Can Corals Recover From Bleaching?

Pocillopora coral bleached due to El Nino in the Pacific ocean, Polynesia, American Samoa

Coral reefs are large underwater structures composed of skeletons of marine invertebrates called corals. The most extensive coral reefs are found in clear shallow waters in the tropics and subtropics with the largest being the Great Barrier Reef in Australia which is 2,400 kilometers long. Coral bleaching is a major environmental concern, a process that can lead to the death of corals present in the reefs. However, bleaching is not a completely irreversible process. The article strives to understand the process of bleaching and its harmful effects and also mentions how the process can be reversed.

Significance Of Coral Reefs

Almost 25% of marine organisms rely on the coral reefs for food, shelter, and as a breeding ground. They form a primary habitat for more than 4,000 species of fish, 700 different species of corals, and thousands of other species of flora and fauna. Due to their significance to marine life, these reefs are sometimes called the rainforests of the sea. Coral reefs also protect the coastlines from storms and erosion and act as medicine. Additionally, corals are used as souvenirs for home decoration and in making jewelry.

Bleaching Of A Coral Reef

Coral and algae have a symbiotic relationship. The microscopic algae called zooxanthellae live embedded in the tissues of the coral. Like most plants, the zooxanthellae can undergo photosynthesis. This process entails the conversion of simple inorganic substances such as water and carbon dioxide in the presence of sunlight to glucose and oxygen. The glucose produced forms the coral’s primary food which provides energy to it. As a result, the reefs grow much faster and healthier than if they relied on planktons for food. It is the presence of the algae that give the corals their characteristic brown color. In turn, the corals supply the algae with chemical components such as ammonia and phosphates that are very crucial for their survival.

However, this perfect mutual relationship between the coral reefs and the algae may be short-lived if the corals detect stress in their environment. The stress can come as result of change in conditions such as elevated temperatures due to global warming, run-offs, and pollution, overexposure to sunlight, oceanic acidification or lack of nutrients. To survive under these conditions, the reefs will expel the photosynthetic zooxanthellae living in their tissues causing them to turn completely white. In this state, the corals are vulnerable to diseases.

Causes Of Coral Bleaching

The factors that result in bleaching of the coral reefs are called stresses, which can either be natural or artificial. The natural causes of stresses include diseases, storms, and predation. On the other hand, the artificial stresses caused mainly by human activities include global warming, ocean acidification, pollution, sedimentation, and unsuitable fishing practices as discussed below;


Many stressful environmental conditions can lead to bleaching of the corals. However, the increase in water temperatures as a result of global warming is the leading cause of bleaching. The microscopic zooxanthellae are quite sensitive to higher temperatures. Such temperatures result in the algae developing heat stress which prevents photosynthesis from taking place. Consequently, no carbohydrates are formed to feed the corals. In 1998 and 2002, the Great Barrier Reef (GBR) experienced severe bleaching known as “mass bleaching” like never witnessed before. This incident was caused by the warmer sea temperatures linked to the El Nino events. More than 55% of the corals were destroyed by the catastrophe. The water temperatures along the GBR are likely to rise by 1.2 to 1.3°C by 2050. Consequently, concerns have been raised over possibilities of more mass bleaching in the future.

Ocean Acidification

The increase in acidification of the ocean occurs when the ocean waters absorb large volumes of carbon dioxide released into the atmosphere through the burning of fossil fuels. This condition creates an acidic medium in the waters that in turn lowers the pH. The weak carbonic acid formed reacts with and destroys the calcium carbonate which is the primary shelter for the corals.


The stress caused by water pollution also causes massive destruction on the coral reefs. Sources of water pollution include runoffs from agricultural pesticides and fertilizers, oil and gasoline spills, sewage discharge, and sediments from the eroded landscapes. The corals, photosynthetic algae, and other marine animals in the reefs’ ecosystem are easily damaged by water pollution.

Fishing Practices

Cyanide fishing, blast fishing, or overfishing using trawlers can be destructive to huge numbers of coral reefs. The Great Barrier Reef which is home to thousands of different species of fish, sea urchins, turtles, snakes, whales, dolphins, mollusks and many more is under threat of coral bleaching due to destructive fishing practices.

Recovery Of Coral Reefs From Bleaching

The reaction of the coral reefs exposed to stressful conditions depends on their resistance. Coral resistance is the extent to which the corals can withstand the elevated stress without bleaching. On the other hand, coral recovery is the ability of the corals and algae’s symbiotic relationship to be replenished after coral mortality. When corals bleach they are not necessarily dead yet. The majority of the corals manage to survive the bleaching event. If the stress-causing the bleaching is not very severe, the coral reefs recover from bleaching action and regain their algal bloom. However, if the algae loss is prolonged and the stress prevalent continues for a relatively long period, then the corals will eventually die leaving behind a pillar of white crystalline calcium carbonate skeleton.


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