Coral reefs are dependent on microscopic symbiotic algae that live inside the cells of corals. These symbionts are spheres with a diameter of about 10 µm with a golden-brown color. When they are healthy, they provide the host coral cells with sugar via photosynthesis. When the ocean temperature rises, the symbiont’s photosynthetic molecule can become damaged and produce harmful reactive oxygen. In response, the host coral cells eject the symbionts, and the whole coral reef lose their brownish color and become white. This process is called bleaching. Corals that bleach for extended period of time usually die.
To learn more about coral bleaching, and other threats to biodiversity, watch the 2014 Holiday Lectures on Science: Biodiversity in the Age of Humans at http://www.holidaylectures.org .
Excerpted from an animation developed by Fabian de Kok-Mercado (HHMI), Satoshi Amagai (HHMI) and Stephen Palumbi (Stanford University)