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Updated: Mar 9, 2023

What is coral bleaching? Coral bleaching looks exactly like what you’re probably envisioning right now: white, bleached-out coral reefs. The good news is corals can survive a bleaching event. The bad news is when bleached, corals are highly stressed and subject to mortality. And there’s more bad news, humans are probably more responsible for coral bleaching than any other factor. The first thing to understand about coral bleaching is that corals get their beautiful colors from tiny algae, zooxanthellae, that live in the coral’s tissues, with the color of a coral depending largely on the species of the symbiont. The corals and algae have a mutualistic, symbiotic relationship. The coral provides the algae with a protected environment and the compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove waste. Most importantly, zooxanthellae supply the coral with glucose, glycerol, and amino acids, which are the products of photosynthesis. The coral uses these products to make proteins, fats, and carbohydrates, and produce calcium carbonate. The next thing to understand is that corals will expel the algae due to certain stresses:

• oxygen starvation

• increases in solar radiation (UV and visible light)

• changes in water chemistry

• increased sedimentation

• bacterial infections

• herbicides

• low tide and exposure

• elevated sea levels

• mineral dust from sand storms

• increased or decreased water temperatures

Stony corals have calcium carbonate skeletons and most have transparent tissues, so expulsion of the zooxanthellae causes them to lose their color and become white. Although the coral polyps feed on zooplankton and other food particles, the majority of reef-forming corals rely for a large proportion of their nutritional requirements on their zooxanthellae. This means that without them they are liable to starve. Coral growth and reproduction are reduced and the coral becomes increasingly susceptible to disease. Coral bleaching is a natural phenomenon; ejection increases the polyp's chance of surviving short-term stress. If stress factors reduce and the zooxanthellae return, the coral can recover e.g. If the coral is exposed to higher intensity light, photosynthesis may increase, which can produce dangerously high levels of oxygen in the coral's tissue. So, if coral bleaching is a natural phenomenon, why is it bad? The corals need the algae to survive and if the stressful conditions persist, the polyp eventually dies. Ejection of the algae was not nature’s idea of a long-term plan, but a short-term solution to an immediate problem such as low tide and exposure, for example. Let’s take another look at some of those stress factors that cause the coral to eject their symbiotic buddies: Oxygen starvation - is primarily caused by an overabundance of zooplankton, typical in areas that suffer from overfishing. Changes in water chemistry - such as acidification of the ocean due to increased carbon emissions. Increased sedimentation - caused by slash and burn and shifting cultivation of tropical forests. Herbicides - commonly called weed killers, enter rivers and streams and, unsurprisingly, end in the ocean. Elevated sea levels - thought to be due to global warming Increased or decreased water temperatures - again, thought to be due to global warming These stresses, if you haven’t noticed, are all caused by human activities and they are not going to disappear after a few hours like a falling tide. Unless we change the way we manage the earth’s resources, we will see a dramatic loss of coral reefs and, while reefs make up less than 1% of the ocean’s ecosystems, their importance shouldn’t be underestimated:

• They provide shelter for 25% of all known marine species

• They protect shorelines, acting as natural sea breaks

• They support fishing industries

• Reef fish feed between 30 and 40 million people per year

• They provide jobs (particularly in the tourism industry)

• They generate tourist dollars

• They absorb carbon dioxide in the water

It’s not all doom and gloom. Here are 6 ways you can help the coral reefs:

1. Conserve water - the less you use, the less runoff and waste will pollute the ocean.

2. Reduce pollution - reduce your carbon emissions by walking, riding a bicycle, or using public transport.

3. Use ecological or organic fertilizers - you may live far from the sea, but these products flow into the water system and pollute the ocean.

4. Plant a tree or two - trees reduce runoff into the ocean and, as a bonus, you’ll contribute to reversing global warming and the rising of the ocean’s temperature.

5. Practice safe and responsible diving and snorkeling practices - this helps reduce the stress on the corals.

6. Contact your government and local MP - demand they take action to protect coral reefs, reduce sewage pollution, expand or introduce marine protected areas, and take steps to reverse global warming.

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