Factors Causing Coral Bleaching and the Symbiotic Relationship with Zooxanthellae

1. Factors Causing Coral Bleaching and the Symbiotic Relationship with Zooxanthellae By Veronica Rodriguez Ecol 475

2. Coral Bleaching Coral Bleaching is a stress condition in coral reefs that involves the breakdown of zooxanthellae.

3. Zooxanthellae • Zooxanthellae are dinoflagellate algae of the genus Symbiodinium, and live in coral tissue. • They provide nutrients such as sugars and oxygen that are essential for the production of calcium carbonate (coral reef skeleton). • Provide coral with beautiful coloration (without them corals are clear or white; phenomenon known as coral bleaching). • More than 50% of the corals’ nutrients are derived from photosynthetic products produced by zooxanthellae.

4. Coral Reefs • Coral Reefs are most diverse and productive communities on Earth. • They are referred to as the “Tropical Rainforest of the Ocean” • Found in warm, clear, shallow waters. • Provide zooxanthellae with a protected environment, steady supply of carbon dioxide, and nitrogen.

5. Factors Affecting Coral Bleaching Temperature: Fluctuation of low and high sea temperature accompanying intense upwelling. • Winter: Change from -3 C to -5 C for 5 to 10 days • Summer: Change from 1 to 2 C for 5 C to 10 days (more common) Salinity: Zooxanthellae are sensitive to low salinity due to precipitation and runoff. Therefore, they tend to live near shallow, clear waters (no deeper than 100m) with plenty of sunlight. UV radiation: High levels of photosynthesis leads to high amounts of nitric oxide production by zooxanthellae.

6. More Factors Natural Phenomenon: Violent storms, flooding, ENSO (El Niño Southern Oscillation), predatory outbreaks, and tsunami are devastating to coral reefs. Anthropogenic Phenomenon: Overexploitation, overfishing, increased sedimentation, and nutrient overloading. Map of coral bleaching over the past 15 years, and major coral bleaching events (yellow dots). www.iper1.it/.../ index/cerca/reef

7. Research Study done by Rosenberg and Ben-Haim (2002): • The actual agent leading to bleaching of hard coral, Oculina patagonia, is caused by an infectious bacteria called Vibrio shiloi. • Purpose of the experiment was to test the growth of bacteria within two different sea water temperatures in an aquaria environment. • Results showed bacteria produced an extracellular toxin that blocked photosynthesis within the zooxanthellae • The toxin would bind to the algal membranes and prevent the ammonia and lead to the destruction of pH gradient across the thylakoid membrane.

8. Research 2 Study done by Rosenberg and Falkovitz, 2004: • Similar study done with Oculina patagonica, hard coral and Vibrio shiloi, bacteria. • Results showed the inability of the bacteria to bleach the coral at cold temperatures due to increased resistance of the coral and in hot temperatures the bacteria was able to produce. • Laboratory experiment demonstrated high temperature would cause bleaching by the bacteria. • Temperature at 29 C there was rapid and complete bleaching • At 25 C bleaching was slower and incomplete • Below 20 C there was no bleaching, even with large amounts of the bacteria exposed.

9. Conclusion • Both studies showed the bacteria was unable to grow in cold temperatures. • A theory was the zooxanthellae provided protection to the coral from the intracellular bacteria. • Based on an alternative explanation of the zooxanthellae production and concentration of oxygen to the coral was toxic to the bacteria. • Results suggested the zooxanthellae are essential in providing nutrients to coral, as well as, providing protection to coral from the bacteria, V. shiloi. • Future studies are to be done on the symbiotic relationship between zooxanthellae and coral reefs. • Human interaction should also be taken in consideration.

10. Global Warming • Global warming is a major concern: • If temperatures continue to increase to 1-2 C, for the next 20 years there will be mass coral bleaching worldwide. • The high seawater temperature elevation will affect over 95% of the species living within the coral and lead species to become extirpated or extinct.