EarthLabs > Corals > Lab 6: Using Data to Identify Hot Spots and Predict Bleaching Events > 6A: Understanding Coral Bleaching

Using Data to Identify Hot Spots and Predict Bleaching Events

Part A: Understanding Coral Bleaching

Under normal conditions, the zooxanthellae algae living in coral tissue absorb energy from the sun and use it for photosynthesis. However, when the water gets too warm, zooxanthellae can produce toxins, which are harmful to both the algae and their coral hosts. For self-preservation, the coral polyps must expel the zooxanthellae, even though they rely on these algae for key life processes such as eating and calcification. Because coral tissue is transparent, coral reefs appear white (the color of their aragonite skeletons) without the zooxanthellae algae. This is why we call this process coral "bleaching."

Diagram of coral bleaching process. Image source: Great Barrier Reef Marine Park Authority.


  1. Go to the NOAA Coral Kingdom Photo Gallery search page. Scroll through the photos of bleached corals.

    Checking In

    Answer the following questions about bleached corals.

    • How do the colors of bleached corals compare to those of healthy corals?
    • In addition to color change, what other irregularities did you see in the bleached corals? Explain.


    Stop and Think

    1:Approximately 1 million species worldwide depend on coral reefs for survival. Based on what you observed in the photographs of bleached corals, what impact do you think coral bleaching has on biodiversity and the overall health of coral reef ecosystems? Explain your answer.


  2. Without the zooxanthellae, corals are without their main source of energy, and are left in a weakened state. Just like a stressed person is more likely to get sick, stressed coral is more likely to become infected with coral diseases. To learn more about diseases that affect corals, read NOAA's descriptions of common coral diseases.

  3. Working with a partner or in a small group, come up with a way to model coral bleaching. Use available craft materials to modify, reinvent, or extend the coral polyp model you made in Lab 2. Include as many details as you can to fully describe the process.

  4. Paired with another group, or in front of the class, use your models to demonstrate and explain coral bleaching.

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