We think of corals as vibrant habitats, providing sheltered homes to countless marine organisms. Famous coral reefs like Australia’s Great Barrier Reef draw millions of visitors every year, drawing people with the promise of seeing scenery from colorful underwater photos in real life.
However, those colors and the healthy ecosystems associated with them are under threat. When experiencing stress from environmental conditions, corals turn an eerie white.
Coral bleaching, as this phenomenon is known, is the visible result of a microscopic process. The macroscopic corals we see are actually colonies of tiny individual organisms called polyps. The polyps of many coral species have an important symbiotic relationship with zooxanthellae, a type of photosynthetic algae. Polyps provide the algae with shelter and ingredients they need for photosynthesis; meanwhile, the algae provide nutrients the polyps need to survive and grow.
Along with their role in providing nutrients, zooxanthellae also give color to hard, stony corals. Coral polyps are themselves translucent, and the algae they host are responsible for the spectrum of colors found in reefs.
While the symbiotic relationship between coral polyps and algae is usually stable, it can be threatened should the coral become stressed by environmental factors. When surrounding waters change dramatically in temperature, coral polyps expel the algae they host.
Without the algae, corals lose their main source of necessary nutrients, damaging their health. They also lose their color and turn completely white
Unfortunately, bleaching events have become widespread in coral reefs around the world. The famous Great Barrier Reef, for instance, experienced four mass bleaching events in the past seven years due to abnormally high water temperatures. And in 2005, half of the United States' corals in the Caribbean were bleached due to warm waters.
Due to climate change, more abnormal temperature changes will likely occur in the oceans. This poses a threat that goes beyond coral reefs. A quarter of marine species need corals at some point in their lives, and healthy reefs protect millions of people along shorelines from dangerous currents.
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