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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
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper3
Typical marks2–4

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

November 2024Paper3 ["SL"] · TZ213[ 2 ]D4.3.7—Coral reef threats
November 2023Paper2 ["SL"] · TZ17(a)[ 4 ]D4.3.7—Coral reef threats
May 2023Paper3 ["SL"] · TZ213(b)(ii)[ 2 ]D4.3.7—Coral reef threats
November 2018Paper3 ["SL"] · TZ015(b)[ 4 ]D4.3.7—Coral reef threats

Coverage 2018–2024 · Updated 16 Jul 2026

Coral Reefs Depend on a Temperature-Sensitive Mutualism

Coral supplies to zooxanthellae Zooxanthellae supply to coral
protected, illuminated cells and inorganic nutrients photosynthetic organic carbon and oxygen
CO₂ and nitrogenous waste from metabolism much of the energy supporting coral growth and calcification

Reef-building corals live near their upper thermal tolerance. Sustained heat stress disrupts photosynthesis and cellular regulation in the symbiosis.

The coral animal is not a plant. Its photosynthetic partner is the zooxanthella living within coral tissues.

Heat Stress Causes Bleaching by Separating Coral from Zooxanthellae

  1. Unusually warm water stresses the coral–algal association.
  2. Damaged photosynthetic processes produce harmful reactive molecules.
  3. Coral expels zooxanthellae or loses their pigments.
  4. Transparent tissue reveals the white calcium carbonate skeleton.
  5. Energy supply falls; prolonged or repeated bleaching increases disease and mortality.

Bleaching is not instant death. Corals can recover if stress ends and compatible symbionts return, but repeated events leave less time to rebuild energy reserves and reproduce.

White colour is the visible symptom; the central mechanism is loss of the mutualistic photosynthetic partner and its energy contribution.

Ocean Acidification Reduces Carbonate Available for Coral Skeletons

  1. Atmospheric CO₂ dissolves into seawater.
  2. Dissolved CO₂ forms carbonic acid and releases hydrogen ions.
  3. Hydrogen ions combine with carbonate ions.
  4. Carbonate availability and calcium carbonate saturation fall.
  5. Corals must spend more energy calcifying and skeleton growth weakens.

Warming and acidification are different pressures from the same CO₂-driven problem: heat disrupts symbiosis; altered carbonate chemistry suppresses skeleton formation.

Ocean acidification means falling pH, not that seawater necessarily becomes below pH 7. The biologically important change is the shift in carbonate chemistry.

Repeated Bleaching and Weak Calcification Can Collapse Reef Habitat

  1. Heat events reduce coral energy and survival.
  2. Acidification slows new skeleton formation and reef repair.
  3. Dead framework erodes faster than living corals rebuild it.
  4. Three-dimensional refuges, feeding sites and nursery habitat disappear.
  5. Fish and invertebrate diversity and food-web functions decline.

Pollution, sedimentation and overfishing can further weaken recovery. Ecosystem collapse arises from interacting pressures and loss of habitat-building coral, not temperature in isolation.

Two pathways show heat stress expelling zooxanthellae and dissolved CO₂ lowering pH and calcium carbonate formation; both converge on degraded reef structure and lower biodiversity.

Climate Effects Follow Forcing, Feedback, Exposure and Biological Response

human greenhouse-gas source → enhanced infrared retention → climate variable changes → feedback may amplify the change → habitat or physiological mechanism alters survival, production or distribution → population and ecosystem consequences

Impact Mechanism that must appear
boreal sink becomes source carbon release exceeds uptake through drought, mortality, fire and thaw
ice-dependent species decline required resting or breeding platform is lost at the critical time
marine productivity falls stratification reduces nutrient upwelling to phytoplankton
range contracts or shifts climate space moves but dispersal and habitat constrain tracking
reef degrades bleaching removes symbiont energy and acidification suppresses calcification

Do not jump directly from “warming” to “species decline.” The biological mechanism and the relevant time or place make the explanation causal.

Coral reef threats

[ 5 ]

Outline the reasons that climate change is a threat to coral reefs.

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.