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Coral reefs are one of the most fragile, biologically complex, and diverse marine ecosystems on earth. This ecosystem is one of the fascinating paradoxes of the biosphere: how do clear and thus nutrient-poor waters support such prolific and productive communities? Part of the answer lies within the tissues of the corals themselves. Symbiotic cells of algae known as zooxanthellae carry out photosynthesis using metabolic wastes of the coral thereby producing food for themselves, for their corals, hosts, and even for other members of the reef community. This symbiotic process allows organisms in the reef community to use sparse nutrient resources efficiently.
But coral reefs are imperilled by several threats. The increasing acidification of the ocean inhibits coral's ability to produce the calcium carbonate exoskeletons they rely on for shelter. Water pollution, too, is wreaking havoc on coral reefs. Agricultural pesticides and fertilizers, oil and gasoline, sewage discharge, and sediment from eroded landscapes make it difficult for coral to thrive. As a consequence of temperature rise due to global warming, coral polyps expel the zooxanthellae they depend on for food. Once the zooxanthellae are gone, the coral loses its brilliant color, and all that can be seen is the white exoskeleton; this is referred to as coral bleaching. Coral colonies subject to bleaching usually die off. Fishing practices such as cyanide fishing (spraying cyanide in the water), "blast fishing" with explosives, and overfishing with trawlers can destroy a thousand-year-old coral reef in a matter of minutes.

The largest coral reef in the world, the Great Barrier Reef, is home to at least 400 individual species of coral and thousands of different species of fish, mollusks, sea snakes, sea turtles, whales, dolphins, birds, and more. As with the other coral reefs of the world, this incredible ecological hotspot is under threat. A heatwave in 2016 caused a large percentage of the corals in the Great Barrier Reef to undergo severe bleaching and death. A 2018 study in the journal Nature Communications found that in just the northern third of the reef, over 60 percent of the shallow-water corals experienced some degree of bleaching, and 30 percent of the coral died.

The Australian government has put forth a long-term plan to sustain the Great Barrier Reef. The plan recapitulates efforts to greatly reduce and eventually eliminate dumping materials and chemicals, reduce fishing and poaching, and monitor the water quality of run-off directed toward the reef. There are also many attempts to rebuild the reef. Scientists are working to breed stronger species of coral that are less susceptible to the warmer waters and grow at an accelerated rate. They grow various coral species in the lab and place them in experimental environments designed to reflect the predicted temperature and acidity of the ocean decades from now. Another group of coral reef ecologists is experimenting with growing corals on steel frames placed over the damaged parts of a reef. Electrical currents sent through the steel frames accelerate the corals' growth by three to four times, reported New Scientist. It's possible this technique could help rebuild the reef and make the coral more likely to survive bleaching events.