Although corals can capture prey, many tropical species are dependent on photosynthetic algae for nutrition—in other words, the algae that live within the cells lining the digestive cavity of the coral. The symbiotic relationship between coral and algae is mutually beneficial: The algae provide the coral with oxygen and carbon and nitrogen compounds while the coral supplies the algae with ammonia (waste product), from which the algae make nitrogenous compounds for both partners.

Coral bleaching is the stress-induced loss of colorful algae that live in coral cells. In coral bleaching, the algae lose their pigmentation or are expelled from coral cells. Without the algae, coral becomes malnourished and dies. The causes of coral bleaching are many, but it is believed the most adverse one involves environmental factors. Pollution, invasive bacteria, salinity changes, temperature changes (associated with global climate change), and high concentrations of ultraviolet radiation (associated with the destruction of the ozone layer) all seem to contribute to coral bleaching. And if the healthy coral reef becomes diseased or dying, it slowly erodes away. When this happens, marine life disappears, floodwaters are not absorbed, the waves pass over the reef and break directly onshore, and most importantly for areas prone to such events as hurricanes, the coastal structures bear the brunt of any storm surge.