Elevated sea water temperatures and bright sunlight cause thermal stress in corals, which can lead to the disruption of normal photosynthetic processes in the corals’ zooxanthellae.
When temperatures become too warm, the photosythetic system of the zooxanthellae can not effectively process incoming light. This results in production of “superoxides,” such as hydrogen peroxide, toxic by-products of this process. These toxins contribute to coral stress reactions (described below), which lead to bleaching.
When corals bleach, their body tissues become transparent and they appear white. Some zooxanthellae remain, and if the coral survives they may propagate to reestablish their presence. Sometimes the new varieties of zooxanthellae that infect the corals become dominant.
While they are bleached, corals are in starvation mode without zooxanthellae to support their metabolic processes. Although some corals can survive for extended periods without zooxanthellae, others can become weakened, more susceptible to disease, and may die of these stresses.
Some corals, however, do survive to regain their color, new zooxanthellae, and restored health. Other corals that appear to have died may regenerate from deep tissues within the colony and form new growth. These corals are called “phoenix corals.”
A single coral colony may host different varieties of zooxanthellae, each adapted to different conditions and associated with different parts of the coral colony. Some of these parts may be more vulnerable to bleaching conditions than others, creating patches of severely bleached and less severely bleached areas on a single colony.
Heat Resistant Zooxanthellae (2:54)
Andrew Baker discusses heat resistant zooxanthellae.
When corals bleach, zooxanthellae migrate from the corals on their own, or the corals evict them or the zooxanthellae die within the coral. Some scientists believe this action by the corals may be a high-risk, adaptive survival technique to remove zooxanthellae that are not suited to stressed conditions, and permit other free-swimming zooxanthellae, those with higher tolerances to high temperatures and light, to associate with them.