Ocean acidification changes the seawater surrounding phytoplankton, and that can alter the work cells do to acquire carbon, maintain internal pH, and—in some species—build calcium-carbonate structures. The effects are not uniform: species, light, nutrients, and experimental conditions can shift whether a measured response is positive, negative, or negligible.
What ocean acidification changes in seawater
Ocean acidification is the long-term shift in seawater chemistry caused primarily by the ocean absorbing carbon dioxide (CO2). Dissolved CO2 reacts with seawater to produce hydrogen ions, lowering pH and changing the balance among dissolved inorganic carbon forms, including bicarbonate and carbonate. NOAA describes the process this way: “Ocean acidification occurs when the ocean absorbs carbon dioxide. This causes a fundamental and global change in the chemistry of the ocean.” NOAA Ocean Acidification Program
The term does not mean that the ocean as a whole has become acidic: surface seawater remains generally alkaline, with pH above 7. NOAA reports that, on average, the global ocean has become about 26% more acidic over the past 250 years; this describes a relative change in acidity, not a fall below neutral pH. NOAA’s overview of observations
For a phytoplankton cell, this is a change in its external chemical environment. It is not the same thing as the cell’s interior becoming equally acidic. The cell regulates its internal conditions, and the resulting physiological response depends on its species and surroundings.
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How cells get carbon and control internal pH
Carbon concentrating for photosynthesis
Many marine phytoplankton use carbon-concentrating mechanisms (CCMs) to supply inorganic carbon to photosynthesis. These can include bicarbonate transport and carbonic anhydrase, an enzyme that helps convert between CO2 and bicarbonate. The details and efficiency differ among groups; phytoplankton do not all use the same cellular machinery. A review describes coccolithophores’ CCMs as generally less efficient than those of diatoms and Phaeocystis, with dinoflagellates intermediate. Annual Review of Marine Science
When more CO2 is available, some cells may need to spend less energy concentrating carbon for photosynthesis. That potential saving is only one part of the response: lower external pH can also make it harder or more costly to maintain intracellular pH. A study examining ocean acidification alongside phosphate limitation found that the two conditions can co-shape phytoplankton physiology and community structure, so CO2 effects should not be considered separately from nutrient conditions. Nature Communications (2023)
Maintaining the cell’s interior
External seawater pH and intracellular pH are distinct. Cells regulate their internal chemistry rather than passively matching seawater, but that regulation takes energy and can be challenged by changes in the surrounding carbonate chemistry. The balance between any reduced cost of acquiring carbon and the cost of pH regulation helps explain why higher CO2 does not guarantee faster growth.
Two examples show why responses differ
Coccolithophores: calcification and proton management
Coccolithophores build calcite plates, called coccoliths, inside an intracellular compartment and then secrete them. Making calcite creates an acid-base challenge: the cell must manage and export protons while keeping its internal environment suitable for calcification. A 2022 study linked reduced H+ channel activity at low ocean pH with disrupted pH homeostasis and calcification in coccolithophores. This is a cellular mechanism by which external seawater conditions can affect a process occurring inside the cell; it is not evidence that every coccolithophore species responds identically. Annual Review of Marine Science (2017) PNAS (2022)
Emiliania huxleyi: cell components do not all shift alike
In a 2021 experiment, researchers exposed the coccolithophore Emiliania huxleyi to dissolved inorganic carbon (DIC) concentrations from 900 to 4,930 μmol kg−1 and pH values from 8.04 to 7.70. Under the experiment’s high-DIC, low-pH condition, pigment, particulate organic carbon, and carbohydrate content increased significantly. Growth rate, maximal relative electron transport rate, particulate organic nitrogen, and protein content were less affected. These results describe this species under those experimental conditions, not a universal forecast for ocean phytoplankton. Frontiers in Microbiology (2021)
Why the same change can produce different outcomes
Studies can report different effects because both the organism and the conditions matter. A review of nearly 20 marine-diatom studies found growth responses to elevated CO2 ranging from stimulation to no change to inhibition. Low-to-moderate light generally accompanied stimulation in acidification treatments in the reviewed studies, while excess light could coincide with inhibited growth. These patterns do not establish a single response for all diatoms, let alone all phytoplankton. Gao and Campbell, Functional Plant Biology (2014)
When comparing results, the relevant details include species or strain, carbonate-chemistry treatment, light, temperature, nutrient availability, experimental design, duration, and the endpoint measured. Growth, calcification, photosynthetic performance, and cell composition are different outcomes and need not move in the same direction. NOAA notes that algae may benefit from more CO2 for photosynthesis, but that possibility does not establish that phytoplankton broadly will grow faster: pH regulation, nutrients, light, and community interactions can change the result. NOAA Education
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What cellular changes can mean for the carbon cycle
Phytoplankton influence ocean carbon cycling through photosynthesis and, for calcifying groups, the production of calcium-carbonate structures. If acidification changes carbon fixation, cellular composition, or calcification, it can alter biological contributions to the cycling of carbon and carbonate chemistry. The direction and scale depend on which organisms respond and how communities change.
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A 2025 study reported that surface-ocean total alkalinity increased by 0.072 ± 0.023 μmol per kilogram per year. Its authors estimated that this increase would have caused human-emitted carbon in the ocean to rise by about 0.20 PgC since the 1990s, and proposed a link between reduced biotic calcification and increased surface alkalinity. They also said more total-alkalinity data are needed to quantify the feedback and its impacts. This is a broader carbon-cycle finding, not a direct measurement of intracellular phytoplankton chemistry or a settled prediction. Barrett et al., Global Biogeochemical Cycles (2025)
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