Record temperatures around the world have left animals, including humans, in dire straits. Of them, corals are particularly vulnerable: when the water around them becomes too warm, they are susceptible to bleaching.
When corals lose their vibrant colours and turn white, they have bleached. This appearance-based definition is valuable because, just by the corals’ pallor, an observer can say that the surrounding water has changed somehow.
There is more to bleaching, however: most corals are home to a type of algae called zooxanthellae, which give the corals their colours as well as have a symbiotic relationship with them. The zooxanthellae provide amino acids and sugars, and receive many minerals and carbon dioxide in return.
When the ocean environment changes – for example, if its temperature rises beyond a point, it becomes too acidic, or it becomes too bright – the zooxanthellae living within the coral leave. As they do, the coral fades until it appears to have been bleached; if the corals continue to be stressed, they won’t welcome the algae back and eventually die. Other stressors include low tides and water pollution, as well as ecosystem changes wrought by the climate crisis.
Bleaching is not always a death knell. Some colonies have been known to survive a bleaching event, like, famously, one near Japan’s Iriomote Island: it was bleached in 2016 but showed signs of recovery in 2020.