Coral bleaching

Coral bleaching is the loss of color of corals, due to stress-induced expulsion of symbiotic unicellular algae or due to the loss of pigmentation within the 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 or reduced water temperatures (often attributed to global warming)
o increased solar irradiance (photosynthetically active radiation and ultraviolet band light)
* changes in water chemistry (in particular ocean acidification)
* starvation caused by a decline in zooplankton levels as a result of overfishing.
* increased sedimentation (can be contributed to silt runoff)
* pathogen infections
* changes in salinity

High sea surface temperature (SST) coupled with high irradiance is known to be the primary factor in summer coral bleaching. Wind, exposure at low tide, and weather conditions can contribute to coral bleaching. Some of these factors are anthropogenic, while others occur naturally. The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots," areas where SST rises 1 degree Celsius or more above the long-term monthly average. Some argue this system detected the massive 1998 bleaching event that was worldwide. the same time, NOAA Coral Bleaching "Hotspot" program uses a 50k satellite resolution at nighttime, which some argue covers too large of a spatial area and does not incorporate the max SSTs occurring usually around height of daytime, noon.

Bleached corals

Healthy corals

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. 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 thermal stress 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.

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, and also in the southern GBR in 2006. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed on IPCC 2007 assessment, coral reefs will be highly suspecptible to increase and more frequent bleaching events with the additional problem of acidification from increase carbon dioxides within the next twenty to thirty years.

Intergovernmental Panel on Climate Change in its Fourth Assessment: Working Group II: Chapter 11 Australia and New Zealand,


"Greatest threat to GBR (i) rising sea temperatures, which are almost certain to increase the frequency and intensity of mass coral bleaching events (ii) ocean acidification, which is likely to reduce the calcifying ability of key organisms such as corals. GBR has experienced eight-mass bleaching event since 1979 ( 1980, 1982, 1992, 1994, 1998, 2002, and 2006 ) The most widespread and intense events occurred in the summers of 1998 and 2002, with about 42% and 54% of reefs affected, respectively (Done et al 2003) Berkelmans 2004). The effects from thermal stress are likely to be exacerbated under future scenarios by the gradual acidification of the world’s oceans."

Even under a moderate warming scenario (A1T, 20C by 2100), corals on the GBR are very likely to be exposed to regular summer temperature that exceed the thermal thresholds observed over the past 20 years. (Done et al 2003)"

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.

Pathogen infection


In 1996, Kushmaro, et al. reported that the agent for bleaching of the coral Oculina patagonica in the Mediterranean 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 Mediterranean Sea appeared to gain resistance to the pathogen, and further infection was not observed . The main hypothesis for the emerged resistance is the presence of 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.