The big idea: The ocean absorbs a large share of the extra carbon dioxide (CO₂) that humans put into the air.
When CO₂ dissolves in seawater it makes the water slightly more acidic — its pH falls. This is called ocean acidification.
More acidic water makes it harder for animals to build shells and skeletons out of calcium carbonate — and that is what threatens coral reefs and other marine life.
- Ocean acidification
- The gradual fall in the pH of seawater caused by the ocean absorbing extra carbon dioxide from the atmosphere.
- pH
- A scale of how acidic or basic a liquid is. A LOWER pH means MORE acidic (more H⁺ ions).
- Carbonic acid
- The weak acid formed when carbon dioxide dissolves in water; it releases hydrogen ions (H⁺) that lower the pH.
- Carbonate ions (CO₃²⁻)
- Ions dissolved in seawater that organisms use to build calcium carbonate shells and skeletons.
- Calcification
- The process by which corals, molluscs and some plankton build hard parts out of calcium carbonate (CaCO₃).
'Acidification' does not mean acid: Seawater is still slightly basic — acidification means its pH is moving down toward acidic, not that the ocean becomes an acid.
A lower pH = more H⁺ ions = harder to make calcium carbonate. Keep that link in your head: more CO₂ → lower pH → less calcification.
Read this as a chain of cause and effect. Each step causes the next — that is exactly how a 7-mark 'Explain' answer scores marks, so learn the chain in order.
We describe the chemistry in words (there is no equation to memorise for Biology).
The cause-and-effect chain
- Humans burn fossil fuels (and clear forests), so atmospheric CO₂ rises.
- The ocean absorbs a large share of this extra CO₂ at its surface.
- Dissolved CO₂ reacts with water to form carbonic acid, which releases H⁺ ions.
- More H⁺ ions means the seawater becomes more acidic — its pH falls (acidification).
- Those extra H⁺ ions react with carbonate ions (CO₃²⁻), so fewer carbonate ions are left.
- Calcifying organisms (corals, molluscs, some plankton) cannot get enough carbonate, so they build calcium carbonate skeletons and shells more slowly.
- Existing skeletons and shells can even dissolve in the more acidic water, so structures weaken.
| Step | What happens | Result |
|---|---|---|
| 1. CO₂ rises | More carbon dioxide is in the atmosphere (mostly from burning fossil fuels) | More CO₂ is available to dissolve into the sea |
| 2. CO₂ dissolves | The ocean absorbs a large share of the extra CO₂ at its surface | Dissolved CO₂ reacts with seawater |
| 3. Carbonic acid forms | Dissolved CO₂ + water → carbonic acid, which releases hydrogen ions (H⁺) | More H⁺ ions in the water |
| 4. pH falls | More H⁺ ions means the water becomes more acidic (lower pH) | This is ocean acidification |
| 5. Fewer carbonate ions | The extra H⁺ ties up carbonate ions (CO₃²⁻) that organisms need | Less carbonate is available to build shells and skeletons |
Why calcifying organisms are hit hardest: Corals, mussels, snails and some plankton all build hard parts from calcium carbonate (CaCO₃) — they need a steady supply of carbonate ions.
Acidification uses up carbonate ions and makes the water corrosive to calcium carbonate. So these organisms:
Grow their shells/skeletons more slowly (calcification is harder), and
Lose existing material as it slowly dissolves.
Animals without carbonate skeletons are far less affected — that is why the exam keeps the focus on calcifying organisms.
| Organism | What it builds from calcium carbonate | Why acidification harms it |
|---|---|---|
| Corals | A hard reef skeleton (the framework of a coral reef) | Less carbonate → skeletons grow more slowly and can dissolve, so reefs weaken |
| Molluscs (e.g. mussels, snails) | Their shells | Shells form more slowly and can become thinner or pit/dissolve |
| Some plankton (e.g. foraminifera, pteropods) | Tiny carbonate shells/plates | Shells weaken; these are a key base of marine food chains |
| Sea urchins and some crustaceans | Calcium carbonate parts of their bodies | Slower, weaker growth of those structures |
A memory hook: C–A–C–C: more CO₂ → Acid (carbonic) → Carbonate ions fall → Calcium carbonate is harder to build.
If you can recite that chain, you can answer almost any ocean-acidification question.
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How this is tested: On Paper 2 a 4-mark Describe asks how rising CO₂ may alter a coral-reef ecosystem — score separate points along the chain (CO₂ dissolves → carbonic acid → lower pH → fewer carbonate ions → slower/dissolving coral skeletons → reef decline and lost biodiversity).
A longer 7-mark Explain can combine the causes of rising CO₂ (fossil fuels, deforestation) with its effect on the ocean and marine life — so be ready to link the carbon source to the acidification chain in one answer.
IB-style question — describe how acidification affects a reef
Atmospheric carbon dioxide levels are rising. Describe how this rise may alter a coral reef ecosystem. [4]
How to score all four marks
- Get the CO₂ into the water. More atmospheric CO₂ dissolves into the ocean, where it forms carbonic acid and lowers the pH (ocean acidification).
- Link pH to carbonate. The lower pH means fewer carbonate ions are available for organisms to use.
- Hit the coral. Corals build their calcium carbonate skeletons more slowly, and existing skeletons may dissolve, so the reef structure weakens.
- Reach the ecosystem. As the reef framework breaks down there is less habitat and shelter, so biodiversity falls — many fish and invertebrate species decline or are lost. (Award 1 mark per distinct point, up to 4.)
Final answer
More CO₂ dissolves into the sea, forming carbonic acid and lowering the pH (acidification); this leaves fewer carbonate ions, so corals build their calcium carbonate skeletons more slowly and existing skeletons dissolve; the reef weakens and provides less habitat, so biodiversity falls.
✓ Why this scores full marks: Each sentence is a separate link in the chain — dissolving CO₂/lower pH, fewer carbonate ions, slower/dissolving coral skeletons, and the ecosystem knock-on (less habitat → lower biodiversity).
A 'Describe' worth 4 marks needs four distinct points, not the single idea 'corals are harmed' written four ways.
A healthy reef (normal pH)
- Plenty of carbonate ions available
- Corals build their skeletons quickly
- Strong reef structure = lots of habitat and shelter
- High biodiversity — fish, invertebrates and algae
An acidified reef (lower pH)
- Fewer carbonate ions for calcification
- Coral skeletons grow slowly and may dissolve
- Reef weakens and erodes = less habitat
- Species are lost → lower biodiversity and fewer fisheries