the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ocean Alkalinity Enhancement (OAE) does not cause cellular stress in a phytoplankton community of the sub-tropical Atlantic Ocean
Abstract. A natural plankton community from oligotrophic subtropical waters of the Atlantic near Gran Canaria, Spain, was subjected to varying degrees of ocean alkalinity enhancement (OAE) to assess the potential physiological effects, in the context of the application of ocean carbon dioxide removal (CDR) techniques. We employed 8.3 m3 mesocosms with a sediment trap attached to the bottom, creating a gradient in total alkalinity (TA). The lowest point on this gradient was 2400 μmol · L-1, which corresponded to the natural alkalinity of the environment, and the highest point was 4800 μmol · L-1. Over the course of the 33-day experiment, the plankton community exhibited two distinct phases. In phase-I (days 5–20), a notable decline in the photosynthetic efficiency (Fv/Fm) was observed. This change was accompanied by substantial reductions in the abundances of picoeukaryotes, small size nanoeukaryotes (nanoeukaryotes-1), and microplankton. The cell viability of picoeukaryotes, as indicated by fluorescein-di-acetate hydrolysis by cellular esterases (FDA- green fluorescence), slightly increased by the end of phase-I whilst the viability of nanoeukaryotes 1 and Synechococcus spp . did not change. Reactive oxygen species levels (ROS-green fluorescence) showed no significant changes for any of the functional groups. In contrast, in phase-II (days 21–33), a pronounced community response was observed. Increases in Fv/Fm in the intermediate OAE treatments of ∆900 to ∆1800 μmol · L-1 and in chlorophyll-a (Chl-a), chlorophyll-c2 (Chl-c2) , fucoxanthin and divinyl-Chl-a were attributed to the emergence of blooms of large size nanoeukaryotes (nanoeukaryotes-2) from the genera Chrysochromulina, as well as picoeukaryotes. Synechococcus spp. also flourished towards the end of this phase. In parallel, we observed a total 20 % significant change in the metaproteome of the phytoplankton community. This is considered a significant alteration in protein expression, having substantial impacts on cellular functions and the physiology of the organisms. Medium levels of ∆TA showed more upregulated and less downregulated proteins than higher ∆TA treatments. Under these conditions, cell viability significantly increased in pico and nanoeukaryotes-1 in intermediate alkalinity levels, while in Synechococcus spp., nanoeukaryotes-2 and microplankton remained stable. ROS levels did not significantly change in any functional group. The pigment ratios DD+DT : FUCO, and DD+DT : Chl-a increased in medium ∆TA treatments, supporting the idea of nutrient deficiency alleviation and the absence of physiological stress. Taken all data together, this study shows that there is minimal evidence indicating a harmful impact of high alkalinity on the plankton community. The OAE treatments did not result in physiological fitness impairment, thus OAE did not cause cellular stress in the phytoplankton community studied.
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RC1: 'Comment on egusphere-2024-847', Katherina Petrou, 26 Apr 2024
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In this paper, Ramirez et al undertake a physiological assessment of a natural oligotrophic plankton community to the effects of ocean alkalinity enhancement. The work is interesting and presents an important step towards understanding OAE and its potential effects on marine life. The design unfortunately, doesn’t lend itself particularly well to bulk physiology measurements in that it is an un-replicated design and therefore relies somewhat of a gradient response for significance. The study aims to assess community responses, however a better way to interrogate differences within communities across treatments would be to take a single-cell approach to the analyses. The authors do undertake some community separation through size fractionation, but unfortunately this is not fully resolved to different taxonomic groups, but rather broad, and it is not done for all parameters. The study is novel and rare, in that it involves the use of large in situ mesocosms, which allows for whole community assessment and interactions under a natural setting and while the work is broad in scope, for the reason above, it is difficult to assess some of the links between the data and the subsequent interpretation, particularly given the non-linearity of the responses with treatment.
- It would be helpful to have the nutrient information included or summarised in the main text somehow, as the authors for example mention the link between stress, viability and nutrient limitation (especially Phase 1), but there is no visible measure of any nutrients and their dynamics, making assessment of this assertion difficult. While these data have been published elsewhere and are not specifically included here, it could be nice to integrate them visually somehow. Particularly, it would be interesting to know if any data were correlative, i.e. chlorophyll and nutrient dynamics?
- This trait-based approach lends itself very well to multivariate analyses (PCA, RDA etc) which would allow for community composition to be overlaid with vectors of variables, or a trait-based PCA. I’m aware that time is a co-variable in this experiment, but analyses could perhaps be done through using the identified Phase 1 vs Phase 2 data. There is of course the limitation of n=1 for many of the variables, which I appreciate precludes many analyses, but maybe by using the multiple measurements that form part of phase 1 and phase 2, can provide some level of replication, as the authors have done with the ROS and viability data. Alternatively, could the authors bin treatments to low, moderate and high – using cluster analysis to delineate the treatment groups, based on physiological and pigment data? I just wonder if there’s a way to make the patterns clearer and the links more tenable.
- None of the FlowCAM data is presented in the main paper, as I understand this is included in another publication. It might be nice however, to integrate these data more strongly to validate the HPLC pigment interpretations, as pigment content can vary based on physiological adjustment as well as taxonomic shifts, often making it hard to infer too much about community structure. For example, the increase in ZEA could be from changes to photoprotection, as stated in the results, or equally an increase in cyanobacteria, which is also mentioned. The authors also mention the increase in peridinin in Phase 1, but don’t link that with dinoflagellate abundance (Supp figure). Adding taxonomic data would strengthen this part of the paper.
- Is there a reason only some of the photophysiological data are presented? For example, the authors mention in the methods fitting the RLC to obtain alpha, Ek and Pm, but don’t present these results. Equally, authors could look at the photophysiologically derived NPQ values from these data or photoinhibitory parameters, which could be compared with or link to the pigment discussion around xanthophyll ratios (VAZ and DD:DT).
- Changes in Fv/Fm can be community structure related. In section starting at L540, authors speculate on the reason for decline in FvFm ratios at the highest TA treatments, but conclude no stress, based on no change in ROS etc. There is little discussion about the influence of how community shift may affect these analyses. The authors conclude no effect of TA, but rather that the intermediate TA mesocosms had relief from nutrient limitation. Again, nutrient data would be helpful here.
- From lines 585 to 610: this section of the discussion reads like the results. Please amend to ensure the findings are discussed with relevant literature cited.
Overall, while this study is interesting and well-executed, the non-linear response, makes it difficult to conclude anything with much certainty – while there are some consistencies across some of the mid-range TA treatments, it is highly patchy across treatments and traits. The authors conclude, maintaining their findings within the bounds of their study, that OAE resulted in minimal physiological impairment on the phytoplankton community, however, this conclusion feels somewhat premature. For example, a shift in community composition across treatments, which seems to have happened based on the supp data, would in itself indicates a significant effect on the community, which would likely translate into changes in the productivity and carbon export potential of the community. Most analyses were done in bulk or at ‘functional group’ level (not high enough resolution to see differences in major taxonomic groups – dinos, diatoms, haptos etc) and therefore the differences in stress or photosynthetic efficiency etc could not be taxonomically resolved. I think that there needs to be discussion around these limitations and recognition that further work would need to be done to ascertain impact.
Some minor comments:
- Line 203: ‘was firstly’ is written twice, please delete one.
- Figure 1: consider shading Phase 1 and 2 on plots – background colour? Or mark with dashed vertical line?
- Line 384: what is meant by ‘other’ treatments. It is the start of a new paragraph, so perhaps remind the reader which treatments are being discussed.
- Line 554: possible spelling error “caseated”?
- The authors could consider indicating Phase 1 and 2 on fluorescence data plots
- Similarly, could these data (photophys) be compared statistically? Or possibly by TA grouping Low, Med, High?
Citation: https://doi.org/10.5194/egusphere-2024-847-RC1
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