Coral reefs bleach when stressed, primarily by rising ocean temperatures caused by climate change. This stress leads corals to expel the symbiotic algae living in their tissues, which provide them with food and color, leaving them white and vulnerable.

why do reefs bleach?

The Deep Dive

Coral bleaching is a stark visual indicator of the stress experienced by these vital marine ecosystems. Corals are tiny animals, polyps, that live in colonies and secrete calcium carbonate to form their intricate structures. Within their tissues reside microscopic algae called zooxanthellae, which have a symbiotic relationship with the coral. The algae photosynthesize, providing the coral with up to 90% of its energy needs, and also give the coral its vibrant colors. When corals become stressed, most commonly by elevated sea temperatures, this symbiotic relationship breaks down. The heat damages the photosynthetic machinery within the zooxanthellae, causing them to produce toxic byproducts. In response, the coral expels these algae to protect itself from the toxins. Without the zooxanthellae, the coral's white calcium carbonate skeleton becomes visible through its transparent tissues, a phenomenon known as bleaching. Other stressors like pollution, ocean acidification, and changes in salinity can also trigger bleaching, but temperature is the primary driver globally.

Why It Matters

Coral reefs are biodiversity hotspots, supporting a quarter of all marine life and providing essential coastal protection against storms and erosion. Bleaching events, especially when frequent and severe, can lead to widespread coral death, devastating fisheries that millions of people rely on for food and income. The loss of reefs also impacts tourism and recreation industries. Understanding bleaching is crucial for developing conservation strategies and mitigating climate change to preserve these invaluable underwater cities.

Common Misconceptions

A common misconception is that bleached coral is dead. While bleaching is a severe stress that can lead to death if conditions don't improve, corals can recover if the stressor is removed and temperatures return to normal. The zooxanthellae can repopulate the coral's tissues, and the coral can regain its color and health. Another myth is that bleaching is only caused by rising temperatures; while temperature is the main culprit, pollution, disease, and changes in salinity can also contribute to coral stress and bleaching, often exacerbating the effects of thermal stress.

Fun Facts

Coral bleaching can occur very rapidly, sometimes within days, when water temperatures become too high for the corals to tolerate.
Some corals are more susceptible to bleaching than others, with branching corals often being more vulnerable than massive, boulder-like corals.