Coral bleaching is a phenomenon where corals lose their vibrant coloration and turn white. This change occurs when corals are under stress, causing them to expel the tiny organisms responsible for their hues. A bleached reef, transformed from a kaleidoscope of colors to a stark white landscape, highlights a serious concern for marine ecosystems.
The Vital Partnership: Coral and Algae
Most reef-building corals engage in a mutually beneficial relationship with microscopic algae called zooxanthellae. These single-celled algae reside within the coral’s tissues, providing its colors. The coral offers a protected habitat and compounds like carbon dioxide and water for photosynthesis.
The zooxanthellae use sunlight to produce energy-rich compounds such as sugars, glycerol, and amino acids. Up to 90% of these organic materials are transferred to the coral. This nutrient transfer supports the coral’s metabolism, growth, and calcium carbonate skeleton production.
Ocean Warming as a Stressor
Even slight increases in ocean temperature can disrupt the balance of the coral-algae partnership. Corals are adapted to a narrow temperature range, and temperatures just 1-2 degrees Celsius above their summer maximum for a few weeks can induce heat stress. The primary target of this thermal stress is the zooxanthellae.
Elevated temperatures impair the photosynthetic process within the zooxanthellae. Instead of converting sunlight into nutrients, their photosynthetic machinery becomes overwhelmed. This dysfunction leads to the production of harmful reactive oxygen species (ROS), such as hydrogen peroxide, within the algae’s chloroplasts. These ROS are toxins that cause oxidative stress and damage to both the zooxanthellae and the coral tissue.
The Mechanism of Coral Bleaching
In response to harmful reactive oxygen species and oxidative stress, the coral begins to expel the stressed zooxanthellae. This expulsion can occur through various mechanisms, such as sloughing off cells or direct ejection via the coral’s mouth. Expelling them is a survival strategy to prevent further tissue damage.
The expulsion of zooxanthellae causes the coral to lose its color, leading to “bleaching.” The coral’s colors come from these algae. Once the algae are gone, the coral’s transparent tissue reveals its white calcium carbonate skeleton, making the coral appear stark white. This loss of algae also means the coral loses its primary food source, making it vulnerable.
Survival and Death After Bleaching
A bleached coral is not immediately dead, but it is weakened and susceptible to further harm. Its chances of survival depend on how quickly ocean temperatures return to normal and if other stressors are absent. If conditions improve within a few weeks, corals can potentially recover by re-acquiring zooxanthellae from the water column or by multiplying any remaining algae within their tissues. Some species can also rely on heterotrophic feeding (consuming plankton) for a period, potentially surviving for weeks or months without their algae.
If the stress is prolonged or severe, the coral will likely die due to starvation and increased susceptibility to disease. Branching corals may only survive for about 10 days without them. Mass bleaching events, especially when recurrent or intense, can lead to widespread coral mortality, making natural recovery processes unable to keep pace. Recovery for a reef can take at least 9-12 years, provided there are no new disturbances or additional bleaching events.