Coral Bleaching: Potential Mechanisms and Observed Adaptations

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Coral Bleaching: Potential Mechanisms and Observed Adaptations

Coral reefs are the most biologically diverse marine ecosystems. Driving this diversity are cnidarian corals which are obligate mutualistic symbioses between coral animals and dinoflagellate algae of the genus symbiodinium. These algae are commonly called zooxanthellae. This symbiosis between heterotrophic hosts and photosynthetic symbionts allows coral to thrive in nutrient poor seas and deposit calcium carbonate to build reefs (Toller et al. 2001, 348).

Coral bleaching occurs when there is a loss of zooxanthellae from their coral host. This is accompanied by loss of a coral's normal color that may or may not be detectable to the human eye. Bleaching can lead to coral death when prolonged, although links between bleaching and subsequent coral recovery or death are not well known. Causes of bleaching have been observed to include salinity, exposure to air, and sedimentation. The primary reasons cited to cause most instances of coral bleaching are increased sea temperatures and solar radiation. These two factors have been observed to cause the most damage in combination (Fitt et al. 2001, 51).

The two types of bleaching that result from organism damage or disturbance are algal-stress bleaching and animal-stress bleaching. Algal-stress bleaching is characterized by expulsion of individual zooxanthellae from a host cell. Animal-stress bleaching is characterized by host cell detachment, which is expulsion of zooxanthellae from the coral animal along with the host cell they occupy. Radiation and temperature induced animal-stress bleaching and host cell detachment have not been observed in the field at current sea temperatures. This type of bleaching has only been observed under laboratory conditions (Fitt et al. 2001, 55). A third type of bleaching, physiological bleaching, occurs in an unstressed coral-zooxanthellae holobiont as a method of shedding excess zooxanthellae from host cells. Zooxanthellae density within host cells varies seasonally in this manner. Maximum zooxanthellae density is observed to occur during the coolest season with the lowest solar radiation. The minimum density is observed to occur at the end of the warmest season with the highest solar radiation (Fitt et al. 2001, 54).

The temperature at which coral bleaching occurs has been observed to be from 30 to 34 degrees Celsius (Ralph et al. 2001, 163). Solar radiation exacerbates bleaching in this temperature range and can also cause bleaching at ‘normal’ temperatures when at sufficient intensity.

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