Explain how rising atmospheric carbon dioxide concentrations contribute to ocean acidification and outline one consequence of ocean acidification for marine organisms.

IB DP Biology Standard Level (2023 syllabus) — D4.3 Climate change · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

CO₂ dissolves in seawater to form carbonic acid (H₂CO₃). The carbonic acid dissociates, releasing H⁺ ions and lowering the pH of the water. The increased H⁺ ions react with carbonate ions (CO₃²⁻), reducing their availability. Consequently, organisms that build calcium carbonate shells or skeletons, such as corals, have a harder time forming or maintaining their structures, leading to weakened or dissolving shells.

Examiner tips

  • Use the exact sequence: CO₂ → H₂CO₃ → H⁺ + HCO₃⁻, then H⁺ + CO₃²⁻ → HCO₃⁻
  • Mention the pH drop and carbonate depletion
  • Give a specific organism example (coral) and the effect on its skeleton

Common mistakes

  • Confusing CO₂ with H₂CO₃ as the main acid
  • Failing to state the pH decrease
  • Not linking carbonate depletion to a specific organism’s shell or skeleton

Mark scheme (4 marks)

  1. CO₂ dissolves in seawater to form carbonic acid (H₂CO₃)
  2. Carbonic acid dissociates, releasing hydrogen ions (H⁺), lowering the pH of seawater
  3. Increased H⁺ ions react with carbonate ions (CO₃²⁻), reducing carbonate ion availability
  4. One consequence for marine organisms: reduced carbonate availability impairs the ability of organisms such as corals, molluscs, or echinoderms to form/maintain calcium carbonate shells or skeletons, leading to weakened or dissolving structures

Key terms in this question

ocean acidification · pH

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