Factors Causing Coral Bleaching and the Symbiotic Relationship with Zooxanthellae

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Factors Causing Coral Bleaching and the Symbiotic
Relationship with Zooxanthellae

• By Veronica Rodriguez
• Ecol 475

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Coral Bleaching

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

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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.

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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.

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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.

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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
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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.

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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.

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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.

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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.