Overview

Coral bleaching is a phenomenon where corals lose their vibrant colors and turn white due to the expulsion of symbiotic algae (zooxanthellae) living within their tissues. This process is primarily triggered by environmental stressors, most notably elevated sea temperatures. Bleached corals are more susceptible to disease and mortality, threatening entire reef ecosystems.

What Causes Coral Bleaching?

  • Thermal Stress: The most common trigger is prolonged exposure to water temperatures 1–2°C above the average summer maximum.
  • Solar Irradiance: Increased sunlight, especially ultraviolet radiation, can stress corals.
  • Ocean Acidification: Elevated CO₂ levels lower ocean pH, affecting coral calcification and resilience.
  • Pollution: Runoff containing pesticides, herbicides, and nutrients can disrupt coral-algae symbiosis.
  • Pathogens: Certain bacteria and viruses have been linked to bleaching events.

How Does Bleaching Occur?

Corals rely on zooxanthellae for up to 90% of their energy via photosynthesis. When stressed, corals expel these algae, losing their primary energy source and color.

Process:

  1. Environmental stress damages the photosynthetic machinery of zooxanthellae.
  2. Corals expel the stressed algae.
  3. Loss of algae leads to white appearance (“bleached”).
  4. If stress persists, corals may die due to energy depletion.

Ecological Impact

  • Loss of Biodiversity: Coral reefs support ~25% of marine species.
  • Reduced Fisheries: Many fish species depend on reefs for food and shelter.
  • Coastal Protection: Reefs buffer shorelines against storms and erosion.
  • Economic Losses: Tourism and fishing industries suffer from reef decline.

Recent Trends & Research

A study published in Nature Communications (2021) by Sully et al. found that global coral bleaching events have increased in frequency and severity since the 1980s. The 2014–2017 global bleaching event affected 70% of the world’s coral reefs, with some areas experiencing up to 90% coral mortality.

Citation: Sully, S., Burkepile, D.E., Donovan, M.K., Hodgson, G., & van Woesik, R. (2021). “A global analysis of coral bleaching over the past two decades.” Nature Communications, 12, 6966. Link

Diagrams

Coral Bleaching Process

Coral Bleaching Diagram

Healthy vs. Bleached Coral

Healthy vs Bleached Coral

Surprising Facts

  1. Coral Adaptation: Some corals can swap their algae for more heat-tolerant species, allowing them to survive in warmer waters.
  2. Global Water Cycle: The water you drink today may have been drunk by dinosaurs millions of years ago, as water molecules are continually recycled through Earth’s hydrosphere—including coral reefs.
  3. Sound of Bleached Reefs: Bleached reefs are quieter; healthy reefs produce complex sounds that attract juvenile fish, but bleaching reduces this acoustic diversity.

Interdisciplinary Connections

  • Climate Science: Coral bleaching acts as a barometer for ocean health and climate change impacts.
  • Chemistry: Ocean acidification alters carbonate chemistry, affecting coral skeleton formation.
  • Economics: Coral reef degradation has direct financial impacts on tourism, fisheries, and coastal communities.
  • Medicine: Coral reefs are sources of novel pharmaceuticals; bleaching threatens these discoveries.
  • Engineering: Artificial reefs and restoration technologies are being developed to mitigate bleaching effects.

Glossary

  • Coral Bleaching: Loss of color in corals due to expulsion of symbiotic algae.
  • Zooxanthellae: Photosynthetic algae living inside coral tissues.
  • Thermal Stress: Stress caused by elevated temperatures.
  • Ocean Acidification: Decrease in ocean pH due to increased atmospheric CO₂.
  • Symbiosis: Close and long-term biological interaction between two different biological organisms.
  • Calcification: Process by which corals build their calcium carbonate skeletons.
  • Ecosystem Services: Benefits provided by ecosystems to humans.

Most Surprising Aspect

The most surprising aspect of coral bleaching is the resilience and adaptability of some coral species. Recent research has shown that certain corals can form new symbiotic relationships with more heat-tolerant algae, offering hope for survival despite rising temperatures. However, this adaptation is not universal and may not keep pace with rapid climate change.

References

  • Sully, S., Burkepile, D.E., Donovan, M.K., Hodgson, G., & van Woesik, R. (2021). “A global analysis of coral bleaching over the past two decades.” Nature Communications, 12, 6966. Link
  • NOAA Coral Reef Conservation Program. Coral Bleaching
  • National Geographic. Coral Bleaching Explained

End of Study Notes