| 26 Mar 2026
Biological thresholds for marine carbon dioxide removal (mCDR): the effect of changes in carbonate chemistry
Abstract. Marine carbon dioxide removal (mCDR) encompasses a variety of approaches to actively remove CO2 from the atmosphere, which, given current and projected emissions, is necessary to keep global average temperature increases to less than 2 °C. In addition to the removal of CO2 from the atmosphere, mCDR projects would have effects on the marine environment, including changes in ocean carbonate chemistry. These changes in carbonate chemistry can affect marine organisms through multiple physiological pathways. Although research on the effects of ocean acidification over the last fifteen years has advanced understanding of the effects of low pH/high CO2 conditions on marine organisms, much less is known about organismal response to the high pH/low CO2 or high alkalinity conditions that can be generated by some mCDR methods. We created a database of available information on marine species response to the carbonate chemistry conditions that can be generated by mCDR with a focus on identifying carbonate chemistry thresholds at which biological responses occur. The database contains 310 studies, from which we estimated 276 thresholds. In addition to all mCDR studies available to date, we reviewed studies not explicitly designed to address mCDR. These studies used natural and artificially manipulated variations in carbonate chemistry, including increasing pH and alkalinity, to explore basic physiological and biological responses as well as approaches to mitigate ocean acidification. In analyzing the database, we considered a variety of biological responses, both positive and negative, and considered sensitivity by taxa, exposure duration, treatment method and other factors. Using practical definitions of a threshold, including one based on the treatment level leading to the first statistically significant biological response, we describe the distribution of pH and alkalinity threshold values. The data reveal a high tolerance for high pH (>12) and high alkalinity for some species responses under some experimental conditions, but negative response to experimental conditions only slightly above ambient (pH 8.3) for other species responses. A positive effect of pH above ambient levels was observed in six cases, just 3 % of the total. We summarize results by presenting the distributions of threshold estimates collected from individual studies. The uncertainty and variation in organismal response represents a challenge for the development of management guidelines for the developing mCDR industry. Building on our review of existing studies, we suggest several paths forward for improved biological threshold estimates for mCDR-relevant carbonate chemistry conditions.
The manuscript by McElhany et al. examines how mCDR involving changes in carbonate chemistry could impact marine organisms and applies a qualitative systematic review approach to answer this question. They identify thresholds, defined as the level at which the first statistically significant result is found to determine when impacts occur. I consider this is a good approach. However, the manuscript itself has some major issues, including missing papers (and therefore responses). If the manuscript is revised to include all relevant work using appropriate methods, then I would be happy to provide more detailed comments on a revised version of the manuscript.
Major comments:
The authors should follow the procedure of PRISMA: https://www.prisma-statement.org/. Here, the search terms are not exactly noted, search dates etc. in the main methods. This has led to other issues. I can see that they are missing a number of studies that I can think of off the top of my head. For example, what about the work of Steeve Comeau? There are a handful of his papers that explore high pH and/or high TA (through high DIC). None of these are here. The manuscript must be reproducible and must be systematic if it is to be published.
I think it is confusing to use the term “studies” and “articles or reports” as being mutually exclusive. Instead of “studies” I would describe them as “responses”, a term typically used in the field of meta-analyses.
The figure quality needs to be improved for this work to be published. The text is currently grainy (and font sizes inconsistent), and the internal gridlines could be removed, colours improved etc.
Minor comments:
The table legends and tables should be on the same page (except table 5, where are the others?). E.g. the one on Diatoms. This could be better suited in the supplementary materials.
The results section is far too long and very list like at time.
The supplementary data set should provide studies in alphabetical order.
Could line numbers please be added in order to facilitate review?
The introduction presently has too much results. This is an odd approach. Introduce the topic, its importance and relevance and then (maybe) discuss the results at the very end of the introduction.
Page 4: a lot more references required. Here and elsewhere. If it is not a novel finding, please use a reference to introduce the point.
Box 2 the figure should be elementary for many readers. The authors should focus on the table, being more specific about what methods have been proposed to induce OAE and what chemical effects this would have for example. Are all the methods equal? These are the things important to the readers.