How Coral Reefs Work and Why They Are Dying

The Rainforests of the Sea

Covering less than one percent of the ocean floor, coral reefs are home to an estimated 25 percent of all marine species. This extraordinary biodiversity density, comparable only to tropical rainforests on land, has earned reefs the name rainforests of the sea. They provide food, income, and coastal protection for more than one billion people. The value of the goods and services they supply, from fisheries to tourism to shoreline protection, has been estimated at hundreds of billions of dollars annually.

Yet coral reefs are among the most threatened ecosystems on Earth. Mass bleaching events, once rare, now occur with increasing frequency and severity. The Great Barrier Reef, the largest reef system on the planet, experienced its worst bleaching event on record in 2022. Scientists estimate that if warming continues at its current trajectory, 70 to 90 percent of coral reefs worldwide will be severely degraded or lost by 2050. Understanding what coral reefs are, how they work, and why they are so vulnerable requires looking at the extraordinary biology at their foundation.

What Coral Actually Is

Most people think of coral as a plant or a rock. In fact, coral is an animal, a tiny invertebrate called a coral polyp that belongs to the phylum Cnidaria, making it a distant relative of sea anemones and jellyfish. Individual polyps are typically only a few millimeters across. They are soft-bodied animals that secrete a hard calcium carbonate skeleton, and they live in colonies of thousands to millions of genetically identical individuals that together build the massive reef structures.

The reef structure we see is largely composed of the accumulated calcium carbonate skeletons of countless past polyp generations. A large reef structure like the Great Barrier Reef is built from coral growth over thousands of years. Individual polyps are relatively short-lived, but the colonies and the structures they build can persist for centuries. The living coral is only the thin outer layer of these structures; the bulk of the reef is dead skeleton.

The Coral-Algae Symbiosis

The ecological productivity of coral reefs in nutrient-poor tropical waters depends on one of the most important symbioses in nature. Living within the tissues of coral polyps are microscopic photosynthetic algae called zooxanthellae (specifically, dinoflagellates of the genus Symbiodiniaceae). This relationship is mutualistic: the algae receive protection, shelter, and access to the coral's metabolic waste products (carbon dioxide and nutrients), which they use for photosynthesis. The coral receives up to 90 percent of its energy needs from the sugars produced by the algae's photosynthesis.

This symbiosis is the engine of reef productivity. The algae's photosynthesis also accelerates the coral's calcification rate, helping the reef grow faster. The golden-brown color of most healthy coral comes from the zooxanthellae. When the symbiosis breaks down, the consequences are catastrophic for the coral, because it loses its primary energy source.

Coral Bleaching: The Breakdown of Symbiosis

Coral bleaching occurs when corals are stressed, most commonly by elevated water temperatures, and expel their zooxanthellae. Without the algae, the coral's tissues become transparent, revealing the white calcium carbonate skeleton beneath. This is not the coral dying, at least not immediately. A bleached coral is a stressed coral under threat: it has lost its energy supply and is surviving on its own limited metabolic reserves.

If temperature stress is brief and modest, the zooxanthellae can return and the coral recovers. But if stress is prolonged, severe, or repeated, the coral starves to death. Mass bleaching events, affecting large areas of reef simultaneously, are driven by marine heat waves in which ocean surface temperatures exceed the coral's thermal tolerance by one degree Celsius or more for several weeks. The threshold varies by species and location, but most corals bleach at temperatures roughly one to two degrees Celsius above their local seasonal maximum.

Ocean warming due to climate change is increasing the frequency and severity of these heat waves. The interval between bleaching events that would allow recovery is shrinking. Many reefs that might have recovered from a single bleaching event are now being hit again before recovery is complete, preventing the regeneration of coral populations and the structural complexity of the reef.

Ocean Acidification

Temperature stress is not the only threat from carbon dioxide emissions. The ocean absorbs approximately 30 percent of the carbon dioxide emitted by human activities. When CO2 dissolves in seawater, it forms carbonic acid, lowering ocean pH in a process called ocean acidification. Since the industrial revolution, average ocean pH has dropped from 8.2 to approximately 8.1, a small-seeming change that represents a 26 percent increase in acidity because pH is a logarithmic scale.

This acidification directly threatens coral reef construction. Corals build their calcium carbonate skeletons from carbonate ions in seawater. As acidity increases, carbonate ion concentration decreases, making calcification harder, slower, and more energetically costly. At the same time, increasingly acidic water begins to dissolve calcium carbonate structures. At projected acidification levels for 2100 under high emissions scenarios, the chemistry of surface ocean waters in tropical regions may become corrosive to coral skeletons, meaning reefs would begin dissolving faster than they can be built even if corals survived the thermal stress.

Local Threats Compounding Global Ones

Climate change is the dominant long-term threat to coral reefs, but local stressors significantly reduce reef resilience and accelerate decline. Runoff from agriculture and urban areas carries excess nutrients (nitrogen and phosphorus) into coastal waters, triggering algal blooms that smother corals by blocking light and competing for space. Once coral cover is lost and algae dominate, recovery is extremely difficult because algae prevent coral larvae from settling and growing.

Overfishing disrupts reef ecology by removing herbivorous fish that graze on algae, allowing algae to grow unchecked. Destructive fishing practices, including blast fishing and cyanide fishing common in Southeast Asia, physically destroy reef structures. Coastal development increases sediment runoff that smothers corals and reduces the water clarity corals need for photosynthesis. Crown-of-thorns starfish outbreaks, which may be connected to excess nutrient runoff, periodically devour large areas of reef across the Indo-Pacific.

Reef Recovery and Conservation

Despite the grim trajectory, some reefs show remarkable resilience under the right conditions, and conservation efforts are having measurable effects. Marine protected areas (MPAs) that reduce fishing pressure and runoff allow coral populations to maintain higher cover and diversity, and these healthier reefs show greater capacity to recover from bleaching events. Some coral populations are also showing signs of genetic adaptation to warmer temperatures through the selection of naturally more heat-tolerant individuals.

Coral gardening and restoration programs, in which fragments of heat-tolerant coral genotypes are grown in nurseries and transplanted to degraded reefs, are operating in Florida, the Caribbean, and the Indo-Pacific. These programs can restore local reef patches but operate at far smaller scales than the rates of natural loss, particularly under ongoing warming. Some researchers are pursuing more ambitious approaches including assisted evolution, selectively breeding corals for thermal tolerance, and introducing heat-tolerant symbiotic algae to coral populations. These approaches raise ecological questions about unintended consequences but are being seriously investigated as options for the future.

Conclusion

Coral reefs are marvels of evolutionary symbiosis that have built some of the most biodiverse ecosystems on Earth over millions of years. They are now being destroyed by the dual impacts of ocean warming and acidification, amplified by local stressors that reduce their capacity to withstand and recover from heat stress. Their fate depends overwhelmingly on how quickly the world reduces greenhouse gas emissions and how effectively local stressors are managed in the meantime. The loss of coral reefs would be an ecological catastrophe that would impoverish both the ocean and the hundreds of millions of people who depend on them for food, livelihood, and coastal protection.