When corals lose their color, it is known as coral bleaching. The skeletal structure of hard corals is usually white, 
but due to the zooxanthellae algae, which are small plants called dinoflagellates, residing in the soft tissues of corals, they have color and, during the day, they photosynthesize.
The coral polyps benefit from the products of photosynthesis, and in turn, the algae benefit from the nitrogen, phosphorus, and carbon dioxide waste produced, which they need to grow. At night, the polyps feed on plankton, capturing it with their tentacles. These microalgae are photosynthetic, and their relationship with some corals, as well as with other marine species, 
is an example of endosymbiosis (symbiosis is the coexistence of two mutually beneficial organisms, and endo means inside).
If the rate of photosynthetic production is too high, corals have the ability to control the amount of zooxanthellae by expelling them. However, when corals are excessively stressed, this causes them to expel more zooxanthellae than necessary, and therefore, the loss of color results from the expulsion of many zooxanthellae, and the concentration of photosynthetic pigments in these organisms is decreased.
Although long-term bleaching can cause partial or 
total death of coral colonies, if the situation is not too severe and the stressful conditions are altered, it is possible for affected colonies to recover their symbiotic microalgae and start growing again.
As stress seems to be the key to the occurrence of this problem, let’s evaluate which events are reported as the cause of bleaching in coral reefs around the world.
Climate change in the Earth’s atmosphere, due to greenhouse gas emissions, correlates with the increase of hurricanes, tornadoes, floods, sand and other storm activities, rising sea levels, and so on.
• Water temperature;
• Water salinity;
• Air temperatures;
• Carbon dioxide and methane gas;
• Increased ultraviolet radiation;
• Exposure to high light levels;
• Increased turbulence or high water;
• Decrease in light levels;
• Pollution.
A change in ocean currents, related to changes in phytoplankton and zooplankton populations, as well as the amount of other nutrients present in the water.
This coral bleaching effect is an indication that severe changes are occurring on the planet and can lead to the extinction of various species.
Source: www.portalsaofrancisco.com.br, by Camila Correia