Ocean acidification changes the structure of an Antarctic coastal protistan community

Hancock, Alyce M.; Davidson, Andrew T.; McKinlay, John; McMinn, Andrew; Schulz, Kai G.; van den Enden, Rick L.

doi:https://doi.org/10.5194/bg-15-2393-2018

Articles | Volume 15, issue 8

https://doi.org/10.5194/bg-15-2393-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The Ocean in a High-CO₂ World IV

https://doi.org/10.5194/bg-15-2393-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 8

Research article

|

20 Apr 2018

Research article |

| 20 Apr 2018

Ocean acidification changes the structure of an Antarctic coastal protistan community

Alyce M. Hancock, Andrew T. Davidson, John McKinlay, Andrew McMinn, Kai G. Schulz, and Rick L. van den Enden

Download

Final revised paper (published on 20 Apr 2018)
Supplement to the final revised paper
Preprint (discussion started on 07 Jun 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

- Printer-friendly version

- Supplement

RC1: 'Review to Ocean acidification changes the structure of an Antarctic coastal protistan community by Hancock et al.', Anonymous Referee #1, 06 Jul 2017
RC2: 'Review of Hancock et al. - Ocean acidification changes the structure of an Antarctic coastal protistan community', Anonymous Referee #2, 15 Jul 2017
AC1: 'Response to Anonymous Referees', Alyce Hancock, 31 Aug 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (18 Sep 2017) by Elizabeth H. Shadwick

AR by Alyce Hancock on behalf of the Authors (31 Oct 2017) Manuscript

ED: Referee Nomination & Report Request started (08 Nov 2017) by Elizabeth H. Shadwick

RR by Anonymous Referee #3 (08 Jan 2018)

RR by Anonymous Referee #4 (08 Jan 2018)

Suggestions for revision or reasons for rejection

Species specific analyses of prokaryotic community response are rare and important additions to our understanding of how ecosystems will change in the face of anthropogenic CO2 enrichment. This study used 6 microcosms to determine dose dependent responses of various members of the prokaryotic community to CO2. Due to space and time limitations, there was a lack of replication. The results were, however, appropriately statistically analyzed based on the experimental design and suggest that for some important taxonomic groups (micro-sized diatoms) there is a u-shaped response curve – perhaps helping to resolve previous discrepancies in the literature and revealing a natural tipping point in community response. This is valuable information (both from an intellectual and policy standpoint) and is a welcome contribution to the literature.

The addition of the carbonate chemistry is important, but remains slightly incomplete. For table TS2 is this the mean + standard error or standard deviation? Also, in the text the authors state that they report omega as well (P4, L31). This calculation is missing. What is PA? Is this calculated Total Alkalinity? If so why is the Alkalinity of T6 different?

The authors state that P is replete throughout the experiment in the response to reviewers, but it reaches 0.1 uM in some treatments. I am not a phytoplankton person and have no idea what is normal for these speceis, but this seems pretty low? Even if a nutrient does not reach non-detectable levels there could still remain some nutrient stress in acquisition (right?). In the text the authors state P “remain[ed] above detectable levels throughout the entire experiment.” As P was a significant predictor of community (22.68% in the reduced CAP analysis) it might just be useful to mention that there could be co-stress. This is not to say that the entire response is associated with nutrients, but it could be an important co-variate and this could have implications for community response under ice-covered conditions when CO2 naturally begins to build up under the ice and air/land based P deposition is decreased and mixing is decreased. Please ignore if I am totally off about this being slightly limiting.

As a note from a zooplankton person – although 200 microns is the typical way of excluding metazoan zooplankton, in reality some will be retained in this fraction (it is rather impossible to get rid of them all without also influencing protistan or chain forming prokaryotic species). Likely to remain are, for example, juveniles or copepods of the Oithona group. I am not questioning the method – it is a standard size fractionation, and this is a community level analysis – I just wish to remind the authors to remember that these types of studies may have an embedded metazoan grazing effect that is unavoidable.

More food for thought – in the response to reviewers the authors state that fCO2 is the most important variable since it is what is changing with human activity. For phytoplankton I can see where this is a valid argument as it is associated with potential “fertilization” of RuBisCO’s photosynthetic function. For the tipping point on the higher side – the trade off with acidity based on increased H+ ion concentration, it is important, however, to think about pH. Concurrence or difference with previous studies near that tipping point the 600-800 uatm level from page 12 could be associated more with acidity as the phytoplankton exert effort to retain homeostasis. It could be useful to compare your pHs to these other studies in this context.

Sentence p 12 L 2-5 is confusing. Maybe break it up and add contrast “Unlike Tortell et al. (2008b), however, there was…”

Sentence p 12 L 6 has an incorrect use of ; use : instead. This study also cites Tortell et al. (2008b) twice. Just polish it a bit.

Sentence p 12 L 10: For those of us that don’t know can you quickly summarize why it is different? Add just a few words before the citation.

Sentence p 12 L 15: You need to remove the extra () within your citation

Sentence p 13 L 6 & 8: This is the first – repeats itself.

Hide

ED: Publish subject to minor revisions (review by editor) (29 Jan 2018) by Elizabeth H. Shadwick

AR by Alyce Hancock on behalf of the Authors (12 Feb 2018)

ED: Publish as is (13 Mar 2018) by Elizabeth H. Shadwick

AR by Alyce Hancock on behalf of the Authors (14 Mar 2018)

Short summary

Absorption of carbon dioxide (CO₂) realized by humans is decreasing the ocean pH (ocean acidification). Single-celled organisms (microbes) support the Antarctic ecosystem, yet little is known about their sensitivity to ocean acidification. This study shows a shift in a natural Antarctic microbial community, with CO₂ levels exceeding 634 μatm changing the community composition and favouring small cells. This would have significant flow effects for Antarctic food webs and elemental cycles.