Coral Bleaching - What is it and why should we care?

Coral Bleaching - What is it and why should we care?

Photo Source: Jurgen Freund, World Wildlife Fund

Coral and coral reefs are exquisite, awe-inspiring, and essential to the health of the ocean. Without coral, over 25% of marine life would cease to thrive. Global climate change and coral bleaching are directly linked. The increase of global climate change and changing temperatures in the ocean have caused mass coral bleaching events and die-offs. Let’s dive in and learn more about coral, zooxanthellae, bleaching, marine productivity, and what’s being done right now to help stop coral bleaching.

Coral 101

First off, let’s talk coral. A lot of people assume (and rightfully so) that coral is a plant. However, coral is actually a sessile animal. They’re invertebrates in the Anthozoa class and phylum Cnidaria. A colony of polyps makes up what we call corals. This means that polyps are the small animals in the group, and coral is a group that is made up of individual polyps. These colonies take root in the ocean floor and along reefs which gives them a plant-like structure and appearance. Countless animals, plants, and life forms live in and around coral, which can make up habitats such as coral reefs. Below is a figure of a polyp and different parts of the polyp. Fun fact: corals are nocturnal! They will stick out their tentacles and become active at night for feeding. 

Photo Source: Emily M. Eng via Coral Reef Alliance

What Is Bleaching?

The National Oceanic and Atmospheric Administration (NOAA) describes coral bleaching, “When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.” The figure below shows a bit of that process and examples of healthier coral vs. coral without the algae. There are some smaller bleaching “events” where coral can eventually survive, heal, and recover, but generally, it is difficult and the coral is left vulnerable and malnourished. In regards to coral bleaching, there are many stressors, changes, and triggers that can cause it. Coral is a living organism and it doesn’t always react to stress well. This stress can include changes in temperature, light, and nutrients (NOAA). Let’s learn more about the symbiotic algae, zooxanthellae, and their role. 

Photo Source: the National Oceanic and Atmospheric Administration (NOAA)

Life In Color With Zooxanthellae

When it comes to the beautiful color of corals, it’s the zooxanthellae that are the true MVPs. Zooxanthellae are photosynthetic algae that live within the tissues of coral. If you read our previous blogpost on symbiotic relationships, specifically mutualisms, you can see why coral and zooxanthellae are a prime example of this! The zooxanthellae receive a safe home to function, photosynthesize, and stay protected; and the coral receives nutrients (such as oxygen, glucose, glycerol, amino acids) and a way to conduct waste removal. The zooxanthellae are responsible for the vibrant and iconic colors of coral, so when they are expelled or leave, it reveals the coral’s white skeleton. When a coral becomes “bleached,” it is not necessarily dead yet. However, when a coral becomes bleached, it is under much more stress and if it goes too long without zooxanthellae, it will starve and die. To learn more about zooxanthellae and the processes and exchanges that occur between coral and these algae, click here

Coral’s Club

Coral reefs are essential for thousands of marine animals. A few examples of species that need coral reefs are sea turtles, fish, crabs, shrimp, jellyfish, sea birds, and starfish (World Wildlife Fund). Marine wildlife depends on coral reefs as their habitat, shelter, breeding, and spawning grounds. Animals use coral reefs to hide, hunt, and rest. As the ocean faces considerable anthropogenic threats, coral reefs may become extinct.

Photo Source: Smithsonian Ocean

Not only is coral important to marine wildlife, but it is important to humans too. People that live in coastal homes and towns rely on coral reefs as barriers for safety from storms, elements, and weather. Additionally, the loss of coral reefs brings collapse in food chains that fishermen and communities use for consumption. Ecotourism is also heavily affected as more coral reefs become bleached and devoid of biodiversity (World Wildlife Fund).

Stop the Bleaching

To reduce and stop coral bleaching, we need to address the cause of it. Below is a photo of a few examples of causes of coral bleaching. The Australian Marine Conservation Society states, “It can take decades for coral reefs to fully recover from a bleaching event, so it is vital that these events do not occur frequently.  If we continue burning fossil fuels at our current rate, severe bleaching events are likely to hit reefs annually by the middle of the century. This would be devastating for coral reefs as they would have no chance to recover.”

Photo Source: The Australian Marine Conservation Society

Several conservationists, researchers, biologists, and companies working hard to save, protect, and recover coral reefs. On a global level, scientists are experimenting and implementing new techniques to benefit coral reefs. For example, scientists are trying to build artificial reefs in the Red Sea to help save their coral reefs and pave the way for them to recover while satisfying their tourism and current reef wildlife’s needs. Read about it here. In Hong Kong, scientists are using 3D printing to help complement their dying reefs with the increased pressure from global climate change. Read about it here!

There are infinite ways to help coral reefs and stop bleaching, but it starts with our daily decisions and carbon footprint. When we reduce our impact on the planet, we can slow the degradation of incredible ecosystems within nature. Small choices such as reef-safe sunscreen (read about it in our past blog, here), reducing stormwater, reducing fertilizer runoff, and avoiding pesticides and herbicides can help coral reef health. Thanks for reading about coral bleaching and doing your part to take care of the ocean, have a great week!

Written By: Bailey Higa



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