Kell et al., report an incubation based study to test whether primary producers in an Antarctic coastal environment respond to increased Zn availability. Whereas light, Fe and to a lesser extent Mn, are well established as drivers of productivity in Antarctic coastal ecosystems, any effects of Zn have not been well explored. The authors use multiple lines of argument to show that a state of co-limitation by Fe and Zn is possible. I am a trace metal chemist so cannot comment in depth on the metaproteomic or metatranscriptomic analyses. Overall I think the subject is topical and the text provides some interesting insights into Zn dynamics.

Minor comments (by line number):

I have not read much about dZn concentrations around Antarctica, I assume because it has not been measured much, if some values are reported in the literature I would find it interesting to refer to them in a few sentences just to understand what sort of range and normal profile should be expected in these coastal environments.

42-43 The concept of Zn limitation mainly applies to low pCO2 environments which arise in various coastal areas for different reasons, it is not clear to me how pCO2 in these "low" CO2 zones will respond to future climate change as this likely depends on shifts in productivity, upwelling and freshwater discharge in addition to a slow increase in atmospheric pCO2, so I didn't find this framing of changes in global CO2 to be particularly relevant to the main story. I would have been more interested to know why these low pCO2 zones exist, but maybe even this is getting a little away from the main focus of the text and I think the text would be fine without it.

50 I would refer instead to the later Browning and Moore work (2023) if referring mainly to secondary limitation

68 I would suggest avoiding the term 'prejudice' as this implies unreasonable deductions. Consider that incubations to assess trace metal (co)/serial limitation are generally limited by the number of bottles that can be incubated simultaneously, so inevitably experiments lean towards designs which focus on the most deficient element, which is usually Fe, and perhaps include some combination of Mn, Co and Zn. This isn't unreasonable, but yes I agree with the notion that it means that co- or serial limitation by trace metals other than Fe has probably been under-appreciated to date. Perhaps the authors could rephrase.

105 Not sure what 'total dissolved Fe' is, would just 'dissolved Fe' (and 'dissolved Zn') throughout not be clearer?

128 I assume N+N means nitrate plus nitrite? Maybe define at first use (apologies if I missed this)

 142 Apologies if my terminology is wrong - is there a possibility of independent co-limitation i.e. both Zn and Fe produce positive, independent responses in the same species/groups?

280-286 Do lab culture metal:P ratios diverge from field ratios? If so a comparison to whatever natural Zn:P ratios are available would be more convincing.

327-330 Not sure I agree with the logic of the connection here. Yes atmospheric pCO2 is rising, but what are the drivers of low pCO2 in these coastal areas where CO2 is low? If productivity or freshwater discharge in these regions increases (which is quite plausible in some of the low pCO2 areas highlighted), this may well maintain these regions in a state of low CO2 in the future even with increasing atmospheric pCO2.

400 (and elsewhere in the methods), reference format is duplicated

463-469 I assume the authors know this is not ideal, leaving samples unacidified for months usually lowers recovery, although having said that the effects of this on dZn appear to be not too bad, maybe add a comment (see Jensen et al., 2020, Assessment of the stability, sorption, and exchangeability of marine dissolved and colloidal metals)

This is an excellent study focused on teasing apart the complexities of phytoplankton nutrition, with a specific emphasis on overlooked and traditionally difficult-to-study trace metal nutrients in the environment, i.e., zinc. This study provides several lines of evidence that, in addition to iron (Fe), zinc (Zn) can also limit phytoplankton growth. Most of my comments are minor and largely relate to the accessibility and readability of the text.

• To make the manuscript accessible to researchers outside marine environmental studies, it would be useful to define terms, such as “polynya”.
• It would also help the reader when describing taxonomic classifications to give a little more information. An example, is line 89. For readers who are not experts in algal phylogeny to add “and the haptophyte Phaeocystis” or something similar, when first mentioning this alga. Is "Phaeocystis" used to refer to this alga (or algae) because the species is unknown or there are likely multiple species?
• Sentences, such as “Pronounced and progressively deepening total dissolved Zn (dZnT) depletion over time was observed, with dZn depleted down to an average of 0.82 ± 0.47 nM at 10 m over all TNB stations (Fig. 1g)”, are difficult to read and understand without re-reading. For instance, I had initial confusion about the use of “deepening” and “down”. For instance, it appears “deepening” refers to depth in the water column, but the use of “down” refers to the dissolved Zn concentration decreasing? As this is a complex study, the authors need to be careful with word choice.
• Since the first section of the results was largely previously published, this section could be shortened and/or moved to the methods section as a description of the sampled sites.
• Some clarity is needed with respect to whether the same station was measured temporally, or whether each date represents a different station.
• Do the authors have data to estimate the Zn content of the phytoplankton (or at least the plankton community) in sampled waters compared to how this value changes when the cells are fed Fe, Zn, and Fe+Zn? This would perhaps get at a better understanding of the community’s Zn quota in relation to the amount of Zn that would be considered to be limiting, sufficient, or a luxury.
• ZCRP-A belongs to a very large and phylogenetically complex family. Algae, in particular, have an unusual number of paralogs from this family and many of these paralogs have distinct evolutionary origins. Are the authors confident that the “ZCRP-A” proteins identified are orthologs vs. potentially distinct paralogs of the characterized ZCRP-A? Similarly, the ZIPs are another complicated family, with some members expressed during Zn deficiency and others during Fe deficiency.