Coralline Algae Responses to Ocean Acidification

Coralline algae are globally important calcifying red algae that secrete high-Mg calcite skeletons, underpinning reef construction, stabilising benthic habitats and inducing larval settlement. As atmospheric CO₂ rises, shifts in ocean carbonate chemistry reduce seawater pH and lower carbonate ion availability, impairing skeletal density and promoting dissolution. Although elevated pCO₂ can enhance photosynthetic carbon fixation, net calcification typically declines, rendering thalli more susceptible to breakage, erosion and bioerosion. The magnitude and rate of pH change, together with light intensity and hydrodynamic conditions, modulate these effects, with rapid acidification often causing greater structural weakness than gradual decline. Interactions with ocean warming and nutrient regimes further complicate physiological responses, sometimes exacerbating stress. Yet significant intra- and interspecific variability indicates potential for acclimation and local adaptation, evidenced by shifts in mineral composition and cellular morphology. At the ecosystem level, reductions in coralline cover and structural integrity threaten biodiversity hotspots such as rhodolith beds, alter carbonate budgets and compromise reef framework stability, with implications for coastal protection and carbon cycling under future high-CO₂ scenarios.

Research from Nature Portfolio

Recent investigations have demonstrated that crustose coralline algae can rival coral in reef carbonate production, calling for their systematic inclusion in carbonate budget assessments as climate stressors intensify. Integrative modelling combining field and laboratory data reveals that under certain conditions coralline production can match or exceed that of scleractinian corals, despite frequent under-recording in benthic surveys. Complementary studies on British rhodoliths using finite element and geochemical analyses document pronounced spatial differences in cellular strength and mineral composition, with southern populations and contemporary specimens generally exhibiting lower structural rigidity than northern and historical material. These findings underscore both the vulnerability of southern coralline habitats to future acidification and the inherent capacity for structural adaptation across populations.

Coralline Algae Responses to Ocean Acidification publication trend

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

Technical terms

Calcification: The biological deposition of calcium carbonate by organisms to form hard skeletal structures.

Crustose coralline algae (CCA): Encrusting red algae that build rigid calcium carbonate layers on reef and rocky substrates.

Rhodoliths: Free-moving, nodular aggregates of coralline algae forming biodiverse beds on the seafloor.

High-Mg calcite: A variant of calcium carbonate with elevated magnesium content, characteristic of coralline skeletons and highly soluble at low pH.

Diffusion boundary layer: The thin fluid layer adjacent to algal surfaces where pH and chemical gradients differ from ambient seawater, influencing stress under acidification.

References

1. Coralline algae (Rhodophyta) in a changing world: integrating ecological, physiological, and geochemical responses to global change. Journal of Phycology (2015).
2. Coralline algal structure is more sensitive to rate, rather than the magnitude, of ocean acidification. Global Change Biology (2013).
3. Crustose coralline algae can contribute more than corals to coral reef carbonate production. Communications Earth & Environment (2023).
4. Environmental impacts on the structural integrity of British rhodoliths. Scientific Reports (2023).
5. Ocean acidification enhances primary productivity and nocturnal carbonate dissolution in intertidal rock pools. Biogeosciences (2023).
6. Coralline Algae in a Changing Mediterranean Sea: How Can We Predict Their Future, if We Do Not Know Their Present?. Frontiers in Marine Science (2019).