Coral Bleaching Determinants and The Symbiotic Relationship Between Zooxanthellae and Corals

Coral bleaching refers to the biological process that occurs as a result of corals getting stressed by changes in conditions such as temperatures, light, or nutrients, which causes the expelling of algae living in their tissues that makes them turn completely white. Additionally, experiments and observations show that coral bleaching occurs as a result of elevated seawater temperature under high light conditions, which leads to increased rates of biochemical reactions associated with zooxanthellae photosynthesis, producing toxic forms of oxygen that disrupt cellular processes (Coles & Brown, 2003). 

Corals are a type of calcareous rock, which is primarily made off the skeletons of minute sea organisms called ‘polyps. Accumulation of the skeletons of these lime-secreting organisms is what results in the formation of coral reefs and atolls. Classification of coral reefs is done based on the nature of its shape and mode of occurrence in three categories. The fringing reef is a kind of coral reef that develops along the continental margins or along with the islands. Barrier Reef is the largest, most extensive, highest, and widest reef among all kinds of coral reefs. This type of coral reef is formed off the coastal platforms and parallel to them. Atoll reef is a ring of a narrow growing chorus of horseshoe shape and crowned with palm trees. This type of coral reef is found around an island or in an elliptical form on a submarine platform (Anwar, 2017). 

Zooxanthellae are plant-like organisms that live in the tissues of many animals such as some corals, anemones, and jellyfish, sponges, flatworms, mollusks, and foraminifera. Such microscopic algae capture light from the sun and convert it into energy just like in plants, in order to provide essential nutrients to the corals. They are species members of the phylum Dinoflagellata. They usually form a symbiotic relationship with other marine organisms, especially the coral (LaJeunesse, 2020). 

The Symbiotic Relationships Between Zooxanthellae and Corals 

The type of symbiotic relationship that exists between Coral and Zooxanthellae is mutualistic. Mutualism refers to a type of symbiotic relationship where both species benefit. Zooxanthellae which are plant cells live within Coral polyps which are anima as plant cells. The Zooxanthellae utilize the carbon dioxide and water produced by the Coral polyps to help during the process of photosynthesis. Being producers, zooxanthellae make their own food and thus they carry out photosynthesis, as such, the zooxanthellae give the coral its color and oxygen. Also, they help coral reef form their exoskeleton as shown in figure one. For the effective process of photosynthesis to occur among the zooxanthellae, coral reefs provide hard coral protection and compounds required during the process of photosynthesis. Zooxanthellae, in return for the protection offered by corals from being fed on by herbivores and other organisms, they photosynthesis organic compounds from the sun, and then transmit the nutrients, glucose, glycerol, and amino acids, which are the products produced during the process of photosynthesis which are offered to their coral hosts, that essentially gives the coral reef their beautiful colors. These photosynthetic products are then used by the corals to produce proteins, fats, carbohydrates, and calcium carbonate (Titlyanov & Titlyanova, 2020). This process is very vital, in fact, almost 90% of the nutrients produced during the process of photosynthesis in zooxanthellae are transmitted to the coral for their use. Additionally, zooxanthellae help in the excretion of waste products such as carbon dioxide and nitrogen. Thus, without zooxanthellae, the coral would die as a result of coral bleaching and if algae didn’t get protection from corals, then they will be vulnerable to herbivores and other organisms. 

Environmental Factors Affecting the Symbiotic Relationship between Corals and Zooxanthellae 

Zooxanthellae live inside various reef-building corals, which creates a symbiotic relationship between the two organisms where both of them benefit. As a result of their mutual relationship, zooxanthellae are given protection by the coral and as well utilize their waste products during the process of photosynthesis and in return the corals utilize the photosynthetic by-products for their photosynthetic by-products, specifically carbon, to respires, reproduce and build coral skeletons. Therefore, in case of any disturbance of this balanced relationship, both the corals and zooxanthellae will be affected. Environmental factors that may pose a threat to the mutualistic relation between these two organisms include pollution, temperature, and pH levels (Zhou et al., 2018). 

• Temperature: Corals are among the most threatened ecosystems on earth, mostly because of the unprecedented global warming and climate changes, which have led to increased extreme temperatures. In the past three years, corals across the world have suffered mass coral bleaching events due to the increase in global surface temperature which are caused by anthropogenic greenhouse gas emissions (Sully et al., 2019). Due to anthropogenic greenhouse gas emissions, there has been an increase in global surface temperature of approximately 1˚C due to industrialization. Due to this, a change in temperature causes corals to expel symbiotic algae residing in their tissue and responsible for their color. A slight increase in temperature from 1-2˚C for several weeks in the ocean can cause bleaching making corals turn white in color. In case of the corals getting bleached for prolonged periods, they eventually die. Consequently, coral bleaching often leads to the destruction of a large number of corals. In case of their death or expulsion of zooxanthellae from their tissue will ultimately interfere with the mutual relationship between the two. As such, there is a high chance of both of them not surviving since corals will die as a result of bleaching and zooxanthellae will be fed on by herbivores and other organisms. This is illustrated in figure two.
• Pollution: Harmful products deposited in the ocean from coastal development activities, deforestation, agricultural runoff, and oil and chemical spills can hamper coral growth and reproduction, disrupt overall ecological function, and as well cause disease and mortality to sensitive species. This means the symbiotic relationship between corals and zooxanthellae will as be affected.
• pH Levels: As a result of increasing acidity in the ocean, coral reefs are at risk since, at this condition, coral find it hard to build their skeletons (Albright, 2018). Oceans become acidic due to increased levels of carbon dioxide (CO2) in the atmosphere which is mostly from burning fossil fuels. The CO2 is absorbed by seawater resulting in a chemical reaction that generates bicarbonate (HCO3–) and carbonate (CO32-) ions. The soft-bodied coral animals (coral polyps) produce seawater having HCO3–, CO32- and calcium (Ca2+) ions into a “calcifying space” between its cells and the surface of the existing skeleton. Hydrogen ions are (H+) are pumped out of this space by coral polyps to create more carbonate (CO32-) ions, which make it easier to form calcium carbonate (CaCO3). However, when a lot of CO2 is absorbed just like how it’s happening now, more HCO3– and CO32- ions are produced in the seawater, which makes it harder for corals to accrete skeletons. Consequently, thinner skeletons will be produced making the coral vulnerable to pounding waves or get knocked out by eroding organisms. An illustration of how ocean acidification affects coral skeletons is shown in figure three.

 Coral Bleaching 

Coral bleaching or rather whitening of coral occurs as a result of the loss of coral’s symbiotic algae (zooxanthellae) or in simple terms, degradation of the algae’s photosynthetic pigment. Bleaching is usually related to the devastation of coral reefs. Coral reefs offer habitat to approximately 25% of all marine species across the universe. In figure four, coral bleaching can be seen in a shallow coral reef off the coast of the Mariana Islands. Coral bleaching makes corals lose their vibrant colors and turn white as a result of expelling zooxanthellae (Anwar, 2017). 

Several conditions can cause coral bleaching. Coral bleaching can be caused by increases in seawater temperature which resulted from raised levels of solar irradiance. It can as well be caused by ocean acidification or pollution, raised levels of sediments in seawater, or exposure of corals to sodium cyanide (a chemical used to capture coral reef fish). In this condition, zooxanthellae may lose a significant amount of their photosynthetic pigmentation, which reduces the rates of photosynthesis and thus, causes bleaching. 

Additionally, pollutants dumped into seawater can pile up and later result in viral infections in hard corals that will cause bleaching. Also, exposure of corals to increased temperatures and solar irradiance results in zooxanthellae manufacturing unusual large amounts of reactive oxygen species (molecules that have oxygen and at least one unpaired electron), which are poisonous to both the algae and their coral symbionts. These will consequently result in a breakdown of the symbiotic relationship which will inevitably cause the separation of zooxanthellae from their coral symbionts. If the algae zooxanthellae don’t reconcile in time, then corals will have to die because none of these symbionts can survive in absence of another. 

Effects of Coral Bleaching  

Since coral colonies play a substantial role as the foundations of the coral reef ecosystem, their decline due to coral bleaching may translate into a loss of habitat for numerous plants and other organisms that rely on them for their survival. Thus, lack of living space and food sources might negatively impact populations of reef-dependent fishes including other forms of marine life will drop precipitously (Pratchett et al., 2018). Also, a concern has been raised that coral bleaching might cause some species to become extinct locally. As such, the coral ecosystem affected would not be able to recover fully from the disruptions, since critical parts of the ecosystem would not be the same or no longer be available. The affected coral ecosystems are as well vulnerable to invasive species, for instance, seaweed and other types of algae, which may the ability to bring about significant and long-lasting structural changes to degraded reefs. 

Conclusion 

In conclusion, the symbiotic relationship between corals and zooxanthellae is of great importance to the aquatic ecosystem. Corals provide protection to zooxanthellae and in return, the algae provide its by-products of photosynthesis making corals grow large and vibrant. Corals offer shelter to some organisms such as coral reef fishes and a source of food for some aquatic animals. This shows that its devastation might endanger a significant number of aquatic species in the ocean. This calls for a need to protect and restore coral reefs that had been destroyed during coral bleaching through creating coral nurseries and transplanting corals into reef restoration. Thus, it is upon national and international bodies to address the dangers of climate change that might lead to extreme temperatures by prohibiting carbon emissions and use solar energy instead of fuels. 

• Figures

(Figure one ) An illustration showing symbiotic relationship between Corals and Zooxanthellae (kindpng, n.d).

(Figure two). An illustration showing threats to coral reefs ( NOAA, 2021). 

A picture Showing How Ocean Acidification Weakens Coral Skeletons

(Figure three). This is a picture taken when scientists were figuring out the effect of Ocean acidification on coral skeletons ( Barkley, 2018). 

A Picture Showing Coral Bleaching

(Figure four). This a picture showing coral bleaching at the coast of the Mariana Islands (David Burdick, n.d) 

References 

Albright, R. (2018). Ocean acidification and coral bleaching. In Coral Bleaching (pp. 295-323). 

Springer, Cham Anwar S. (2017). Interesting Facts on Coral Reefs and Atolls. 

Coles, S. L., & Brown, B. E. (2003). Coral bleaching—capacity for acclimatization and adaptation. 

Titlyanov, E. A., & Titlyanova, T. V. (2020). Symbiotic Relationships between Microalgal Zooxanthellae and Reef-Building Coral Polyps in the Process of Autotrophic and Heterotrophic Nutrition. Russian Journal of Marine Biology, 46(5), 307-318. 

LaJeunesse, T. C. (2020). Zooxanthellae. Current Biology, 30(19), R1110-R1113. 

Zhou, Z., Zhao, S., Ni, J., Su, Y., Wang, L., & Xu, Y. (2018). Effects of environmental factors on C-type lectin recognition to zooxanthellae in the stony coral Pocillopora damicornis. Fish & shellfish immunology, 79, 228-233. 

Sully, S., Burkepile, D. E., Donovan, M. K., Hodgson, G., & Van Woesik, R. (2019). A global analysis of coral bleaching over the past two decades. Nature communications, 10(1), 1-5. 

Pratchett, M. S., Thompson, C. A., Hoey, A. S., Cowman, P. F., & Wilson, S. K. (2018). Effects of coral bleaching and coral loss on the structure and function of reef fish assemblages. In Coral bleaching (pp. 265-293). 

Springer, Cham. https://www.kindpng.comhttps://oceanservice.noaa.gov/facts/coralreef-climate.html Burdick D. (n.d). Coral Bleaching.