Coral Bleaching

Coral bleaching is defined as the loss of algal symbionts and/or their pigments in response to environmental stress conditions.  Tropical symbiotic corals, cnidarians, and mollusks exhibit signs of bleaching.  Environmental stresses include decreased salinity, sedimentation, darkness, exposure at low tide, fluctuations in temperature, and solar radiation.  One stress, or a comination of several stress factor, can result in a sudden reduction in the density of algae in corals.  Corals turn white because of the loss of pigments from the algae (Fitt, Brown, Warner, Dunne, August 2001, p. 57-65.)

                                 
             Image 12:  Bleached coral                  Image 13:  Bleached coral

Bleaching is a new phenomenon that was identified in the 1970s.  The increase in freqeuncy of coral bleaching incidences worldwide is attributed primarily to increases in sea temperature and levels of solar radiation.  Photosynthetic symbionts of coral become vulnerable to light damage at higher sea temperatures.  The increase in sea temperature results in the expulsion of zooxanthellae from the host, leaving only the white tissue of the coral.

The increase in sea temperatures worldwide has been correlated to increased concentrations of carbon dioxide and greenhouse gases in the atmosphere.   A 0.07 to 0.5 degree Celsisus increase in sea temperature has been reported per decade.  By 2100, scientists expect sea temperature to increase by 1 degree to 2 degrees Celsius per decade.  Currently, coral bleaching occurs ever three or four years, however, bleaching is expected to occur annually over the next thirty to fifty years. 


Image 14:  Map of 1997-1998 Bleaching Event

Scientists Coles and Jokiel were the first to correlate coral bleaching with elevated sea temperatures.  Their research also demonstrated seasonal differences in thermal tolerance of corals and the importance of length of exposure time to upper lethal temperature limits (Fitt, Brown, Warner, Dunne, August 2001, p. 53).

Three Types of Coral Bleaching

There are three different types of coral bleaching: physiological, algal-stress, and animal stress,

Physiological constraints affect the size of population of symbionts living in coral host cells.  Coral tissue can only hold a certain number of symbiotic dinoflagellates, therefore, the density of symbionts is directly related to the amount of host tissue.  Each coral may hold different densitites and changes in the steady-state densities of symbionts in host tissue may vary due to environmental varibales.  In addition, seasonal changes in solar radiation and sea temperature occur throughout reefs worldwide.  Research indicates that seasonal changes affect the density of zooxanthellae in coral tissues as well.  Densities of symbionts are at their highest during cool seasons with low levels of radiation, whereas densities are at their lowest during very warm seasons with the high levels of radiation (Fitt, Brown, Warner, Dunne, August 2001, p. 54).

Algal-stress bleaching occurs because of physiological stresses that results in dysfunction of symbiotic algae.  Photoinhibition contributes to algal-stress bleaching.  Chronic photoinhibition is defined as non-reversible photodamage, involved with the the malfunction of one or more biological pathways involved in the synthesis of components of the photosystems.  Zooxanthellae die as a result of chronic photoinhibition.   The damaged symbiotic dinoflagellates are then released from the coral host by exocytosis or host cell detachment.  Dynamic photoinhibition is reversible photostress that occurs in algae.  Zooxanthellae avert extra photons away from photosystem II through the xanthophyll cycle in order to keep excess light from damaging the photosynthetic components of the cells.  Exposure time to particularly high sea temperatures and solar radiation determine whether chronic or dynamic photoinhibition occur (Fitt, Brown, Warner, Dunne, August 2001, p. 54-55).

Although research shows that symbiotic dinoflagellates have less tolerance to increases in sea temperature and solar radiaton than their coral hosts, animal components of reef systems can be affected as well.  "Bleaching is the result of relative tolerance of the algal versus the animal components" of the symbiotic relationship.  Shedding of host issue that contains symbiotic algae is not prevalent, but laboratory experiments involve extreme temperature changes have induced animal stress bleaching.  Stress on corals, the animal cells, could cause a release of algae, however, it is unlikely that animal stress bleaching would occur on a large scale (Fitt, Brown, Warner, Dunne, August 2001, p. 55).

Scientist employ a variety of both intrusive and non-intrusive techniques to determine what type of bleaching occurs on reefs.  Although bleaching is reversible, corals die in great numbers following bleaching events.  If a coral does recover, its health is compromised for hundred of years.  Widespread coral bleaching has many potential detrimental impacts on not only the health of coral reef ecosystems, but the peoples who depend on them for their livelihood as well.