AP Syllabus focus:
‘Ocean warming can cause coral bleaching when corals lose the algae living in their tissues. Some corals recover, but others die.’
Coral reefs are highly productive ecosystems built by organisms that live close to their thermal limits. When ocean temperatures rise, corals can lose their essential algal partners, turning white and risking starvation and death.
What coral bleaching is
The coral–algae partnership
Reef-building corals are animals that host microscopic algae inside their cells. This relationship supplies much of the coral’s energy and helps reefs grow.
Coral bleaching: the loss (or pigment breakdown) of symbiotic algae in coral tissues, causing the coral to appear white because its calcium carbonate skeleton shows through.
Even when bleached, a coral is not automatically dead; it is a stressed animal with reduced energy income.
This USGS photograph contrasts an apparently healthy coral colony (left) with a visibly bleached colony (right) during a Florida Keys bleaching event. The side-by-side comparison makes the central diagnostic feature of bleaching clear: loss of normal coloration as the coral tissue becomes more transparent and the pale skeleton shows through. Source
Zooxanthellae (symbiotic algae): photosynthetic algae (often dinoflagellates) living inside coral tissues that provide sugars and other nutrients to the coral.
Why the algae matter
Photosynthate supply: algae transfer a large fraction of produced sugars to the coral, supporting metabolism, growth, and reproduction.
Coloration: algal pigments give many corals their brown, green, or golden hues.
Reef building support: higher energy availability supports calcification and skeletal maintenance.
How ocean warming triggers bleaching
Heat stress damages photosynthesis
When sea surface temperature rises beyond a coral’s tolerance, the algae’s photosynthetic machinery becomes inefficient and can produce harmful byproducts.
Key steps (simplified):
Temperature anomaly (often just above the usual seasonal maximum) stresses the algal photosystems.
Damaged photosynthesis increases reactive oxygen species (ROS) inside coral cells.
ROS trigger a stress response: the coral may expel algae, digest them, or the algae may abandon the host.
With fewer algae/pigments, the coral tissue becomes transparent and the white skeleton becomes visible.
What makes bleaching more likely
Bleaching risk increases with the intensity and duration of warming.
This NOAA Coral Reef Watch time-series graphic illustrates how short-term temperature exceedances above a local bleaching threshold (“HotSpot”) accumulate over weeks into Degree Heating Weeks (DHW). The figure helps explain why prolonged, repeated warm conditions can be more biologically damaging than a brief temperature spike, because DHW integrates intensity × duration of thermal stress. Source
Short, mild warming may cause partial bleaching with potential recovery.
Prolonged warming (weeks) can cause widespread bleaching and mortality.
High sunlight can intensify stress during warm periods by increasing light-driven damage in already-stressed algae.
What happens to corals after bleaching
Immediate biological effects
Without their algal partners, corals lose a major energy source.
Lower growth and calcification: less energy available for building and maintaining skeleton.
Reduced reproduction: fewer resources for gamete production and successful spawning.
Higher disease susceptibility: stressed corals are more vulnerable to pathogens and tissue loss.
Starvation risk: corals may survive temporarily by using stored reserves and capturing plankton, but this is often insufficient during extended heat stress.
Reef-scale ecological consequences
When many corals bleach at once, the entire reef community can shift.
Habitat loss: complex coral structures support fish and invertebrates; coral death simplifies habitat.
Biodiversity decline: specialized reef species may decrease as shelter and food webs degrade.
Community reorganization: dead coral surfaces can be colonized by algae or other organisms, altering species interactions and reef function.
Recovery versus death
Why some corals recover
If temperatures return to tolerable levels soon enough, some corals can regain symbiotic algae and rebuild energy stores. Recovery is more likely when:
Heat stress is brief and not repeatedly reintroduced.
Some algae remain in tissues to repopulate.
The coral’s energy reserves were high before the event.
Why others die
Corals are more likely to die when:
Warming is severe or sustained, leaving too little time to restore the symbiosis.
Bleaching overlaps with other stressors (for example, poor local conditions that further strain energy balance).
Bleaching recurs frequently, preventing full recovery between events and reducing long-term resilience.
FAQ
No. Different symbiont types vary in heat tolerance and performance.
Some corals can “shuffle” symbionts after stress, but heat-tolerant types may provide less energy under normal conditions.
Some corals increase production of fluorescent or chromoprotein pigments during stress.
These pigments may act like sunscreens by reducing light damage, but they do not guarantee survival.
Remote sensing can track sea surface temperature anomalies and calculate heat stress metrics.
Managers combine satellite alerts with field surveys to prioritise monitoring and rapid response.
Adaptation can occur via selection for heat-tolerant corals, symbiont changes, or acclimatisation.
However, adaptation is constrained by generation time, trade-offs, and the speed and frequency of warming events.
Experimental approaches include temporary shading, localised cooling, and selective propagation of more tolerant corals.
These methods can be logistically limited and are typically small-scale compared with the size of reef systems.
Practice Questions
State what coral bleaching is and explain how ocean warming can cause it. (2 marks)
1 mark: Defines bleaching as loss/expulsion of symbiotic algae (or their pigments) from coral tissues causing whitening.
1 mark: Links warming to stress/damage to algal photosynthesis leading to expulsion/loss of algae.
Describe how a prolonged period of unusually warm seawater can lead to changes in a coral reef ecosystem through coral bleaching. (6 marks)
(Any six):
Explains that elevated temperature stresses coral–algae symbiosis.
Mentions impaired photosynthesis and/or build-up of harmful byproducts (e.g., ROS).
States algae are expelled/lost, causing whitening (bleaching).
Explains reduced energy supply to coral, leading to reduced growth/calcification and/or reduced reproduction.
States increased susceptibility to disease and/or starvation leading to coral mortality.
Links coral loss to reduced habitat complexity and declines/changes in reef organisms (fish/invertebrates).
Describes potential shift in community composition on dead coral (e.g., increased algal cover).
