFG) relative abundances might have been affected within each size class, especially seeing as how there is PFG overlap among size classes (e.g., Pierella et al. 2020).  As such, stating that “only minor changes in species composition were observed” (line 531) seems a bit premature. If the authors wish to use “community composition” to describe their results, they should note that potential changes in the relative abundances of species or PFGs within size classes might be masked.

Lines 315-316: It’s not clear what criteria were used to differentiate nanoeukaryote (1) and nanoeukaryote (2) populations. Figures 3 and 4 have the nano community as one group, but two populations are discussed in Section 3.3 and presented in Figure 6.

Minor:

Line 51: “Process that is…” – this sentence is not complete and would flow better if it were joined with the previous sentence.

Lines 54 and 59: Should “carbon dioxide removal” and “negative emissions technologies” be capitalized here?

 Line 55 and throughout: “…hard to abate emissions…” should be written as “…hard-to-abate emissions…”. Here – and throughout the manuscript (e.g., line 61: “…carbonate- or silicate-based alkaline…”) – phrasal adjectives are often incorrectly written.

Line 175: Incubation time is represented as “h_D” and “h_L” in the two equations rather than “T” as noted in the text (line 182).

Line 187: I couldn’t find Carmeño et al. 2012 in the reference list – check that all references are included.

Line 284 and following: Panel D in Figure 2 is labeled “E” in the caption. GP:CR is also shown as “GCP over CR”. The authors should verify that figure captions match plot labels and in-text references.

Line 290: The axes in Figure 3 are difficult to read.

Line 400:  At the beginning of this section, the authors’ use of “these results” make it a bit difficult to determine to which study they are referring. For example, is the sentence beginning at the end of line 405 referring to this study or the Ferderer et al. 2022 study?

Line 410: Why is Figure 7 presented here instead of in the Results section?

Line 420: The sentence beginning with “In addition…” should be combined with the previous sentence.

References

Ferderer, A., Chase, Z., Kennedy, F., Schulz, K. G., & Bach, L. T. (2022). Assessing the influence of ocean alkalinity enhancement on a coastal phytoplankton community. Biogeosciences, 19(23), 5375–5399.

Pierella Karlusich, J. J., Ibarbalz, F. M., and Bowler, C. (2020). Phytoplankton in the Tara ocean. Annu. Rev. Mar. Sci., 12, 233–265.