Bleaching colonies of mustard hill coral, Porites astreoides
Bleaching colonies of mustard hill coral, Porites astreoides, both green and brown color morphs, Florida Keys. When corals are stressed, the symbiosis between the coral animal and its photosynthetic algal symbionts (zooxanthellae) breaks down and the zooxanthellae are expelled from the coral tissue. The zooxanthellae’s photosynthetic pigments contribute much of the color we see in corals, so when the symbionts are lost, the coral’s white calcium carbonate skeleton shows through the thin skin of coral tissue. This lighter or white appearance is referred to as bleaching. Common triggers of bleaching are exposure to very warm or very cold water and changes in salinity. However, many other stressors (for example, excess sediment, nutrients, or chemicals) can also cause bleaching. Bleaching makes corals more susceptible to diseases. If the stress continues, the coral may die. However, if the stress is removed, it is possible for the coral to regain its zooxanthellae and return to normal.