AP Syllabus focus:
‘Acidification damages corals because lower pH reduces calcium carbonate availability, making it harder for corals and other organisms to form shells and skeletons.’
Ocean acidification threatens organisms that build hard parts from calcium carbonate. This page focuses on how lower seawater pH reduces carbonate building material, weakening corals and shell-formers and reshaping reef and coastal ecosystems.
Core idea: less carbonate, harder building
Many marine organisms construct skeletons or shells by combining dissolved ions into solid calcium carbonate (CaCO₃). As seawater becomes more acidic (lower pH), the chemical balance shifts so there is less carbonate ion () available for building, even if calcium is still abundant.
Calcification and why it slows
Calcification: The biological process of forming calcium carbonate structures (shells/skeletons) from seawater ions.
Calcification becomes more energy-intensive when carbonate ions are scarce, so organisms may calcify more slowly, produce thinner structures, or divert energy from growth and reproduction to maintenance.
= Calcium ion in seawater (mol L)
= Carbonate ion availability for building (mol L)
= Solid calcium carbonate shell/skeleton
Lower availability can also increase the tendency for existing CaCO₃ structures to dissolve or erode, especially in more corrosive microenvironments (for example, within sediments or at night when respiration raises local CO₂).
This time-series photograph shows a pteropod (a calcifying plankton) shell progressively dissolving when exposed to lower-pH seawater. The sequence illustrates that ocean acidification is not only a problem for building new CaCO₃, but can also accelerate chemical erosion of existing shell material—especially for thin, high-surface-area shells. Source
Effects on corals (reef builders)
Corals rely on CaCO₃ skeletons to create reef frameworks. With reduced carbonate availability, reefs can shift from net growth to net loss.
Individual coral impacts
Reduced skeletal growth: slower extension and/or lower density skeletons.
Weaker structural integrity: more breakage during storms and wave action.
Higher energetic costs: more energy spent maintaining internal chemistry for calcification.
Lower recruitment success: larvae may settle less successfully or build early skeletons more slowly, reducing juvenile survival.
Reef-scale ecosystem impacts
Habitat loss: complex reef structure supports high biodiversity; flattening reduces shelter and feeding niches.
Food-web changes: declines in reef complexity can reduce fish and invertebrate abundance tied to reef habitats.
Coastal protection declines: weaker, eroding reefs dissipate less wave energy, increasing shoreline vulnerability.
Effects on calcium carbonate shells (molluscs and plankton)
Many organisms beyond corals depend on CaCO₃, including molluscs (oysters, clams, mussels), some plankton (for example, pteropods), and various calcifying algae.
Common biological responses
Thinner shells or slower shell deposition, increasing predation risk.
Greater dissolution at shell surfaces, especially in juveniles with high surface-area-to-volume ratios.
Reduced growth and reproduction if more energy is redirected to shell maintenance.
Population and community shifts if sensitive species decline and non-calcifiers gain a competitive advantage.
Why this matters for people
Damage to corals and shell-formers can reduce:
Fisheries productivity (habitat and nursery grounds; shellfish yields).
Tourism and recreation tied to reefs.
Natural coastal defence provided by robust reef structures.
FAQ
No. Sensitivity varies by species, life stage, and the form of CaCO₃ used.
Some organisms precipitate different crystal forms (e.g., aragonite vs calcite), which can differ in susceptibility to dissolution under lower pH conditions.
Early life stages build shells rapidly and have thin, high-surface-area structures.
They may also have less physiological capacity to regulate internal chemistry, so corrosive conditions can cause proportionally greater dissolution and slower development.
Acidification can reduce skeletal density, making colonies more brittle.
Storms and strong wave action then break corals more easily, and rubble can be harder to re-cement into stable reef framework when calcification is slowed.
Potentially, but it depends on generation time and genetic diversity.
Responses may include selection for tolerant genotypes, shifts in energy allocation, or changes in associated microbes; however, adaptation may not keep pace with rapid chemical change.
Local stress reduction can improve resilience:
Limit nutrient pollution to reduce algal overgrowth and oxygen/pH swings.
Reduce sedimentation that smothers corals and juveniles.
Protect herbivores that help maintain reef balance and settlement surfaces.
Practice Questions
Explain how ocean acidification makes it harder for corals to build skeletons. (2 marks)
Identifies that lower pH leads to reduced availability of carbonate ions / calcium carbonate building material (1).
Links reduced carbonate availability to slower/less calcification or weaker skeletal growth (1).
Describe four distinct ecological or biological effects of reduced calcium carbonate availability on (i) corals and (ii) shell-forming organisms. (6 marks)
(Any four, 1 mark each; max 6 with development):
Corals: reduced skeletal growth rate or density (1).
Corals: weaker reefs/greater breakage and erosion (1).
Corals: reduced habitat complexity leading to biodiversity loss (1).
Corals: reduced recruitment/juvenile survival due to slower early calcification (1).
Shell-formers: thinner shells or slower shell deposition (1).
Shell-formers: increased shell dissolution, especially in juveniles (1).
Developed link to consequences (e.g., higher predation, reduced fisheries/coastal protection) earns up to 2 additional marks (max 6 total).
