Cell Biology of Coral Bleaching

Abstract

Corals depend on a mutualistic symbiosis with intracellular dinoflagellates of the genus Symbiodinium for their energetic needs. The high productivity of corals in a challenging environment and the necessity of coordinating the metabolism and growth of each partner mean that severe stresses, such as sustained high temperatures, may destabilize the symbiosis. Coral bleaching is linked to oxidative stress of the coral holobiont, and high temperatures may operate on either partner independently or synergistically to elevate oxidative stress. The photosynthetic performance of Symbiodinium is impaired by photosystem degradation and photoinhibition, overexcitation of the photosynthetic apparatus, or sink limitation, factors exacerbated by high temperatures and irradiance. These effects may cause the generation and release of reactive oxygen species or sensitizing compounds into the host cells, initiating the host’s bleaching response. The host may also be independently damaged, particularly the mitochondrial and endoplasmic reticulum functions, by thermally-induced oxidative stress. Regardless of the source, oxidative stress sensed by the host initiates a series of events that culminate in symbiont loss by several mechanisms, principally host cell apoptosis. Coral bleaching is a cellular process that unfolds at the ecosystem scale, and understanding the mechanisms of the breakdown of the symbiosis, aided by new technologies and model systems, is essential to predicting the effects of anthropogenic climate change on coral reefs.