Coral bleaching happens when corals lose the tiny algae that live inside their tissues and give them most of their color. These algae, called zooxanthellae, also provide much of the coral’s food through photosynthesis. When ocean water stays too warm for too long, this partnership breaks down.
Bleaching matters because coral reefs support huge numbers of marine species and protect coastlines from waves and storms.
During heat stress, zooxanthellae can produce harmful chemicals inside coral tissues, so the coral expels them to protect itself. Without the algae’s pigments, the coral’s white calcium carbonate skeleton shows through its clear tissues. Bleached coral is not always dead, but it is weakened and can starve if stressful conditions continue.
If temperatures return to normal soon enough, algae can return and the coral may recover.
Key Facts
- Coral bleaching is the loss of symbiotic algae from coral tissue, causing the coral to appear white.
- Zooxanthellae use photosynthesis to make sugars that feed the coral: CO2 + H2O + light energy -> sugar + O2.
- Corals build hard skeletons from calcium carbonate: Ca2+ + CO3^2- -> CaCO3.
- A sustained temperature increase of about 1 to 2 °C above the usual summer maximum can trigger bleaching.
- Bleached coral can recover if heat stress ends quickly and zooxanthellae return.
- Mass bleaching events affected reefs worldwide in 1998 and 2016, linked to unusually warm ocean temperatures.
Vocabulary
- Coral
- A small marine animal that often lives in colonies and builds a hard calcium carbonate skeleton.
- Zooxanthellae
- Tiny photosynthetic algae that live inside coral tissues and provide food and color to the coral.
- Symbiosis
- A close relationship between different species, such as coral and algae living together for mutual benefit.
- Bleaching
- The whitening of coral that occurs when it loses its algae or the algae lose their pigments.
- Heat stress
- Harm caused when an organism is exposed to temperatures higher than it can tolerate for too long.
Common Mistakes to Avoid
- Thinking bleached coral is always dead is wrong because bleached coral is still alive at first and may recover if conditions improve.
- Blaming bleaching only on sunlight is wrong because unusually warm water is the main trigger, although strong sunlight can make heat stress worse.
- Forgetting the algae’s role is wrong because zooxanthellae provide much of the coral’s energy and color.
- Assuming one hot day causes mass bleaching is wrong because bleaching usually requires elevated temperatures lasting for days to weeks.
Practice Questions
- 1 A reef’s usual summer maximum temperature is 29 °C. If bleaching risk rises at about 1.5 °C above this level, what water temperature would reach that risk threshold?
- 2 A coral gets 80% of its energy from zooxanthellae. If bleaching reduces this energy source to 25% of its normal amount, what percent of the coral’s original total energy is still coming from the algae?
- 3 Explain why a coral can turn white after heat stress even though the coral animal itself may still be alive.