Explain how rising atmospheric CO₂ concentrations affect the pH of the ocean and outline the consequences of this change for organisms with calcium carbonate structures.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
CO₂ dissolves in seawater forming carbonic acid (H₂CO₃). The acid dissociates, releasing H⁺ ions and lowering pH – this is ocean acidification.
The extra H⁺ reacts with carbonate ions (CO₃²⁻), decreasing their concentration.
Organisms that build calcium carbonate (CaCO₃) structures – such as corals, molluscs and echinoderms – find it harder to precipitate CaCO₃. Their shells or skeletons become weaker, growth slows and existing structures may dissolve.
The extra H⁺ reacts with carbonate ions (CO₃²⁻), decreasing their concentration.
Organisms that build calcium carbonate (CaCO₃) structures – such as corals, molluscs and echinoderms – find it harder to precipitate CaCO₃. Their shells or skeletons become weaker, growth slows and existing structures may dissolve.
Examiner tips
- Show the CO₂→H₂CO₃→H⁺ sequence first; then link H⁺ to carbonate depletion; finally give specific organism examples and effects on CaCO₃.
- Use the exact terms: ‘ocean acidification’, ‘carbonate ion’, ‘calcification’, ‘CaCO₃ structures’.
- Keep answer concise – 4 marks, so one sentence per point.
Common mistakes
- Mixing up CO₂ with O₂ or forgetting the acid dissociation step.
- Using ‘acidification’ without explaining the H⁺/carbonate chemistry.
- Mentioning organisms that do not rely on CaCO₃ (e.g. fish) or giving irrelevant effects.
Mark scheme (4 marks)
- CO₂ dissolves in seawater to form carbonic acid (H₂CO₃)
- Carbonic acid dissociates, releasing hydrogen ions (H⁺), lowering seawater pH — a process termed ocean acidification
- Excess H⁺ ions react with carbonate ions (CO₃²⁻), reducing carbonate ion concentration available for calcification
- Organisms such as corals, molluscs, and echinoderms struggle to form or maintain calcium carbonate (CaCO₃) shells/skeletons, leading to weaker structures, reduced growth, or dissolution of existing structures
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Climate change questions
- Explain how increasing global temperatures can alter the geographical distributi…
- Explain how positive feedback mechanisms in the Arctic contribute to acceleratin…
- Explain how climate change may affect the timing of seasonal biological events, …
- Explain how climate change is predicted to affect the geographic distribution of…
- Explain how elevated atmospheric CO₂ concentrations affect the rate of photosynt…