Ocean Insights

Coral Bleaching: The Ecosystem’s SOS Signal

Coral reefs often require precise parameters for survival. So, as humanity’s impact manifests from increasing emissions and pollution our planet and our ocean experiences dramatic changes to its conditions, coral animals struggle to withstand the transformation. When hard polyps undergo periods of stress, they turn ghostly white in a response called “coral bleaching.” Luckily, though, this shocking transition with the coral doesn’t mean the colony has died just yet – they still have a chance to rebound.

worst coral bleaching event of the Great Barrier Reef, 2024

What conditions initiate coral bleaching?

Corals most frequently inhabit shallow, calm, clear, nutrient-poor, tropical seas. These regions are warm year-round, and corals tolerate a typical 1-2ºC seasonal heat variability. However, when temperatures increase an additional 1-2ºC above the average summer maximum, coral bleaching can be triggered. Heat is the leading cause of coral bleaching and is often associated with prolonged heat stress at or above 32ºC. But other changes, in the form of pollution that alters the water’s nutrient contents, or sediment suspension that clouds the sun’s visibility, can kick off a stress response, too.

How does coral react to these conditions?

Factors like temperature sensitivity, nutrient concentrations, and sun visibility are often associated with plant growth. So, if coral is an animal, why do they respond so strongly to these stressors?

Polyps host a dinoflagellate microalgae within their tissues, called zooxanthellae that give coral their color. They share a mutualistic relationship, where both the zooxanthellae and the polyp benefit from and, in this case, rely on each other’s existence. The microalgae find a safe home within the coral’s structure and perform photosynthesis using sunlight, carbon dioxide, and water. 90% of the oxygen, glucose, glycerol, and amino acids produced by the zooxanthellae are transferred to the host coral that uses the materials to create carbohydrates, fats, and proteins. This process provides up to 90% of the animal’s daily energy requirements and promotes the production of coral’s skeletal material– calcium carbonate. In return, when the polyp performs cellular respiration, it releases carbon dioxide and water as waste, and both products are used by the zooxanthellae to repeat the cycle.

Because coral and their zooxanthellae companions’ survival are so deeply intertwined, the host exhibits the plant-like responses experienced by the microalgae. When conditions become too hostile, their symbiosis breaks down. Corals expel zooxanthellae, expose the colony’s skeleton, and turn bright white.

2024 saw very warm water and the GBR experienced the worst bleaching event in history

When distressing conditions persist, and the coral remains bleached without their zooxanthellae symbionts for a period lasting longer than a few weeks to months, the coral will die. But, if temperatures subside quickly enough, most coral species are able to survive a bleaching event. When they do recover, however, the colony becomes more susceptible to disease or inhibited growth.

Different coral species vary in their resistance to bleaching. Some react to stressors after one to two weeks of elevated temperatures, while others withstand four to six-week periods of extreme heat before bleaching. Fast-growing species with thin tissues are the most vulnerable.

Currently, scientists possess a limited understanding of coral cell biology. Still, they work to develop mitigation techniques like selective breeding, probiotic administration, genetic manipulation, among many other interventions. But, as they advance solutions to this growing global crisis, they must continue to study how to implement strategies most effectively.

We partner with experts like Dr. Emma Camp, who work to understand coral resilience and find ways coral can survive under increased environmental stress. Within their portfolio of research, her team works with local stakeholders to plant and foster coral growth and reintroduce them to reefs. Throughout the process, they monitor scientific success and cost and plan how they can utilize scientific findings to increase coral resilience. In our upcoming episode of “A Sea of Hope,” we explore Dr. Camp’s work and amplify our ocean’s voice by capturing coral bleaching throughout the Great Barrier Reef.

With the lives of countless aquatic animals and people intertwined with the sustained success of coral reef ecosystems, it’s imperative that we address the root cause by lowering global emissions and working toward restoring balance below our ocean’s surface. Earth’s temperatures continue to rise, and we’re observing large-scale coral bleaching more frequently and with increased severity.

What does our coral landscape look like?

In 2023 and 2024, a massive coral bleaching event took place around the world. According to NOAA’s Coral Reef Watch (CRW) data, this marks the world’s fourth global bleaching event since recording began in 1985, but the second instance in the last ten years. Bleaching has been observed by NOAA’s CRW in every ocean on Earth between February 2023 and April 2024. The CRW team has confirmed mass coral bleaching in tropical waters around the world, including (read the full article here):

– Florida in the U.S
– The Caribbean
– Brazil
– The eastern Tropical Pacific (including Mexico, El Salvador, Costa Rica, Panama and Colombia)
– Australia’s Great Barrier Reef
– Large areas of the South Pacific (including Fiji, Vanuatu, Tuvalu, Kiribati, the Samoas and French Polynesia)
– The Red Sea (including the Gulf of Aqaba)
– The Persian Gulf
– The Gulf of Aden
Our co-founders sailed to the Great Barrier Reef to document this historic coral bleaching event in real time. From the frontlines, we’ve captured the impact of increasing temperatures on the largest reef in the world, and with help from leading scientific experts, we’ll look forward to hopeful solutions.