coral bleaching

Two images of the Great Barrier Reef showing that the warmest water (top picture) coincides with the coral reefs (lower picture), setting up conditions that can cause coral bleaching.

Coral bleaching refers to the loss of color of corals due to stress-induced expulsion of symbiotic unicellular algae. The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached".

Coral bleaching is a vivid sign of corals responding to stress, which can be induced by any of:

• increased water temperatures (often attributed to global warming^[1])
• starvation caused by a decline in zooplankton levels as a result of overfishing.^[2]
• solar irradiance (photosynthetically active radiation and ultraviolet band light),
• changes in water chemistry
• silt runoff
• pathogen infections

Some of these factors are anthropogenic, while others occur naturally.

Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survives, it often requires weeks to months for the remaining symbiont population to reach a normal density. Following bleaching, corals may be recolonised by the same species of zooxanthellae, or by a different species. Different types of zooxanthellae respond differently to environmental conditions and may be more resistant to coral bleaching than other species. Some corals are known to host multiple clades of zooxanthellae within an individual coral. ^[3]. Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to dying following a bleaching event. Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching. Factors that protect against mass coral bleaching are bleaching resistance, coral tolerance, reef recovery. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce the risk of bleaching. Also, the health and genetics of both the coral and its zooxanthellae can influence the risk of bleaching.^[4]

Other reef creatures have symbiotic zooxanthellae, which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals, giant Tridacna clams and some sponges.

The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed.

Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.

Bioerosion (coral damage) such as this may be caused by coral bleaching.^[5]

Pathogen infection

In 1996, Kushumaro, et al. ^[6] reported that the agent for the coral bleaching in the Red Sea was an infectious bacteria attacking the symbiotic algae. The agent has been later identified as Vibrio shiloi. The pathogen is infectious only during warm periods; therefore, global warming would increase the occurrence of conditions that promote the spread of infection.

During the summer of 2003, coral reefs in the Red Sea appeared to gain resistance to the pathogen, and further infection was not observed ^[7]. The main hypothesis for the emerged resistance is probiotic: inside the symbiotic communities of bacteria living with the corals. One species capable of lysing V. shiloi has gained prominence. This hypothetical bacteria has not yet been identified.

Notes and references

External links

• Great Barrier Reef Marine Park Authority information on bleaching.
• ReefBase: a global information system on coral reefs.
• More details on coral bleaching, causes and effects.
• Reeffutures.org: causes, effects and monitoring of bleaching on Great Barrier Reef
• Travellers Impressions
• The Link between Overfishing and Mass Coral Bleaching
• Discussion on Overfishing and Coral Bleaching

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