Although coral reefs hold 25% of the ocean’s biodiversity, these ecosystems face environmental hardships that lead to coral bleaching and potentially death.

The world under the ocean is a mystery to so many of us, but once you start exploring it, you will find biodiverse wonders like coral reefs and understand the importance of the intricate relationships between them and the rest of the world.

With 14% of the ocean’s reefs lost in the short period of 2008 – 2019, coral bleaching has become a major concern, and the importance of understanding how it works and what to do about it has become ever more dire.

A Brief Introduction to Coral Reefs

Coral reefs are large underwater structures comprising of individual colonies of approximately 6 thousand different species.

Each coral colony is made up of individual coral polyps that build and live on a hard exoskeleton.

Part of what makes coral so special is their symbiotic relationship with zooxanthellae (a specific type of microscopic algae). The coral-reef ecosystem provides a safe place for the coral algae to live, while the chlorophyll-containing photosynthetic organism provides food for the coral polyps.

Without zooxanthellae, coral reefs cannot survive.

Coral reefs are unparalleled in their biodiversity. These incredible sea animals (yes, corals are animals, not plants) make up only 0.1% of the seabed’s surface while holding 25% of its biodiversity.

Coral reefs hold 25% of the oceans biodiversity
Coral reefs hold 25% of the ocean’s biodiversity

Providing shelter for smaller creatures, breeding grounds, food, a safe place for juvenile development, and important migratory routes, healthy coral reefs are essential to the overall balance of the ocean.

Despite the importance of these underwater cities, they are at major risk due to the phenomenon known as coral bleaching.

Understanding the Dynamics of Coral Bleaching

The phenomenon of coral bleaching is fairly easy to understand, but it’s important to have a full grasp of the relationship between the coral and its algae and how climate change and temperature stress impact them.

Corals’ Symbiotic Relationship With Alge

Corals are fairly immobile. 

Although when we compare soft coral to hard coral, it’s clear that soft coral is more flexible, each colony remains rooted to a solid substrate and therefore can’t venture to search for food.

Most corals feed at night when the polyps stretch tentacles out of their hard exoskeletons and sift the passing water to catch zooplankton. However, this only makes up 5% of the coral’s nutrient intake.

According to the Scripps Institution of Oceanography, the remaining 95% of a coral’s dietary requirements are met by its photosynthesizing symbiotic algae.

It is, however, worth noting that coral living in waters with a higher concentration of chlorophyll gain more of their nutrients from plankton and other organisms, suggesting that some species of coral may be less reliant on zooxanthellae than others.

Coral get 95% of their nutrients from thier zooxanthellae
Coral get 95% of their nutrients from their zooxanthellae

Even so, one cannot survive without the other.

Coral provides a safe space and essential compounds the algae need to photosynthesize, and in return, the algae produce oxygen for the coral and help remove waste.

More importantly, through photosynthesis, they provide the coral polyps with glucose, glycerol, and amino acids turned into proteins, fats, and carbohydrates, which provide energy to the coral.

These amino acids and sugars are also essential to producing calcium carbonate, which forms the coral’s white skeleton.

It is also due to zooxanthellae that coral gets its vibrant colors.

Although not much is known about the color of coral, it has been suggested that they play a major role in attracting different marine life to the reef, thus increasing biodiversity and overall reef health.

Furthermore, discussions have been made around the idea that the color of coral provides a level of protection from the sun, acting as a mild UV blocker. However, this is not conclusive.

The Process of Coral Bleaching and it’s Contributing Factors

Coral bleaching is the process of coral expelling their zooxanthellae due to stress-related factors.

As it’s the algae that’s responsible for the coral’s vibrant color, the coral turns white, displaying only its calcium carbonate skeleton, thus looking as though it has been bleached.

Without its partner algae, coral can survive a short time, but if conditions do not change and the zooxanthellae don’t return to the coral’s tissue promptly, the coral will suffer from nutrient deficiency and eventually die.

Why Do Coral Expell Their Zooxanthellae?

The zooxanthellae living on coral are extremely sensitive to temperature, and when local water temperatures reach above the normal summer maximum, these single-cell organisms get stressed.

This stress is caused by increased temperature through water and overexposure to bright light. The result of this increase in energy is the overproduction of reactive oxygen species (ROS).

These ROS species are toxic to both coral and their algae symbionts.

Coral bleaching occurs when algae is expelled from the coral
Coral bleaching occurs when algae is expelled from the coral

Although the relationship between ROS and the coral polyps is not certain, it has been suggested that it’s due to the increase in ROS and its byproducts (particularly hydrogen peroxide) that the coral expels the algae.

That said, some studies have found little relationship between the increase of ROS and the excretion of zooxanthellae.

Whether the increase in ROS is directly related to coral bleaching or not, it is clear that the expelling of these algae is directly related to temperature increases, which come from various contributors.

Climate Change and the Role of Temperature Stress on Coral Bleaching

As mentioned before, corals live in a very specific temperature range, typically between 73° and 84° F (23°–29°C).

Although some species of coral can tolerate warmer temperatures up to approximately 104°F (40°C) for short periods, when water temperature increases by as little as 1°-2° C above the ideal conditions, coral begins to bleach.

The increase in ocean temperature and its resulting coral bleaching can be attributed mainly to the increase in global temperatures, commonly known as global warming.

That said, coral bleaching can also occur in a drop in water temperature – called cold water stress.

Although the rapid increase of greenhouse gasses can be directly related to the ocean’s temperature rising and an increase in coral bleaching, it is not the only contributing factor.

Causes Of Coral Bleaching

Water temperature rise has been labeled as the leading contributor to coral bleaching, and although this is partly true, the stress experienced by their algae leads to the phenomenon.

This stress can be induced in numerous ways, contributing to the loss of these biodiverse habitats.

  • Climate Change: The burning of fossil fuels and other human-driven activities has increased the release of greenhouse gasses, which in turn raise ocean temperatures.
  • Ocean Acidification: Carbon dioxide, a greenhouse gas, dissolves in seawater, forming carbonic acid. This leads to decreased ocean pH, which hinders corals’ ability to build their skeletons. Ocean acidification results in an increased susceptibility to the stress of corals.
  • Solar Radiation: High levels of solar radiation, particularly during low tides or in calm conditions, can hyper-excite the zooxanthellae on coral, leading to heat stress.
  • Global Weather Phenomenon: Unusual changes in currents and weather patterns can stress coral and lead to localized bleaching. El Niño and La Niña weather patterns have been closely related to coral bleaching events.
  • Change In Salinity: Heavy rainfall, reduced freshwater inflow, and melting glaciers can change the salinity concentration of the ocean, which puts coral and their algae under a lot of stress.
  • Pollution: Agricultural runoff, sewage, and industrial waste products can cause an increase in nutrients in coral-rich areas, which can lead to algal blooms such as those common with the bryopsis species. When invasive algae grow out of control, they can cover the symbiotic zooxanthellae on coral, blocking their light and suffocating them.

What Can Be Done to Protect the World’s Reefs From Bleaching?

Addressing the coral bleaching issue is a complex process, and as it is a worldwide event, it requires intricate agreements and contributions from several global organizations, individuals, and governments.

Governments and scientists are hard at work, trying to alleviate the stress of coral bleaching
Governments and scientists are hard at work, trying to alleviate the stress of coral bleaching

Some key measures include:

  • Improving water quality
  • Promotion of sustainable fishing and tourism practices
  • Coral reef restoration projects such as coral farms
  • Education and public awareness campaigns
  • Reduction in climate change by implementing policies that promote renewable energy and reduce greenhouse gas emissions

That said, it is up to each individual to live in a conscious, responsible manner. 

If you want to help the coral bleaching issue, then the best steps you can take is by reducing your emissions and ensuring that each step you take on the Earth is conducive to the greater ecology and flow of nature.


What are some major coral bleaching events?

There have been numerous mass bleaching events in the past decades, which include bleaching events in 1998, 2010, 2015-2017, and 2020. These severe bleaching events affected the ocean reefs on a global scale.

Can coral reefs recover from coral bleaching?

If environmental stressors are reduced and sea surface temperatures return to normal, then algae will return to the coral, and they will recover. That said, prolonged bleaching will eventually result in coral mortality.

Is coral bleaching natural or a human-induced process?

Coral bleaching can occur naturally, often due to changing climate conditions and rare weather patterns that bring warmer or cooler waters to coastal areas. That said, the process and frequency of these events have drastically increased due to human activity.

Are all corals susceptible to bleaching?

Some corals are more tolerant of environmental stress and can recover from bleaching easier than others. Different coral hosts use different algae, which often determines their resilience.

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