Coral Bleaching

By Katherine Bethell

Marine invertebrates called coral are essential for maintaining the health of the world’s oceans; they provide a habitat for a diverse range of plant and animal species whilst protecting coastal areas by absorbing wave power. However, the so-called bleaching of coral threatens its survival and function globally. 

Bleaching is characterised by the whitening of coral, leaving behind a reef of skeletal structures void of marine life. This can be caused by the loss of algae called zooxanthellae, which usually live-in symbiosis with the coral, or loss of the algae’s photosynthetic pigment (Brown B, 1997). Without the algae or its photosynthetic pigment, the rate of photosynthesis is significantly reduced which causes coral bleaching. 

One of the biggest causes of coral bleaching is associated with rising sea temperatures, largely due to climate change. Large coral bleaching events which were recorded in the late 1990s were more common in coastal tropical regions. However, as sea temperatures have risen so has the number of bleaching events which appear to be more severe and occur increasingly in northern locations away from the equator (Hughes T et al., 2017). Climate change can directly increase sea temperatures due to the ‘greenhouse effect’ whilst damage to the ozone layer can affect coral through solar irradiance. Both visible and ultraviolet radiation received from the sun can induce bleaching as the algae’s photosynthetic pigment is reduced due to DNA and protein damage (Lesser M and Farrell J, 2004). In addition, high water temperatures  and exposure to solar irradiance largely cause bleaching as they cause zooxanthellae to produce toxic reactive oxygen species which cause the algae to separate from the coral, resulting in the whitening of coral. 

Zooxanthellae can also be affected by changes in seawater chemistry. For example, ocean acidification ,which is largely caused by climate change with carbon dioxide absorbed from the atmosphere causing a decrease in the pH of seawater, effects the algae’s ability to photosynthesise. Pollution has a similar effect with increased levels of nitrates and phosphates in seawater, largely caused by industrial waste and agricultural runoff, reducing levels of the algae’s photosynthetic pigments (Baum G et al., 2015). It is also believed that certain chemicals in some sunscreens and other skin care products may accumulate in areas of water heavily used in the marine tourism industry causing changes in the waters chemistry and contributing to coral bleaching events. 

As mentioned earlier coral reefs provide a safe habitat for countless species whilst also protecting the worlds coastlines from things like coastal erosion or even tsunamis by reducing the power of waves. However, when coral is bleached its characteristic whitening does not provide a secure habitat and its weakened structure reduces the absorption of wave power. Coral also becomes more susceptible to disease and has higher mortality levels so whole swathes of reef can be lost.

There are simple things everyone can do to help prevent coral bleaching and these include using reef-safe sunscreens, avoiding touching corals whilst diving and preventing water pollution. However, as the largest cause of coral bleaching is believed to be climate change the only way we will fully prevent bleaching events in the future is to combat the climate crisis now and prioritise reducing carbon emissions. 

Over the past few years there has been interest in the idea of ‘replanting’ damaged or lost coral reefs. These pilot projects aim to form coral nursery’s where asexually derived corals are planted back into the wild where they restore damaged, bleached reefs. Although, coral restoration may help to rebuild the ecosystems lost due to coral bleaching some species of coral are likely to be lost forever as they are unable to adapt to the changing environment. Asexual restoration methods also do not promote genetic diversity leaving new coral reefs vulnerable to disease or extreme conditions. 

To conclude, widespread coral bleaching will have immense consequences as the species which rely on coral reef ecosystems find it more and more difficult to survive without the protection and food provided by healthy corals; this is likely to result in the extinction of several marine species. It is difficult to reverse the effects of bleaching and instead priority has been focused on prevention and restoration by replanting coral. It is hoped in the future these measures alongside efforts to reduce the temperature of the oceans will help protect this vital marine habitat. 

References:

Brown B. Coral Bleaching: Causes and Consequences. Springer. 1997; 16: S129-138. Available from: DOI: 10.1007/s003380050249

Hughes T, Kerry J, Alvarez M. Global Warming and recurrent Mass Bleaching of Corals. Nature. 2017; 543: 373-377. Available from: DOI: 10.1038/nature21707

Lesser M, Farrell J. Exposure to Solar radiation Increases damage to Both Host Tissues and Algal Symbionts of Coral During Thermal Stress. Coral Reefs. 2004; 23: 367-377. Available from: DOI: 10.1007/s00338-004-0392-z

Baum G, Januar H, ferse S, Kunzmann A. Local and regional Impacts of Pollution on Coral Reefs along the Thousand Islands North of the Megacity Jakarta, Indonesia. PLOS ONE. 2015. Available from: DOI: 20.1371/journal.pone.0138271

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