When the water temperature rises too high, corals eject the algae (zooxanthellae) that live in their tissues, causing the coral to turn entirely white. This is known as coral bleaching. Coral does not die when it bleaches. Corals can survive a bleaching episode, although they are stressed and may die as a result. The heat also kills some of the bacteria in the coral's digestive system, which can cause problems eating certain types of food after the event.
Coral is a hardy animal and many species have survived major climate changes in the past: oceans have risen and fallen over the last four billion years, and today's coral reefs were once tropical beaches with lots of sun and waves. If coral can survive these changes then so can we. However, even if coral reefs disappear tomorrow it would not be because of man-made pollution - rather because of natural changes to the environment due to warming temperatures.
Corals depend on a symbiotic relationship with zooxanthellae. The algae provide nutrients that allow the coral to grow, while the coral protects the algae from harmful conditions in its environment. When the water gets too hot for the algae, the coral turns white and loses its ability to reproduce. But unlike land plants, coral can move if there is enough light and water flow around them. So even though they look dead, they can still find cooler waters or move away from intense sunlight.
Coral bleaching occurs when corals' brilliant colors fade and they turn white. However, there is a lot more to it than that. Coral reefs are vibrantly colored due to tiny algae known as zooxanthellae. If the temperature remains high, the coral will not allow the algae to return, and the coral will perish. While some species of coral can withstand higher temperatures for longer periods of time, most species can't handle heat waves for long enough to be threatened by climate change.
Corals depend on sunlight to live. When the sun's rays are blocked out by clouds or darkness, the corals will begin to lose their color. This is why scientists need to pay attention to the weather patterns of coral reefs - if it gets too hot, the corals will bleach. As well as being caused by heat, coral bleaching can also be caused by cold temperatures and pollutants such as nitrogen and phosphorous. Climate change makes these heat waves occur more often, which puts more stress on the corals and could lead to their extinction.
Coral bleaching was first documented by European explorers in the 17th century. At the time, they were surprised by how colorful many coral reefs seemed to be, considering that it was winter in Australia. As technology has progressed, so has our understanding of coral biology. Today, we know that corals are organisms: they grow bodies of calcium carbonate molecules under water using photosynthesis-like processes called "polyps".
Consider the color and form. Old dead corals will be broken down, will lack healthy color, and will occasionally be coated in algae. Corals bleached by rising water temperatures turn white when the symbiotic algae leaves the coral. Without their colorful photosynthetic partners, these corals cannot make food using the sun's energy and must rely on nutrients dissolved in the water for growth. As a result, they become skeletal remains of their former selves. The algae may return to some corals after they have died, but they too are often gone by the time the coral is exposed by high tides or strong waves.
Alive coral grows thickly over large areas. It may be gray or black where exposed to sunlight, but it will usually have colors hidden beneath the surface. Healthy coral also emits gases such as hydrogen sulfide and methyl mercaptan that lead researchers to conclude that it is alive. Dead coral does not emit gases. However, scientists can still determine if coral is alive or dead by looking at the area itself; if living coral is present then it will be growing into the surrounding water, while dead coral will be crumbling away.
Coral reefs cover about 10 percent of the ocean's surface area but contain 25 percent of all marine life. They provide habitats for thousands of species including many that are endangered otherwise. Reefs capture carbon dioxide from the atmosphere and pump it into the sea through natural processes.
Ocean water is warming. While most corals dwell in the tropics, the water can get too warm for them. When the water temperature rises, corals get stressed and undergo a process known as bleaching, which causes them to appear as white as their skeletons. If the heat persists, the coral will die.
Coral depends on algae called zooxanthellae for survival. The algae provide nutrients that allow the coral to grow, and also give it color. As temperatures rise, the algal life-cycle becomes unstable and the coral loses its color. However, some corals are able to survive such attacks if they have already grown during a previous cold period when temperatures were low. But over time, even these hardy corals will be killed off by heatwaves unless global temperatures start to cool down.
Corals exist in many areas where the water is between 55 and 90 degrees Fahrenheit. But if the water gets hotter than 100 degrees F., then it's too hot for most species of coral. Even at lower temperatures, coral can suffer from high levels of carbon dioxide in the water. This can happen when ocean waters absorb more carbon dioxide from the air than they can release back into them. This leads to acidification of the water, which harms the coral.
If you own a beach house or condo unit near the ocean, take care not to let the water get too hot.