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Coral reef bleaching and its connection to coral bacterial diversity

Updated: Jul 10, 2022

- by Stanca Ioana.

Due to the constant increase of temperature in the Pacific Ocean, corals started to bleach in 1998, and as of 2022, the Great Barrier Reef Marine Park authority confirms an unprecedented sixth mass coral bleaching in different regions with a range of impact from low to severe.

The sixth mass coral bleaching event

“The Great Barrier Reef has been hit with a sixth mass coral bleaching event, the marine park’s authority has confirmed, with aerial surveys showing almost no reefs across a 1,200 km stretch escaping the heat.” according to Graham Readfern when he talked about the recent bleaching event in his article in the Guardian.

To be able to understand these events, let`s take a closer look into what composes a coral reef.

Coral reef composition

Reefs are complex ecosystems that consist of a variety of species including animals, plants, microorganisms, and even viruses and they represent one of the largest biological structures to origin from our planet estimated to provide a habitat for over 25% of all marine species according to Connell (1978).

The ecosystem works similar to a complex network, hosting the largest variety of symbiotic associations in the marine environment by providing shelter and food resources for them. Corals hold symbiotic relationships with more than microorganisms and protozoans, including nematodes, trematodes, cestodes, annelids, mollusks, and crustaceans by creating a diverse and complex system of trade.

Scleractinia corals are frequently the dominating species in reefs due to their ability to deposit calcium in their skeletons, allowing them to form a solid base to grow up the ecosystem. They are colonial organisms consisting of millions of individuals connected by living tissue representing the largest order within the phylum Cnidaria, The association of individuals that constitutes a scleractinian coral is called a polyp (Bourne et al., 2013).

Polyps are shaped similar to a cup with tentacles that carry stinging cells all around the opening of the structure. With the help of these specialized cells, scleractinian corals capture zooplankton and defend themselves (Bourne et al., 2013).

Even though they gather energy through zooplankton, the majority of coral energy relies on their symbiotic relationship with photosynthetic microalgae commonly known as zooxanthellae.

Living scleractinian corals on the Great Barrier Reef, Queensland, Australia. (Source: Image by Toby Hudson, Digital Atlas of Ancient Life, )

The importance of prokaryotic and zooxanthellae communities in reefs

Prokaryotic communities occupy a vast and diverse habitat such as sediment, corals, sponges, and the water around the reefs. These microorganisms have important roles in the development and functional structure of the reef such as fixation and passage of nitrogen and carbon to the coral host and the metabolization of organic matter which is later released into the reef`s ecosystem.

Through the ability to capture and recycle nutrients, microorganism along with corals allow the ecosystem to thrive in waters that are poor in nutritive matter through pelagic and benthic processes. Furthermore, corals obtain their fantastic bright colors from the microscopic algae zooxanthellae residing in their gastrodermal cells, that is why when the bleaching effect occurs, corals become white or transparent, losing not only their color but also the dinoflagellates that provide their pigmentation and their means of survival.

Coral-algae symbiosis

Corals thrive off of their relationship with their symbionts, one of the most studied relationships is with the endosymbiotic algae called Symbiodinium which converts sunlight and carbon dioxide into organic carbon and oxygen. This interaction benefits both symbionts as the corals provide the algae with a safe environment and the compounds they need for photosynthesis including metabolic waste such as nitrogen, phosphates, and through respiration carbon dioxide. The algae help corals with growth and calcification as well as benefiting other biodiversity by creating a safe habitat for them in the reefs, by providing oxygen, glucose, glycerol, and amino acids from the photosynthetic process (Bourne et al., 2013).

However, even if this symbiosis is very beneficial, it does have its downsides. The algae have to safely harvest sunlight and dissipate the excess energy which wasn’t used in photosynthesis, otherwise, the oxidative stress can settle in and cause the bleaching of the corals.

Influences of the temperature on the reef symbiosis

Climate change is not without consequences, this constant modification in ocean temperature levels has triggered an imbalance in the microbiome and led to a state of dysbacteriosis which has exposed the corals to diseases and bleaching through the emergence of pathogenic species. With this in mind, it is important to consider the major role of the microbiome in coral reef restoration, as any disturbances to the normal function of the ecosystem are shown early by irregular behavior in the microbial communities leading to the bleaching effect.

Coral bleaching on The Great Barrier Reef

Coral bleaching

As we have already established the meaning of coral-algal symbiosis, Warner, Fritt, and Schmidt studied the effects of this symbiosis and its possible correlation to bleaching. Is it possible corals are starting to bleach because of the malfunctions of the algae photosystem?

Coral bleaching is defined as a general phenomenon in which corals turn visibly pale because of the loss of pigments and their endosymbiotic dinoflagellates during or after exposure to a higher temperature than normal.

Warner, Fritt, and Schmidt tell us in their article on photosystem II (PS II) in correlation with coral bleaching, that heat changes the normal activity of dinoflagellate PS II which creates in return damage through heat-stress. This damage alone disrupts the normal activity of both symbionts, correlating the damage to the photosystem II of the endosymbiont to the loss of protein D1 (Warner et al., 1999). Protein D1 is the reaction center of this photosystem, when damaged or destroyed, the whole photosynthesis mechanism malfunctions. Thus, without a working photosynthetic system, both symbionts deteriorate.

However, a bleached coral does not mean that the coral is dead. In this state, the corals are more susceptive to diseases and starvation. The heat stress induced by climate change, along with the contaminants in seawater and ocean acidification are still the main threats to coral reefs today. Increases of sediments and nutrients in seawater can lead to a build-up on the ocean floors because of massive growth in the algal communities leading to reduced light levels and towards coral asphyxiation.

Hope for the future

The Great Barrier Reef Foundation has deployed a research and development program for coral restoration and adaptation on a large scale. Through their efforts, corals have been protected by heat stress through cooling and shading techniques as well as the identification of genetic marker indicators for coral resilience to heat stress in order to identify and enhance these properties in new generations of corals.

The foundation aspires to restore and protect the reef islands through the Reef Islands Initiative (2018–2025). Their effort has proven successful on Raine Island (2015) which is hosting the world`s largest remaining green turtle nesting population and as well in Lady Elliot Island (2018) by building a nursery for native coral cay species. Many of their initiatives and achievements are slowly bringing back the unique but vulnerable marine life.­­


1. Bourne D.G., Webster N.S. (2013) Coral Reef Bacterial Communities. In: Rosenberg E., DeLong E.F., Lory S., Stackebrandt E., Thompson F. (eds) The Prokaryotes. Springer, Berlin, Heidelberg.

2. Bourne, D. G., Morrow, K. M., & Webster, N. S. (2016). Insights into the Coral Microbiome: Underpinning the Health and Resilience of Reef Ecosystems. Annual Review of Microbiology, 70(1), 317–340. doi:10.1146/annurev-micro-102215-095440

3. Connell, J. H. (1978). Diversity in Tropical Rain Forests and Coral Reefs. Science, 199(4335),1302–1310.doi:10.1126/science.199.4335.1302 10.1126/science.199.4335.1302

4. Roth M. S. (2014). The engine of the reef: photobiology of the coral-algal symbiosis. Frontiers in microbiology, 5, 422.

5. Vanwonterghem, I., & Webster, N. S. (2020). Coral Reef Microorganisms in a Changing Climate. iScience, 23(4), 100972.

6. Warner, M. E., Fitt, W. K., & Schmidt, G. W. (1999). Damage to photosystem II in symbiotic dinoflagellates: A determinant of coral bleaching. Proceedings of the National Academy of Sciences, 96(14), 8007–8012. doi:10.1073/pnas.96.14.8007


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