Coral reefs comprise the exoskeletons of dead coral polyps, layered over periods of up to 10,000 years. Each generation of coral polyps establishes a new layer over the previous one, using seawater's calcium carbonate to create an exoskeleton around a fragile, soft body. Coral consumes zooxanthelae, a type of plankton that lives in coral tissue and provides its coloration. These features make coral especially vulnerable to changes in the temperature and composition of seawater.
Global climate change purportedly has a profound effect on coral. Oceans absorb massive amounts of carbon dioxide from the atmosphere, with one-third of it attributable to carbon emissions. Carbon dioxide lowers seawater's pH, which reduces the availability of the carbon ions needed to build coral exoskeletons. Higher water temperatures resulting from global warming can cause coral bleaching, by which heat-stressed coral polyps expel the zooxanthelae that live in their tissues and provide their color.
Chemical and material pollutants take a toll on coral reefs. Toxins of all varieties from industrial waste, oil and mining byproducts and other sources can kill coral directly or create imbalances in the marine ecosystem. Pollutants dumped near coral reefs increase the water's nitrogen content, causing algae overgrowth. This, in turn, cuts off the sunlight that coral and its food source, the zooxanthelae, need to survive. Floating trash also cuts off sunlight and smothers coral reefs.
Commercial fishing strategies such as cyanide fishing and blast fishing harm coral reefs and their inhabitants. Anglers squirt poisonous cyanide into coral reefs' crevices to stun fish for netting, a practice that kills vulnerable coral polyps. Blast fishing, by which anglers set off underwater explosives to stun or kill fish, also destroys coral and other marine creatures living within the reef. Additionally, coral is harvested for souvenirs, jewelry and use in brick making.