Coral Bleaching: Causes and Mechanisms

Unprecedented changes in coral reef systems have focused attention on a wide range of stressors on local, regional, and global spatial scales but global climate change resulting in elevated seawater temperatures is widely accepted as having contributed to the major declines in coral cover or phase shifts in community structure on time scales never previously observed or recorded in the geological record. The major mechanism of scleractinian mortality as a result of global climate change is “coral bleaching,” the loss of the endosymbiotic dinoflagellates (=zooxanthellae) that occurs as part of the coral stress response to temperature perturbations in combination with several other synergistic factors. Over several years many studies have shown that the common mechanism underlying the stress response of corals to elevated temperatures is oxidative stress that is exacerbated when exposure to high irradiances of solar radiation accompanies the thermal insult. Oxidative stress, the production and accumulation of reduced oxygen intermediates such as superoxide radicals, singlet oxygen, hydrogen peroxide, and hydroxyl radicals can cause damage to lipids, proteins, and DNA. Reactive oxygen species are also important signal transduction molecules and mediators of damage in cellular processes, such as apoptosis, autophagy, and cell necrosis all of which are believed to have roles in coral bleaching depending on the intensity and duration of the environmental insult. This chapter examines the current evidence supporting the hypothesis that the production and accumulation of reactive oxygen species leads to oxidative stress and is the proximal cause of coral bleaching.