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  • Melissa Pappas

Coral Bleaching

What even is it and why should you care?



Most people have never and will never see a coral reef. That is not only because most people don’t have the luxury of living near them or the capacity to travel to them, but also because we, the human race, are doing a pretty good job of destroying them. Of course, we never set out to destroy these awe-inspiring creatures that build entire ecosystems teaming with the most biodiverse underwater life, but their demise is a byproduct of our modern lifestyle in what scientists refer to as the Anthropocene, or the epoch in which human activity has been the most influential over the Earth’s climate. Coral reefs are not the only ecosystem feeling the immense anthropogenic pressures, but they are one of the first and said to be the canary in the coal mine for climate change. This is because corals are extremely sensitive to increases in temperature, a major consequence of climate change. As the ocean warms, we are seeing a consistent increase in the summertime maximum temperatures to which corals and other marine organisms are exposed. Most corals are already living at their thermal threshold, and an increase in 1-2°C is enough to cause major physiological damage and coral starvation through a process called bleaching. The term “coral bleaching” has been popping up more frequently in the media, as it is now starting to occur annually on most tropical reefs. Many people are unfamiliar with these creatures, thus grasping the concept of coral bleaching is challenging and seemingly irrelevant. However, in reality, coral bleaching affects hundreds of millions of people, as reefs provide food, coastal protection from storms, and economic value through tourism. Now, the loss of these ecosystems is at higher risk as bleaching is occurring more frequently and intensely than ever before.


Born and raised in the Arizona desert, I myself had no conception of what lived in the ocean until I was old enough to travel to California and explore the little worlds of life in the tide pools. I’d get sucked into these tide pools containing sea anemones, urchins, and hermit crabs, with little indication that anything in them could harm me. Little did I know, we were doing more harm to these marine creatures than they ever could or would do to us. When I was 14, I decided I wanted to become a marine biologist. Through a series of fortunate events and tons of support from my family, I learned how to SCUBA dive, got a degree in marine science, and now continue to study the fascinating marine world of after diving the most colorful reefs I have ever seen in the Red Sea. I have never taken my path or opportunities for granted, but now I realize I have been taking the health and abundance of coral reefs for granted. The more I learn about how our modern lifestyle driven by consumption of disposable goods, processed food, dirty energy sources, and our mindset that nature is meant to be conquered, is killing the very things I love and the hope for these ecosystems to exist in the future, not only for our kids and grandkids to enjoy, but to rely on as well, I feel compelled to be a translator for these ecosystems, because they do have their own voice, but many chose not to listen or understand. and an advocate for awareness and change.


To speak for the ocean and coral reefs in particular, I must first share the facts of coral bleaching, currently, one of the largest threats to coral reefs. Corals are animals similar to jellyfish, as they are in the same family “Cnidaria,” both containing stinging cells called nematocysts. Corals, unlike jellyfish, are colonial animals, as hundreds to thousands of coral polyps (individual corals) grow together over the top of their calcium carbonate skeleton joined by connective tissue.



Corals like jellyfish use their stinging cells in their tentacles to capture prey in the water column. An organism that consumes other living matter is considered a heterotrophic feeder, while organisms that get their food from photosynthesis, or the power the sun, are considered autotrophic. Corals are both heterotrophic and autotrophic because they have a symbiotic relationship with microscopic single-celled algae called zooxanthellae (literally meaning yellow animals) that live within the coral’s endodermis (outer skin-like layer). These algae, although tiny and short-lived, have provided corals with the ability to survive and thrive in nutrient-poor environments like tropical waters. The relationship between corals and these algae has existed for millions of years and has allowed the coral animal to create such impressive structures and entire ecosystems that the marine world has come to rely on. Unfortunately, the stability of this relationship is being threatened as disturbances in the environment such as poor water quality, changes in pH and salinity, and increases in water temperature disrupt the communication between coral and algae. When the relationship between coral and algae is no longer beneficial, the algae vacate the coral. Because the coral relies on the algae to provide it with up to 99% of its food source through photosynthetically-produced sugar, the loss of the algae leads to coral starvation. Although corals can gather organic matter from the water column with their tentacles, there isn’t much available on the reef. When zooxanthellae leave the coral, it appears white because the algae give the coral its color. Typically, coral tissue is transparent with few cells that exhibit color, thus, when the algae leave, we see straight through the coral tissue to the white skeleton. This is why the phenomenon of algae vacating its coral host is termed coral bleaching.



Once bleached, a coral does not die immediately, however, it does become compromised as it is not receiving nearly enough carbon to sustain itself. Bleached corals are more susceptible to disease just like we are more susceptible to sickness if we are starving. Once bleached, corals do not have much time to recover before they die. It is in this critical stage where water temperatures need to return to normal to allow corals to take up their algae from the water column or repopulate their cells with remnant algae in their tissue. Corals have an amazing potential to recover from environmental disturbance, but if these disturbances interrupt the recovery process, the disturbance becomes chronic and full recovery is no longer possible. To put things into perspective for coral recovery from bleaching, coral colonies have been found to recover their algal population in less than a year (spanning from 5 to 10 months), but entire reefs require decades to recover from bleaching. Currently, mass bleaching events are predicted to occur annually on most reefs by 2050 and some reefs, including the Great Barrier Reef (GBR), by 2020. We have already started to see these predictions come true as shown by a major bleaching event on the GBR in February 2020. Effects of climate change are staring us in the face and if we hope to save these ecosystems, we have to act now.



The problem of climate change can seen overwhelming and it is true that efforts to combat this global issue must be executed at a global scale, we as individuals can make a difference. Coral reefs and all other ecosystems need many imperfect eco-warriors more than just a few perfect ones.





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