Changing Ocean Chemistry to Mitigate Climate Change: How Do Phytoplankton Respond?
Primary source:
Guo J. A., Strzepek R. F., Yuan Z., Swadling K. M., Townsend A. T., Achterberg E. P., Browning T. J. and Bach L. T. (2025) Effects of ocean alkalinity enhancement on plankton in the Equatorial Pacific. Commun Earth Environ 6.
Featured Image Source: NOAA MESA Project, Public domain, via Wikimedia Commons
Secondary sources:
https://oceanvisions.org/ocean-alkalinity-enhancement
IPCC (2023) Climate Change 2023: Synthesis Report. eds. P. Arias, M. Bustamante, I. Elgizouli, G. Flato, M. Howden, C. Méndez-Vallejo, J. J. Pereira, R. Pichs-Madruga, S. K. Rose, Y. Saheb, R. Sánchez Rodríguez, D. Ürge-Vorsatz, C. Xiao, N. Yassaa, J. Romero, J. Kim, E. F. Haites, Y. Jung, R. Stavins, A. Birt, M. Ha, D. J. A. Orendain, L. Ignon, S. Park, Y. Park, A. Reisinger, D. Cammaramo, A. Fischlin, J. S. Fuglestvedt, G. Hansen, C. Ludden, V. Masson-Delmotte, J. B. R. Matthews, K. Mintenbeck, A. Pirani, E. Poloczanska, N. Leprince-Ringuet, and C. Péan,
The Anthropocene is the unofficial geological epoch, beginning in the mid-20th century and continuing to the present day, in which human activity is the dominant force shaping Earth’s climate and ecosystems. The emission of immense amounts of CO2 and other greenhouse gases, year over year, has led to global warming and environmental changes at rates unheard of in the geologic record. The most important and impactful solution to this problem is to reduce CO2 emissions. However, even with reduced emissions, carbon dioxide removal (CDR) techniques will be necessary to limit the impact of “legacy” CO2, or the cumulative load of CO2 we have emitted since the start of the Industrial Revolution. Otherwise, this legacy CO2 will continue to drive global warming for many years to come. One such CDR technique is to artificially increase ocean alkalinity, referred to as Ocean Alkalinity Enhancement (OAE).
Alkalinity is broadly defined as the ability of a solution to neutralize acid. When CO2 from the atmosphere dissolves into the ocean, it is transformed into carbonic acid resulting in ocean acidification. So, increasing the alkalinity of the ocean effectively increases the ocean’s capacity to take up CO2 from the atmosphere and sequester it for a long time (100-10,000 years). Proposed OAE techniques vary widely, from spreading alkaline minerals along beaches to electrochemically altering sea water chemistry. No matter the technique, the ocean’s chemistry will be changed significantly, which is both a solution to our CO2 issue and a potential problem in and of itself.
Phytoplankton are very important organisms potentially impacted by OAE. These microscopic, plant-like organisms living in the surface ocean are the base of almost the entire marine food web and are extremely sensitive to changes in ocean chemistry. Scientists worldwide have been studying how OAE affects different phytoplankton communities, focusing on both the materials used to enhance alkalinity and the amounts added. Recently, one group tested the impact of three alkalinity sources on phytoplankton communities across the Equatorial Pacific Ocean. They traveled from Ecuador to Australia in the spring of 2023, collecting sea water along the way. During this research cruise, they added alkalinity to bottles of sea water in the form of liquid sodium hydroxide (NaOH), solid steel slag (a byproduct of the steel-making process rich in alkaline compounds like CaO, aka lime), and solid olivine (an abundant, fast-dissolving silicate mineral). The altered sea water was then incubated for 48 hours, after which the phytoplankton community was assessed to determine if and how it was affected by the alkalinity addition.
To determine the extent of the impact, if any, these scientists measured the concentration of chlorophyll (a proxy for phytoplankton growth) with and without OAE. They also measured the variable fluorescence/maximum fluorescence, which is a method for determining the phytoplankton’s photosynthetic efficiency and therefore is interpreted as a measure of fitness, and the composition of the phytoplankton community before and after incubation. Flow cytometry was used to establish the phytoplankton community, which rapidly analyzes the size, shape, and auto-fluorescence signal of cells in the water samples. These scientists classified the phytoplankton into 3 eukaryote groups based on size: picoeukaryotes (0.2-2 µm in diameter), nanoeukaryotes (2-20 µm in diameter), and microeukaryotes (20-200 µm in diameter). Eukaryotes are organisms that have cells with a nucleus and specialized organelles, and, for reference, the largest eukaryote group included in this study, microeukaryotes, are roughly as thick as a human hair. They also measured 3 bacteria groups: Prochlorococcus, Synechococcus, and heterotrophic (non-photosynthesizing) bacteria.
The three alkalinity sources differed markedly in their impacts on phytoplankton. NaOH did not significantly impact any metric used to assess the fitness or composition of the phytoplankton community. The steel slag additions had variable impacts on the different classes of organisms. Chlorophyll increased following slag addition, indicating that overall phytoplankton growth accelerated relative to the control incubations. However, slag did cause a reduction in Prochlorococcus. The increase in phytoplankton growth overall was likely an expression of “biotic” CDR. Here, the release of very small amounts of metals (iron and manganese) from the slag likely acted as a fertilizer for the larger phytoplankton. The growth of these phytoplankton led to CDR removal through increased photosynthesis as the phytoplankton take CO2 and convert it into organic matter. Olivine certainly had the most negative impact on the phytoplankton community, strongly reducing the number of picoeukaryotes, Prochlorococcus, Synechococcus, and heterotrophic bacteria in the incubations. The scientists leading this study highlight two potential causes of the ecological harm of olivine: 1) olivine dissolution releases small amounts of metals (nickel, cobalt, copper) that are harmful to organisms, and 2) the olivine particles physically disturb the plankton.
Ocean alkalinity enhancement is a promising CDR technique and, based on the results of this study, the benefits of NaOH addition may exceed the risks. Steel slag did have some co-benefits besides alkalinity addition in terms of phytoplankton growth, but these benefits could potentially result in the reallocation of nutrients in the surface ocean and even deoxygenation- both of which could be very bad for the ocean. More research will be necessary before large-scale NaOH or slag addition can proceed. Olivine addition was both inefficient at adding alkalinity and had pronounced negative impacts on the phytoplankton communities, making it the least viable option of the three alkalinity sources according to this study.
The climate problem we are all facing necessitates action. Most importantly, we need to reduce the amount of greenhouse gases we are emitting into the atmosphere. Subsequently, CDR must be implemented in such ways that the environments we all rely on are not adversely affected. To ensure our solutions do not cause more problems, a lot of research must be done. Luckily for all of us, many great minds around the world are working on a plethora of CDR techniques to determine their efficacy, scalability, and safety. Ocean alkalinity enhancement is just one of those techniques, but it is one that has been exhibiting very promising results.
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