the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Biological impact of ocean alkalinity enhancement of magnesium hydroxide on marine microalgae using bioassays simulating ship-based dispersion
Stephanie Delacroix
Tor Jensen Nystuen
Erik Höglund
Andrew L. King
Abstract. Increasing the marine CO2 absorption capacity by adding alkaline minerals into the world’s oceans is a promising marine carbon dioxide removal (mCDR) approach to increase the ocean’s CO2 storage potential and mitigate ocean acidification. Still, the biological impacts of dispersion of alkaline minerals needs to be evaluated prior to its field deployment. In this study, the toxicity effect on marine microalgae of two commonly used alkaline minerals, calcium hydroxide (Ca(OH)2) and sodium hydroxide (NaOH), was compared with magnesium hydroxide (Mg(OH)2), by applying the same concentration of hydroxyl radicals (OH-) for each component. Cultures of marine green microalgae Tetraselmis suecica were exposed to NaOH, Ca(OH)2 or Mg(OH)2 in concentrations mimicking dispersion scenarios from a ship which included short-term exposure with high alkaline mineral concentration called “dispersion phase” followed by a dilution and “regrowth” phase over six days. There was no detectable effect of Mg(OH)2 treatment on algae growth either after the dispersion phase or during the regrowth phase, compared to control treatments. The Ca(OH)2 treatment resulted in very few living algal cells after the dispersion phase, but a similar growth rate was observed during the regrowth phase as was for the Mg(OH)2 and control treatments. The NaOH treatment resulted in no surviving algae after the dispersion phase and during the regrowth phase. Standardized whole effluent toxicity (WET) tests were carried out with a range of Mg(OH)2 concentrations using a sensitive marine diatom, Skeletonema costatum, which confirmed the relative low toxicity effect of Mg(OH)2. Similar biological effects were observed on natural microalgae assemblages from a local seawater source when applying the same Mg(OH)2 concentration range and exposure time used in the WET tests. The results suggest that Mg(OH)2 is relatively safe compared to Ca(OH)2 and NaOH with respect to marine microalgae.
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Stephanie Delacroix et al.
Status: open (until 28 Sep 2023)
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CC1: 'Comment on bg-2023-138', Chris Vivian, 22 Aug 2023
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Stephanie,
The dilutions you simulated in section 2.1 lines 104-106 are a significant underestimate of what can be achieved in the wake of a vessel. Consequently, your assessment of the safety of discharging alkaline materials from vessels is overly conservative.
I worked at the Cefas Burnham Laboratory for many years and was responsible for advising the Ministry of Agriculture, Fisheries and Food on the licensing of liquid industrial wastes dumped at sea from 1986-1992, including the setting of discharge rates based on toxicity tests. The UK dumped industrial wastes at sea up until 1992 including high strength alkaline wastes. In order to avoid undesirable effects on the marine environment, the wastes were discharged through twin pipes at the stern of converted tankers and calculated dilutions of up to 10,000 times within 5 minutes could be achieved. The dilutions and discharge rates were determined based on toxicity tests.
There are models for the dilution of wastes in the wake of vessels. Paper MEPC III/7 (copy attached) from the 3rd session of IMO MEPC that provided a method for calculation of dilution capacity in a ship’s wake. The methodology was derived from field experiments carried out by the United States, Netherlands and Norway, as well as an experiment by the Netherlands using a model in the Netherlands Ship Model Basin. This methodology was used by European countries dumping liquid industrial wastes under the Oslo Convention from the 1970’s through to the early 1990’s. There are also models for the discharges from exhaust gas cleaning systems on ships (using the MAMPEC-BW model originally developed for ballast water discharges - https://www.deltares.nl/en/software/mampec/). Also, see the attached paper by Caserini et al. (2021).
In the UK’s case, the MEPC paper methodology was used to ensure that dilution of the waste was such that the 96-hour LC50 concentration derived from toxicity testing of the wastes was achieved within 5 minutes of discharge. In practice, a number of field experiments carried out by the UK found that the methodology in MEPC III/7 underestimated the dilution achieved in practice in these and other experiments (see attached papers). Speed of vessels, waterline length and discharge rate were the most important factors.
Chris Vivian.
Chris.vivian2@btinternet.com
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CC2: 'Reply on CC1', Michael Tyka, 30 Aug 2023
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I'll add that it would be quite interesting for the reader to directly see the results of the simulations carried out in section 2.1 (in particular the dilution-vs-time curves) directly compared with the empirical formulas used by IMCO, Chou et al and others. See for example - section 3.3 in https://bg.copernicus.org/articles/20/27/2023/ and Box 2 in https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000533 and also the above-mentioned Caserini et al. (2021). Can we gain any insight from your simulations about the accuracy of these different formulas (which already deviate from each other quite significantly) ?
Citation: https://doi.org/10.5194/bg-2023-138-CC2
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CC2: 'Reply on CC1', Michael Tyka, 30 Aug 2023
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RC1: 'Comment on bg-2023-138', Anonymous Referee #1, 07 Sep 2023
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Delacroix et al. test the effect of three potential OAE source materials (Mg(OH)2, Ca(OH)2, NaOH) on phytoplankton. They expose Tetraselmis to extremely alkaline conditions (~3.4 mol/kg added alkalinity, pH 13-14) for one hour and test their survival and regrowth. The extremity of the treatment (and other critical issues in the experimental design) impede drawing meaningful conclusions for OAE. The ecotox tests presented alongside the main experiments are potentially informative for OAE as these may reflect protocols used in permitting procedures in environmental agencies. However, those limited tests with just one species seem too slim to justify a stand-alone publication. Due to these shortcomings (further detailed below) I can unfortunately not recommend publication.
Major comments:
- The authors estimated an application rate of 500 kg of alkaline material per second in a ship wake. It is not entirely clear how this number was derived but this is perhaps not the most critical problem. More critically is that this leads to an experimental design where e.g., 141 g of NaOH pellets are added to 1L of Tetraselmis culture for one hour. This seems deadly by design. I am not sure how reliable carbonate chemistry software is under such conditions (i.e., alkalinity = 3.4 mol/kg, DIC = 0.0021 mol/kg) but this translates into a pH of >>13, possibly 14. As such, it is not surprising that even a very robust species like Tetraselmis dies, as the water was essentially sterilized. The fact that Tetraselmis survives the Mg(OH)2 treatment (despite the extremely large amounts added) is most likely due to slower dissolution so that the extreme pH effect cannot fully unfold within an hour of experiment. It could be argued that this is just what happened under such a OAE application and the data is robust (I totally trust the data that all cells died under pH 13-14). However, would anyone consider such application? It would be like doing an ocean acidification experiment under pH 4. Yes, OA reduces pH but pH 4 is still not ocean acidification. Indeed, previous OAE studies have already carefully assessed dosing rates in order to not increase coastal pH beyond a ∆pH of 0.1 on average and pH 11 on the order of seconds assuming a ship wake scenario (He and Tyka, 2023). As such, the way the main experiments are designed will lead to misleading conclusions as what they tested has very little to do with OAE.
- The second major critique is the dilution scenario that follows the 1 hour exposure to extremely alkaline conditions. In the oceans, a small volume directly affected by alkaline substances (where some cells may indeed die) would mix with the same water mass and pH would decline to much lower values (e.g. pH 9) within seconds to minutes (see Fig. 7 in He and Tyka, 2023). The not immediately perturbed water body, with which the highly perturbed water mass is quickly mixed, still contains the same unperturbed phytoplankton community. However, in the experiments presented here, deepwater is used because it is essentially free of phytoplankton. As such, the experiments exclude the natural seed population that has not been affected by extremely high pH for an hour. The conclusions drawn from this experiment can therefore not be used to assess an OAE perturbation in a ship wake.
Other comments:
Title: There seems to be a logic issue. Is the impact from OAE or MgOH2? Also, I would refrain from the word “impact” as it implies bias. Influence?
Line 12: Toxicity effect implies by default detrimental outcomes and thus suggests bias. Influence of XXX on YYY?
Line 13: Title mentions MgOH2 but the study seems to address other alkalinity sources.
Line 14: Hydroxyl radical: Isn’t that hydroxide ions? The former seems to be used in atmospheric sciences.
First part of introduction: The lengthy introduction on OA seems off-topic and is also not tailored towards the subject of the study, i.e., phytoplankton. The three main negative consequences are also not backed by references and the text seems biased towards considering only negative effects of OA. Pretty much all text until line 51 could be condensed to one sentence or fully deleted, since OA is of little relevance for the research here.
Line 53: I would refer to IPCC 2022, I think the relevant working group is either 2 or 3.
Line 55: There are many more, refer to GESAMP 2019.
Line 58: Not all aim to accelerate: Some aim to establish a new C-sink in the earth system that currently does not exist, e.g. seaweed farming.
Line 60: OAE, when done properly and the generated CO2 deficit is matched with atmospheric CO2, has very little influence on ocean pH. This narrative of OA remediation does therefore make little sense.
Line 66/67: Solubility or dissolution? Have not worked with MgOH2 myself but all data I’ve seen suggests rather fast dissolution.
Line 69: Durability is unclear. In principle, all alkalinity should have the same durability.
Line 70ff: As mentioned above, I would refrain from “toxicity” in the context as there are likely pros and cons or different organisms. Toxicity implies everything will suffer or die, which is not what current research suggests.
Line 92: is there a reference that confirms Skeletonema to be more sensitive?
Line 105: Explanation of dilution is unclear. Does that mean a dilutin of 1/1000 after two minutes, 1/7000 after 5 hours, 1/154000 after 10 hours? Please clarify, ideally with an illustrative example.
Line 109: 100g/L appears unrealistically high, which appears to be a critical problem in the study design (see major comment).
Line 174: How was pH calibrated? It seems that best practices for seawater carbonate chemistry measurements (Dickson et al., 2007) were not applied. This may have been necessary but a justification would be required here.
Line 209: Were carbonate chemistry changes through CO2 exchange with the atmosphere considered. These may have altered conditions, especially when cells are grown in beakers.
Line 242: The statistical approach seems to fully counter the experimental design. Why was a gradient established if a range of concentrations were then put together?
Line 292: The WET test results are somewhat unclear.
Line 321: It is quite obvious that pH was similar at the onset of the regrowth phase because the initial water could diluted. The gradient between treatments is still totally as one would expect it, i.e., highest in NaOH and CaOH2.
Line 322: The regrowth conditions were extremely different because in some treatment (those that were inadvertently sterilized with NaOH) all cells were dead, as one would expect from a treatment under pH 13-14. (See major comment 2).
Line 362: This statement lacks any evidence, including in the cited references.
Citation: https://doi.org/10.5194/bg-2023-138-RC1
Stephanie Delacroix et al.
Stephanie Delacroix et al.
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