Coral bleaching has already caused widespread death of huge areas of coral reefs around the world. Even Australia's prestigious Great Barrier Reef is massively affected.

The bleaching of corals is a natural response to environmental stress. Coral bleaching occurs when the relationship between the coral host and its dinoflagellate symbionts breaks down, typically prompted by extreme environmental conditions such as successive warming or cooling or excessive levels of light/UV or wind exposure (at low tide). Consequently, bleached corals are less fit and vulnerable to mortality and/or disease.

When entire communities of corals become stressed at the same time it is called a mass-bleaching event. Mass bleaching is becoming increasingly prevalent and severe worldwide, and in some locations, thousands of square kilometers of coral reef have been affected.

Coral Bleaching at Lizard Island

Coral bleaching off Lizard Island

Image: Lyle Vail
© Australian Museum

Symbiosis & Affects

Many types of coral have evolved to form a special symbiotic relationship with a type of tiny marine algae (called zooxanthellae) in order to survive. Photosynthetic pigments of the zooxanthellae are responsible for the stunning colours of hard corals. Zooxanthallae live inside the coral tissue to avoid predation and like other plants the tiny algae produce energy by photosynthesis.

  • If a coral becomes stressed it will expel its symbiotic algae. Without zooxanthellae, the tissue of the coral animal appears transparent and the coral’s white (calcium carbonate) skeleton is revealed.

Coral bleaching is the whitening of corals, due to stress-induced expulsion of the corals' symbiotic algae.

Once a coral bleaches it begins to starve. While coral can obtain some nutrition from the plankton they capture with their tentacles, most shallow-water corals struggle to survive without their zooxanthellae, so bleached corals often die if the stress persists.

If conditions return to normal, corals can regain their zooxanthellae, return to normal colour and survive. This stress, however, is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.

Coral reefs suffering severe mortality following bleaching can take many years or decades to recover.

What causes coral bleaching?

The primary cause of coral bleaching is high water temperature. Temperature increases of only 1.5 –2°C lasting for six to eight weeks are enough to trigger bleaching.

When high temperatures persist for more than eight weeks, the coral's symbiotic algae produce highly corrosive radicals within the coral. That damages the photosynthesis system of the algae. The coral can't prevent the division of the algae cells - that leads to an even higher carbon production, to be diverted into the algae rather than the coral. Due to this imbalance the coral expels the algae.

The ability for corals to tolerate stress is influenced by the identity and diversity of algal symbionts (Symbiodinium spp.). Some strains of Symbiodinium are more thermally tolerant than others.
Fluorescent proteins in the zooxanthellae and coral tissue also reduce the severity of physiological damage caused by bleaching by reducing phototoxic effects of light in conditions of elevated temperature.

Other physiological defences that help reduce the severity of bleaching include: non-fluorescent proteins; mycosporin-like amino acids; antioxidants and heat-shock proteins.

Relationship of water temperature to coral bleaching along the Great Barrier Reef

Two images showing the relationship of water temperature to coral bleaching along the Great Barrier Reef. Warm pink and yellow tones show where sea surface temperatures were warm in the top image. The warmest waters are the shallow waters over the reef near the coast, where coral bleaching was most severe in the summer. The lower image shows chlorophyll concentrations, where high concentrations (yellow) generally point to a high concentration of phytoplankton in surface waters of the ocean. In this image, the bright yellow dots actually represent the coral reefs, and not surface phytoplankton.

Image: unknown
© Public Domain (NASA)

Are there more reasons for coral bleaching?

Yes, there are all kinds of "stress factors". Apart from higher water temperatures, also long term lower-than-normal sea temperatures cause the expulsion of the corals' symbiotic algae.

Whether corals recover or not from this stress will also be determined by other factors such as water quality. Major water quality variables affecting coral reef health include water temperature, salinity, nutrient and suspended sediment concentrations, as well as toxicants including pesticides.

Sedimentation caused by human activities such as land clearing and coastal construction can result in high rates of erosion and a higher density of suspended silt particles which can lead to reef destruction through burial, disruption of recruitment success or deleterious community shifts.
Sediment affects coral by: smothering when particles settle out (sedimentation), reducing light availability (turbidity) and potentially reducing coral photosynthesis and growth.

Elevated nutrient concentrations affect corals by promoting phytoplankton growth, which in turn supports increased numbers of filter feeding organisms such as tubeworms, sponges and bivalves that compete with coral for space. Macroalgal blooms can also result under enhanced nutrient regimes and macroalgae may overgrow coral structures, out-competing coral for space and shading coral colonies to critical levels.

Excessive phosphorus concentrations result in coral colonies with weakened skeletons, which make colonies more susceptible to damage from storm action. Elevated nutrients can inhibit fertilization rates and embryo formation of corals, as well as causing direct coral mortality.

Shipping accidents resulting from groundings, vessels sinking, illegal and accidental oil and diesel spills, and loss of litter overboard can pollute coral reefs.

Hence, increased temperature is just one of many effects of climate change on the Reef but it is of great concern because it can affect reefs at regional or global scales. The level of additional stress at the local scale (i.e. local water quality) determines the potential for corals to recover from bleaching events.