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Why Coral Reefs Are Bleaching
Zooxanthellae loss, thermal stress, and acidification
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Coral reefs are living ecosystems built by tiny animals called coral polyps, and they support an enormous variety of marine life. Their bright colors come mostly from microscopic algae living inside coral tissue. When ocean conditions become too stressful, especially during unusually warm water events, corals can lose these algae and turn white. This process is called bleaching, and it matters because repeated bleaching can weaken reefs, reduce biodiversity, and damage coastal economies.
Bleaching begins when heat and other stressors disrupt the partnership between coral and zooxanthellae. The stressed algae produce harmful reactive oxygen molecules, so the coral expels them to protect its cells, but this also removes the coral’s main food source. Ocean acidification adds another challenge by making it harder for corals to build calcium carbonate skeletons. Keeping global warming near 1.5C, reducing local pollution, protecting herbivores, and restoring resilient corals can improve the chances that reefs survive future heat waves.
Key Facts
- Corals are animals that host symbiotic algae called zooxanthellae, which provide much of the coral’s food through photosynthesis.
- Bleaching happens when corals expel zooxanthellae or lose their pigments, exposing the white calcium carbonate skeleton beneath clear coral tissue.
- A common thermal stress rule is that water about 1C to 2C above the usual summer maximum for several weeks can trigger mass bleaching.
- Photosynthesis in zooxanthellae can be summarized as 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- Ocean acidification occurs as CO2 + H2O = H2CO3, which lowers pH and reduces carbonate ions needed for coral skeleton growth.
- At 1.5C of global warming, most warm-water coral reefs are projected to decline sharply, but impacts are expected to be much worse at 2C.
Vocabulary
- Coral polyp
- A small soft-bodied marine animal that builds reef structures by secreting a calcium carbonate skeleton.
- Zooxanthellae
- Microscopic photosynthetic algae that live inside coral tissues and provide energy-rich sugars to the coral.
- Bleaching
- The whitening of coral caused by the loss of symbiotic algae or their pigments during environmental stress.
- Thermal stress
- Stress caused when water temperatures stay above the normal range that corals can tolerate.
- Ocean acidification
- The decrease in seawater pH caused by absorption of atmospheric carbon dioxide.
Common Mistakes to Avoid
- Thinking bleached coral is always dead. Bleached coral is still alive at first, but it is weakened and may die if stressful conditions last too long.
- Blaming bleaching only on sunlight. Strong light can worsen stress, but unusually warm water is the main trigger in many mass bleaching events.
- Forgetting that corals are animals. Corals build reef structures, but each coral colony is made of animal polyps that depend heavily on algal partners.
- Assuming acidification causes the white color directly. Acidification mainly reduces skeleton-building ability, while bleaching is mostly caused by the breakdown of coral-algae symbiosis.
Practice Questions
- 1 A reef’s usual summer maximum temperature is 29.0C. During a heat wave, the water stays at 30.5C for 4 weeks. By how many degrees is the reef above its usual summer maximum, and is it within the common 1C to 2C bleaching risk range?
- 2 A coral colony receives 80 percent of its energy from zooxanthellae. If bleaching causes it to lose three-fourths of that algal energy supply, what percent of its original total energy supply is lost?
- 3 Explain why reducing local pollution and protecting herbivorous fish can help a reef recover from bleaching, even though these actions do not directly lower global ocean temperature.