Responses of Marine Phytoplankton to Ocean Acidification

Marine phytoplankton, the microscopic photosynthetic organisms at the base of the ocean food web, are experiencing rapid changes in seawater chemistry as increasing atmospheric CO₂ dissolves in surface waters. Ocean acidification alters carbonate speciation, reducing pH and the availability of carbonate ions, with cascading effects on physiological processes such as carbon acquisition, nutrient uptake, photophysiology and biomineralisation. Different taxonomic groups respond variably: calcifying coccolithophores often exhibit reduced calcification and altered growth, whereas many diatoms show species-specific shifts in photosynthetic efficiency, silica deposition and lipid composition. Non-calcifying flagellates and cyanobacteria may benefit from enhanced CO₂ availability through stimulated carbon fixation and down-regulation of energy-intensive carbon concentrating mechanisms. At the community level, acidified conditions can drive shifts in species composition, with potential impacts on food-web dynamics, biogeochemical cycles and the efficiency of the biological carbon pump. Moreover, interactions with co-occurring stressors such as warming, ultraviolet radiation, deoxygenation and nutrient limitation can modulate, amplify or buffer individual responses, underscoring the need for multifactorial studies to predict ecological outcomes in a changing ocean.

Research from Nature Portfolio

Recent studies have revealed that elevated CO₂ concentrations projected for the end of this century trigger a substantial increase in the synthesis of phenolic compounds in phytoplankton assemblages, accompanied by a pronounced rise in mitochondrial respiration rates. This metabolic shift leads to the transfer of higher toxin loads to grazers, resulting in elevated phenolic content in zooplankton and suggesting potential repercussions for marine food-chain integrity and seafood quality. The findings highlight the capacity of acidified conditions to reconfigure secondary metabolism across trophic levels, raising concerns over ecosystem services and fishery resources.

Responses of Marine Phytoplankton to Ocean Acidification publication trend

The graph below shows the total number of articles in responses of marine phytoplankton to ocean acidification across all publications each year (not limited to Nature Index journals).

Technical terms

Ocean acidification: The long-term decrease in seawater pH and alteration of carbonate chemistry due to uptake of atmospheric CO₂.

pCO₂: The partial pressure of dissolved carbon dioxide in seawater, reflecting CO₂ availability for photosynthesis.

Carbon concentrating mechanism (CCM): Biochemical or biophysical processes used by phytoplankton to elevate CO₂ levels around the enzyme RUBISCO to enhance carbon fixation efficiency.

Photosystem II (PSII): A multi-protein complex in the thylakoid membrane that uses light energy to split water and initiate electron transport in photosynthesis.

Non-photochemical quenching (NPQ): A photoprotective strategy by which excess excitation energy in PSII is dissipated as heat to avoid photodamage.

Phenolic compounds: Secondary metabolites characterised by one or more hydroxyl groups attached to aromatic rings, often involved in stress responses and with potential toxic effects.

References

1. Effects of pH/pCO2 fluctuations on photosynthesis and fatty acid composition of two marine diatoms, with reference to consequences of coastal acidification. Biogeosciences (2025).
2. Ocean acidification increases the accumulation of toxic phenolic compounds across trophic levels. Nature Communications (2015).
3. Effects of Ocean Acidification on Marine Photosynthetic Organisms Under the Concurrent Influences of Warming, UV Radiation, and Deoxygenation. Frontiers in Marine Science (2019).