Coral Bleaching

April 22ndis Earth day, where we share our love for the wonderful planet we live on and raise awareness on how we can contribute to Earth more.

Via Giphy

Lately, much attention has been drawn on the awareness that global warming is occurring on our planet. This can have rippling effects on our planet, especially to small things such as zooxanthellae. As I mentioned in the previous post, zooxanthellae have a symbiotic relationship with coral reefs. The coral is the hosts of zooxanthellae, yet they rely on each other to survive. The coral is able to perform cellular respiration, creating carbon dioxide and water, which is given to the zooxanthellae. Using these products, the zooxanthellae are able to perform photosynthesis that creates sugars, oxygen, and lipids for the coral.  Thus, zooxanthellae and coral reefs utilize each of the products they make, contributing to a continuous cycle and dependent on each other to survive. In terms of the zooxanthellae reproduction, the host coral is in charge of whether the new zooxanthellae stay or leave.

            While the coral is in charge of this process, there is a possibility for the zooxanthellae to leave on their own will. This process is known as coral bleaching, a process that occurs when the zooxanthellae leave the coral due to not having the correct environment to function in. The corals become stressed by the changes in the environmental conditions causing it to expel the zooxanthellae. The main reasons as to why they leave consists of sudden extreme high or low temperatures in the water or changes in the light they are receiving. As the figure below explains, climate change, pollution, low tides, and too much sunlight stress out the coral leading to the bleaching of it.           

Once the coral loses its zooxanthellae, it begins to starve. Approximately 60- 70% of the zooxanthellae are lost when coral bleaching begins. In these cases, the coral has lost its source of nutrients, resulting in the corals losing its color and becoming white or pale, emphasizing the name coral bleaching.

            Studies have shown that it is possible for corals to recover from the bleaching. If not too much time has passed by and the environment returns to normal conditions, zooxanthellae are able to return. However, if nothing changes and much time has passed since bleaching occurred, the coral will die. This is a sad process for the corals to deal with, and unfortunately, it is going to become more prominent if the climate changes or pollution does not stop. “In 2005, the U.S lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event”. It was caused by the extremely high temperatures in the water that year. Since that year, the temperature is consistently increasing and drastically due to the harmful actions being done to the planet. Organizations such as Coral Restoration Foundation and SECORE foundation has partnered up with multiple aquariums, such as the Georgia Aquarium, to try and preserve the corals.

            Hopefully, we will be able to fight back and conserve the many wonderful corals our oceans have to offer. This is a reminder that we can all play a part in helping the corals by caring for the earth and giving it the love it deserves. By doing so, not only will we be able to preserve the corals, but also preserve our planet that needs our help

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Life Cycle

he life cycle of a Zooxanthellae consist of two stages; the coccoid stage and the motile masticate phase. The most common phase in which zooxanthellae are found in the coccoid phase. In this stage, they become intercellular symbionts inside of the coral. They let go of their flagella and reproduction occurs. Zooxanthellae undergo asexual reproduction by a division called meiosis. This occurs in the dark and once the mother cell is exposed to light, it divides by cytokinesis. The two daughter cells released are two motile cells, which transitions to the other stage of their life cycle. Depending on the zooxanthellae, it determines how long they stay in this stage.

At this time, the zooxanthellae either stays inside of the coral and goes back to the coccoid stage or they leave the coral and stay in the motile phase until they enter a new coral. Corals are in charge of determining this for the zooxanthellae. They can choose whether to let go of the zooxanthellae, obtain new ones, or maintain the ones they have.

            Zooxanthellae are photoautotrophs, meaning that they perform photosynthesis using the benefits that the coral provide for them. Corals are able to provide them with carbon dioxide and water of cellular respiration. Once the zooxanthellae perform photosynthesis, they provide the coral with sugars, oxygen, and lipids. This helps the cycle continue and makes the relationship between coral and zooxanthellae symbiotic.

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Structure of Zooxanthellae

Zooxanthellae are a brownish color unicellular dinoflagellate that lives within Cnidaria Anthozoa, such as corals, to maintain a symbiotic relationship with. They have a spherical shape and typical range around the size of 6 um to 12.5 um

There are many different types of zooxanthellae that vary depending on the type of coral it is living within. Each type has its own genomes as well as different sizes. They tend to vary from the host they are in; however, their genomes typically range from 3,000 to 215,000 Mbps (base pairs).

They contain two flagella; one is known to trail in the water while the other wraps around the cell. These flagella fall off the zooxanthellae when it enters a host.

The zooxanthellae have photosynthetic chloroplast bounded by three membranes to produce nutrient for the host. They also include a nucleus, nuclear membrane, vacuole, lysosomes and thylakoid bodies.

It is interesting to learn about the zooxanthellae, as they are essential to the lives of corals. Without them, it would be impossible to have the wonder various corals that exist today. 

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Georgia Aquarium

One of the wonderful attractions that the city of Atlanta offers is the Georgia Aquarium. I decided to choose this place for my blog, as it has always been a fascinating site for me to observe and learn more about marine life.  I took my time to explore many of the areas in the Aquarium; in order to see and experience all the exhibits had to offer. After walking around to see all the whales, penguins and otters, I started to focus more on the inside of the tanks. I was focusing on other marine life growing in the tank that many people tend to overlook.  Seeing all the algae, coral reefs, and kelp in the background made me notice how overlooked they tend to be, especially with larger creatures swimming around them constantly.

To be honest, I also overlooked them, and it was not until I entered “The Coral Kingdom” exhibit that I started to appreciate the beauty of these corals. 

The Aquarium has a huge glass tank in the middle of this exhibit where many of the corals’ habitat.  It is a wonderful place to stop and take pictures, as well as learn about the many different species of corals the aquarium houses. I was able to talk to an employee who works there to learn more about the exhibit. They told me about all the research they conduct and showed me an interactive touch screen that displays information about the different corals.


After learning all this new information, I instantly knew I wanted to choose a microbe relating to the corals. That is when I read a sign, which read. “ Most other corals have algae called zooxanthellae living in their tissue, helping the corals obtain nutrition by converting energy from the sun”. That is when I decided to conduct my project on zooxanthellae.

I was unable to get a photo of the zooxanthellae as they live in the tissue of the coral, yet I was able to get photos of some the corals they reside in.


It was an all-around fun experience exploring the Aquarium and I am excited to learn more about zooxanthellae.