Coral bleaching is typically defined as the breakdown of the coral-algal symbiosis through either loss of symbiotic algae or a reduction in their per cell pigment concentrations. It is considered a generalized pathological response that can occur via impacts to a number of physiological pathways. Risk factors other than extremes in solar radiation or temperature that have been linked to coral bleaching include environmental factors (e.g., salinity extremes, sediment, low temperatures, desiccation, low light or darkness, ultraviolet light, and elevated pCO2 leading to reduced pH) and environmental contaminants (e.g., herbicides, copper, cyanide oil, sunscreens). Bleaching is often viewed as a response of the coral host to algal stress however, depending on the stressor it may be initiated primarily because of algal stress (e.g., herbicides, cyanide), animal stress (e.g., oil, metals) or to both partners (sedimentation, salinity extremes). Bleaching can lead to coral mortality if prolonged or severe, but mild or short-lived bleaching is often fully reversible, within weeks to months. Chlorophyll fluorescence techniques are used as an indicator of metabolic pathology in the algal symbionts under various stress conditions. Reductions in Fv/Fm typically lead to coral bleaching, and have become an important pathophysiological metric of bleaching stress.
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