How do Coral Reefs get bleached and what does it do to them?

Corals are basically composed of a symbiosis of two organisms: polyps (which create the hard skeleton) and zooxanthellae (photosynthetic algae). Bleaching occurs when corals get stressed due to certain environmental events (one example being global climate change). The corals often then expel these algae, which leads to a whitening of color. The coral relies on these algae and this symbiotic relationship to live; often corals do not survive bleaching, and when they do, it can take weeks or months for that amount of algae to grow again.