While corals create habitat used by a diversity of marine life, coral bleaching can put that marine life at risk. Coral reefs cover less than 1% of the planet, yet over 1 billion people are estimated to depend on coral reefs for food. When colorful corals turn pure white, the sudden change is cause for alarm. A bleached coral's white skeleton is fully exposed making the animal look dead. While bleached corals are still alive, their color loss is a symptom of intense stress: a desperate effort by an immovable animal to survive.

What Causes Coral Bleaching?

A healthy coral's brownish base color comes from tiny, plant-like critters known as zooxanthellae. While these colorful inhabitants are each less than 1 millimeter in size, over one million zooxanthellae typically live in each square centimeter of coral. Zooxanthellae congregate in the coral's clear polyps where their combined color is visible to the outside world. Yet the zooxanthellae's colors are simply a side-effect of their main function to the coral: to provide food.

How Algae Provide Corals With Food

Zooxanthellae are actually tiny pieces of algae. Like plants and other seaweeds, zooxanthellae capture energy from the sun through photosynthesis to produce food. The zooxanthellae capture light using chlorophyll, which is also what gives corals their brown tone. In return for the shelter and carbon dioxide the coral provides, the zooxanthellae share certain nutrients that are hard for the coral to come by on its own.

The amount of food a coral receives from its zooxanthellae varies quite a bit, with some coral species lacking these partnerships altogether. For these independent corals, the animal must rely entirely on its polyps to catch food. Like tiny sea anemones, the coral's polyps use sticky tentacles to catch food as it floats by. Some corals use their tentacles during the daytime, but most tropical corals only extend their polyps at night.

Corals evolved to partner with zooxanthellae may have a competitive advantage over species with entirely independent feeding strategies. While the amount varies greatly between coral species, corals that work with zooxanthellae can obtain over 50% of their daily nutritional needs directly from their photosynthesizing tenants. Unfortunately, coral bleaching may turn this competitive edge into a catastrophic weakness for these work-sharing corals.

Bleached Corals Lack Their Zooxanthellae

A bleached coral lacks its colorful, photosynthetic inhabitants, leaving the coral alone with its bare white skeleton and see-through polyps. Without its zooxanthellae, a bleached coral must rely on its own tentacles for food. For corals that are used to providing most of their food for themselves, this may be quite manageable, but for corals that normally have a tight-knit relationship with their zooxanthellae, the loss of these photosynthetic allies not only strips these corals of their competitive advantage — it also puts these photosynthesis-reliant corals in danger.

On the left, the coral's polyps are full of zooxanthellae, giving the coral color. On the right, the coral lacks zooxanthellae, revealing the coral's bright white skeleton.

The unfortunate breakup between a coral and its zooxanthellae is initiated by the coral landlord when the animal is under intense stress. Most often, this stress comes in the form of abnormally warm water. Other known culprits include drops in seawater's saltiness, nutrient overload, excessive sun exposure, and even unusually cold water.

These stressful situations are thought to cause serious damage to the coral's zooxanthellae, preventing the algae from photosynthesizing properly. Normally, the coral digests damaged zooxanthellae as part of the animal's natural maintenance process, but when large swaths of zooxanthellae are damaged all at once, the coral cannot keep up. The build-up of non-functional zooxanthellae can cause damage to the coral itself, leading a coral to forcefully release its algal inhabitants in a desperate attempt at self-preservation.

A close-up view of a coral's polyps, showing the coral's color. The zooxanthellae are what give corals their color.

Heat stress is also thought to damage the coral's tissues directly. Under these stressful conditions, coral hosts are known to release apparently healthy zooxanthellae, too. The removal of these healthy, food-producing algae may be an unintentional side-effect of heat stress. In addition to damaging the zooxanthellae, heat stress may cause the coral's own tissues to lose their grip on the coral's skeleton, causing the coral to lose its own cells with healthy zooxanthellae inside. In this way, coral bleaching may actually be a symptom of stress instead of just a protective measure.

The mechanisms behind coral bleaching are not yet fully understood and may vary depending on the source of the coral's stress. Nevertheless, it is clear that a coral turns pure white when times are tough.

The Far-Reaching Detriments of Coral Bleaching

In addition to hurting the coral animal itself, coral bleaching greatly affects the fish that depend on corals for food or shelter. In fact, nearly one-quarter of all known fish species live among coral reefs. Many studies have documented losses in reef fish abundance and diversity following coral bleaching events. Fish the primarily or solely feed on corals are thought to be the most susceptible to coral bleaching events, whereas fish with broad feeding habits have been shown to actually increase in abundance in the years following a massive bleaching event. Fish that live within corals are also thought to receive the brunt of the coral's stress response, as these fish are easier for predators to spot when hiding within a bright white coral. Similarly, crabs and other marine animals that live within the coral structure experience immediate, severe declines when during bleaching.

The devastating effects of coral bleaching extend to humans, too, since coral reefs are considered major food sources. Tourism linked to coral reefs makes up an estimated $36 billion-dollar industry upon which many economies are built. The complex, 3D structure created by corals also protects adjacent shorelines by dampening the impact of incoming waves. When coral reefs bleach, these benefits are greatly diminished. A bleached reef has fewer fish available for human consumption. Similarly, a reef lacking its world-famous colors and diverse marine life provides a blow to the tourism industry.

Can Our Coral Reefs Recover?

Coral bleaching was first documented in the 1970s. Since then, it's become a common phenomenon for the world's coral reefs and is often linked with massive coral die-offs.

When a coral dies, algae takes over the animal's leftover skeleton.

Fortunately, there are signs of hope. When models used for predicting coral bleaching are applied to historic temperature data, the models tend to over-predict the amount of coral bleaching, expecting more bleaching to occur than what was actually observed. Scientists interpret this as a sign that some corals are adapting to climate change.

Scientists have also discovered pockets of corals already adapted to extremely warm waters, including mangrove corals in the Great Barrier Reef and back-reef pools in American Samoa. These corals already live in extreme environments, making them "ahead of the game" when it comes to adjusting to increases in ocean temperature. The hope is that pre-adapted, heat-tolerant corals like these will be able to populate future coral reefs should today's main reef-building coral species be unable to adapt to climate change fast enough.

Nonetheless, the best course of action to ensure the longevity of the world's coral reefs, and the livelihoods of the many reef creatures that rely on these corals, is to slow down the rate at which coral reef environments are changing due to climate change. Corals can adapt, but only if they are given enough time for evolution to occur before they are wiped out.