Bleaching - The Offline Version

Although the calcium carbonate skeleton produced by the coral animal is white, under normal circumstances, the healthy tissues of most hard corals have a brownish coloration. This coloration results from the presence of zooxanthellae, symbiotic microalgae, that live within the coral tissues.

An important sign of disease in corals and some other reef organisms is "bleaching." Bleaching is the phenomenon in which the coloration disappears or becomes noticeably less intense, due to the loss of zooxanthellae and/or to the loss of photosynthetic pigments from these microalgae. As a result of the loss of the zooxanthellae, the white of the skeleton shows through the translucent coral tissue. Bleaching involves the change in the color of the tissue attached to the skeleton, but not the loss of actual coral tissue. Bleaching of corals has been recognized at least since the turn of the century; however, since 1980 extensive outbreaks of bleaching affecting numerous species have occurred on reefs worldwide.

Bleaching on a symmetrical brain coral, Diploria strigosa, Puerto Rico. Photo by L. Bunkley- Williams.
Appearance	The loss of the microalgae or pigments from corals results in the white skeleton showing through the translucent tissue, thus it looks whitened or "bleached." (Note: The tissues of western tropical Atlantic massive starlet coral, Siderastrea siderea, can appear pinkish or bluish when bleaching occurs. Similarly, the finger coral, Porites porites, can appear bluish when bleached.) Bleaching can take several forms. The entire coral colony can become progressively lighter brown in color. Partial bleaching can occur, where some patches of tissue still retain the microalgae and photosynthetic pigments. At its most extreme, bleaching can result in all of the tissue appearing translucent white. Patterns of bleaching on a colony can vary, with only the upper surface or lower surface of the colony being affected, or bleached tissue can appear as a circular patch or in the shape of a ring or a wedge. The pattern of bleaching may give important clues as to the cause of bleaching. The nature and extent of bleaching varies between individuals within a species and among species at the same location.
Partial bleaching of tissues on a mountainous star coral, Montastraea faveolata, Panama. Photo by E.C. Peters.
Cause	Coral bleaching has been attributed to exposure to high light levels, increased solar ultraviolet radiation, temperature or salinity extremes, high turbidity and sedimentation resulting in reduced light levels, and other factors. It appears to be a generalized stress response of the coral and some species are more susceptible to bleaching than others under the same conditions. More recently, studies indicate that bacteria can cause bleaching (as reported by Kushmaro et al., 1996) and that changes in the kinds of bacteria living on the surface mucus of corals can occur when the tissue is bleached (Ritchie and Smith, 1995), but bacteria have not been implicated in other bleaching events thus far. Patchy and extensive bleaching of coral tissue has also been found to be associated with a coccidian (protozoan) microparasite that infects the coral cells in which the symbiotic algae live (Upton and Peters, 1986). Bleaching that consistently occurs on one part of the colony in relation to other parts may also be due to the presence of physiologically different symbiotic algae (Edmunds, 1994; Rowan et al., 1997).
Distribution	Bleaching has been reported on reefs around the world. Numerous mass bleaching events, in which most corals on a reef were affected, have been observed since the early 1980s, often following exposure to elevated water temperatures, some of which have been associated with El Niño climate fluctuations. Reports of bleaching are available at: the NOAA Coral Health and Monitoring Program (ftp://coral.aoml.noaa.gov/pub/champ/bleach) - this is NOT a link in this offline version, but you may copy this URL to your browser when you have web access.
Impact	Loss of symbiotic microalgae, for whatever reason, signals a disturbance in the normal metabolism of the host and can lead to changes in reproduction, tissue degeneration, reduced growth, and death of the affected tissue. Widespread partial mortalities of coral colonies have occurred as a result of bleaching Williams and Bunkley-Williams, 1990; Glynn, 1993; and Birkeland, 1997).