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Coral Bleaching

Author: Vanessa Salazar

Coral reefs are the most diverse aquatic ecosystem in the world, supporting 25% of marine species. But from 2008 to 2019, 14% of the world’s coral reefs died as a result of coral bleaching (Global Coral Reef Monitoring Network, 2020).

What is Coral Bleaching?

Coral bleaching refers to the ejection of the symbiont zooxanthellae (algae) living on coral tissue, which resultsin the white color. This does not necessarily mean coral reefs are dead but, as these algae are their primary food source, it leaves the reefs more susceptible to disease and mortality. Coral bleaching occurs when reefs are under stressful conditions: often warmer temperatures, polluted water, overexposure to air and sun, and more (National Oceanic and Atmospheric Administration, n.d.).

A coral reef in American Samoa showcasing what a bleaching event can look like for reefs (Catlin Seaview Survey, n.d.).

Why Does This Happen?

A common misconception regarding coral bleaching is its relationship with chemical sunscreens. A mainstream idea has been that chemical sunscreen is the leading cause of coral loss through runoff. This is simply untrue. While chemical sunscreens have “potential hormonal/oestrogenic activity and non-hormonal effects, including acting as teratogens, altering gene regulation, inducing changes in antioxidant and free radical production, and inducing coral bleaching,” (Wheate, 2022) these reactions are only possible in an oceanic setting in concentrations that far exceed “(10-10 000-fold)” levels found in sea samples.

In fact, increased ocean surface temperatures was identified as the primary cause of coral loss. This correlates with rising sea temperatures due to carbon emissions changing global temperature averages. Increased carbon dioxide can also dissolve into waters and acidify the ocean. Elevated sea temperatures disrupt the photosynthetic abilities of algae, interrupting their relationship and resulting in bleaching. During bleaching, reduced chlorophyll can cause solar radiation in coral tissue, resulting in permanent damage to photosynthetic algae (Galindo-Martínez et al., 2022). Not only extreme heat temperatures but extremely cold temperatures have resulted in coral death, like in the case of the Florida Keys in which the temperature dropped -6.7°C (National Oceanic and Atmospheric Administration, n.d.).

Hope for the Future

Coral bleaching rates are directly proportional to climate change. Increased extreme oceanic temperatures are a result of the exponential rise in carbon emissions globally. Maintaining climate change effects to 1.5°C in accordance with the Paris Climate Agreement would provide security for the remaining reefs, ("Coral reefs and climate change", 2021) while addressing smaller issues like local pollution and unethical fishing can alleviate coral bleaching to some extent, it would be fruitless without regulated emissions. There also needs to be increased research on genetic editing for heat-resistant corals.

These efforts have been applied to previous coral reefs and studies have shown that after the removal of stress, reefs were able to recover. In 2019 a slight improvement was seen in 2% of reefs regaining their coral covers. Coral reefs are not gone yet; they continue to be resilient and, with proper action, their once-flourishing state can be regained.


Catlin Seaview Survey. A coral reef in American Samoa, photographed before, during, and after a bleaching event [Image]. Retrieved 14 May 2022, from

Coral reefs and climate change. IUCN. (2021, March). Retrieved 15 May 2022, from,of%20large%20amounts%20of%20corals.

Galindo-Martínez, C., Weber, M., Avila-Magaña, V., Enríquez, S., Kitano, H., Medina, M., & Iglesias-Prieto, R. (2022). The role of the endolithic alga Ostreobium spp. during coral bleaching recovery. Scientific Reports, 12(1).

Global Coral Reef Monitoring Network. (2020). Status of Coral Reefs of the World: 2020. [Ebook]. Retrieved 15 May 2022, from

National Oceanic and Atmospheric Administration. (n.d.). What is coral bleaching?.,This%20is%20called%20coral%20bleaching.

Wheate, N. (2022). A review of environmental contamination and potential health impacts on aquatic life from the active chemicals in sunscreen formulations. Australian Journal Of Chemistry, 75(4), 241-248.

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