Assessing the effects of ocean alkalinity enhancement on marine protozoa: physiological dynamics and transcriptomic responses

[Geoengineering News]

https://journals.asm.org/doi/full/10.1128/aem.00298-26

Authors: Zuyuan Gao, Mengwen Pang, Mingjie Li, Yuzhen Ming, Hongbin Liu, Kedong Yin 

https://doi.org/10.1128/aem.00298-26

30 June 2026

Abstract

Ocean alkalinity enhancement (OAE) is proposed as a potential tool to remove atmospheric CO2 and mitigate climate change. However, the effects of OAE on marine protozoa remain poorly understood. In this study, we conducted acute and acclimated experiments on two heterotrophic nanoflagellates, i.e., Cafeteria burkhardae and Paraphysomonas longispina, to investigate their responses to two substances (NaHCO3 and NaOH) at low (set ~2,600 µmol L−1) and high (set ~4,000 µmol L−1) levels, respectively. Our results showed that the two species had similar negative reactions under acute exposure. However, the two species showed different tolerances after acclimation. The growth rates of C. burkhardae decreased under all OAE treatments, while its reactive oxygen species accumulated only at the high OAE level. The low-level OAE treatments had no significant effects, but high-level OAE was more adverse for P. longispina, which grew more slowly and grazed more, indicating lower growth efficiency. Underlying transcriptomic mechanisms were only analyzed for low-level OAE, which were consistent with the physiological responses. Replication and repair and metabolism-related pathways were significantly inhibited, with translation-related pathways stimulated in C. burkhardae. For P. longispina, in addition to translation-related pathways, replication and repair, and metabolism pathways were significantly upregulated. Overall, our findings suggest the potential negative effects of OAE on marine protozoa, and the effects can vary depending on the species, level, and substances. Given the critical role of protists in marine ecosystems, these adverse effects raise important concerns about the broader implications of OAE for marine biodiversity and ecosystem stability.

Source: Applied & Environmental Microbiology 

[Greg Rau]

While the authors did an amazing amount of careful measurement and analysis, this paper is yet another example of bio effects being observed with alkalinity addition, but with a duration of high alkalinity exposure that is completely unrealistic with respect to how alkalinity rapidly dilutes after discharge in a real OAE application (e.g. https://www.nature.com/articles/s43247-024-01506-4 ). 

 

Depending on the size of discharge to the ocean, the instantaneous increase of +600 uM to +2,000 uM alkalinity into a parcel of seawater (1-2L used in the study) would, due to dilution, be undiscernible from background alkalinity hours if not minutes after discharge to the ocean. Also, a coastal OAE discharge of +2,000 uM alkalinity would never be allowed and should never be done because it violates the ocean discharge pH<9 restriction common in most coastal jurisdictions (in some, pH must be <8.5). Plus, a 100% increase in seawater alkalinity as in the +2000 uM treatments greatly increases the chance spontaneous CaCO3 precipitation [remarkably, no loss of alkalinity, indicating CaCO3 precip, was reported (their Fig. 2)!?].  Anyway, instead of incubation under rapidly declining (diluting) alkalinity as is typical in real OAE, the critters were subjected to 5 days of exposure to constant, high alkalinity concentration and high pH. 

 

The authors then have the nerve to make the seeping conclusion that  "... the adverse effects of OAE raise serious concerns regarding marine biodiversity and ecosystem stability, and our findings emphasize the need for careful consideration of in-situ OAE implementation." In other words, to all policymakers/investors/stakeholders and the public: "OAE looks unsafe".  Meanwhile, Mother Nature adds 60 Tmols/yr of alkalinity to the ocean via (impure) rock weathering and river discharge, safely(?) performing about 1Gt CDR/year. Where's the concern/outrage here, and how "unsafe" would it be to increase this otherwise natural CDR? How "unsafe" for the planet and the ocean would it be not to have an OAE CDR option (in time) simply because of the unjustified extrapolation of negative experimental results like the preceding?

  

Given the limited amount of time and resources we have to determine true OAE safety, effectiveness and capacity, I ask that marine biologists incorporate realistic ocean physics, dilution and exposure times (and starting concentrations) into their experiments/interpretations rather than continuing to extend the results of unrealistically long, static, high conc, beaker/mesocosm incubations to represent real-world OAE effects. This way we can make decisions/policies/investments on OAE/mCDR that are informed by relevant testing and observation rather than by false narratives, undue fear and speculation.

 

I am not alone in these concerns: https://cdrxiv.org/preprint/457

 

Regards,

Greg Rau 

 

 

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[Greg Rau]

For a more nuanced and appropriate testing of OAE effects on biota, check out this example from Adam Subhas' group at WHOI: https://academic.oup.com/icesjms/article/83/4/fsag057/8662360

Greg

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