17 Aug 2023
 | 17 Aug 2023
Status: this preprint is currently under review for the journal BG.

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, and 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.

Stephanie Delacroix et al.

Status: open (until 28 Sep 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on bg-2023-138', Chris Vivian, 22 Aug 2023 reply
    • CC2: 'Reply on CC1', Michael Tyka, 30 Aug 2023 reply
  • RC1: 'Comment on bg-2023-138', Anonymous Referee #1, 07 Sep 2023 reply

Stephanie Delacroix et al.

Stephanie Delacroix et al.


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Short summary
The addition of alkaline minerals into the oceans might reduce the excessive anthropogenic CO2 emissions. Magnesium hydroxide can be added in large amount because of its low seawater solubility without reaching harmful pH levels. The toxicity effect results of magnesium hydroxide, by simulating expected concentrations from a ships’ dispersion scenario, demonstrated low impacts on both sensitive and local assemblages of marine microalgae when compared to calcium hydroxide and sodium hydroxide.