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D4.3.7—Coral reef threats

Coral reefs are threatened by warming-driven bleaching and carbon-dioxide-driven acidification that reduce symbiosis, calcification, habitat complexity, and marine biodiversity globally.

Syllabus
First assessment 2025
Objective
D4.3.7
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper2
Typical marks1–3

Common command terms

  • Describe
  • Suggest
  • Outline
  • Discuss
  • Deduce
  • Explain

Scoring notes

Common mistake
Treating bleaching as colour loss only, without explaining zooxanthellae expulsion and reduced nutrient supply.

Recent exam appearances

May 2024Paper2 ["HL"] · TZ18(b)[ 5 ]D4.3.7—Coral reef threats
November 2016Paper2 ["HL"] · TZ01(i)[ 3 ]D4.3.7—Coral reef threats
November 2016Paper2 ["HL"] · TZ01(h)[ 2 ]D4.3.7—Coral reef threats
November 2016Paper2 ["HL"] · TZ01(g)[ 1 ]D4.3.7—Coral reef threats
November 2016Paper2 ["HL"] · TZ01(f)(iii)[ 1 ]D4.3.7—Coral reef threats
Practice this objective

Coverage 2016–2024 · Updated 16 Jul 2026

Concept essentials

  • Coral bleaching occurs when heat stress disrupts the coral-zooxanthellae symbiosis.
  • Ocean acidification lowers pH and reduces calcification in corals and other calcium-carbonate organisms.
  • Coral decline reduces habitat complexity and biodiversity across reef ecosystems.
ConceptIB Biology HL