Factors Causing Coral Bleaching Factors Causing Coral Bleaching and the Symbiotic Relationship and the Symbiotic Relationship

with Zooxanthellaewith Zooxanthellae

By Veronica RodriguezBy Veronica RodriguezEcol 475Ecol 475

Coral BleachingCoral Bleaching

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

ZooxanthellaeZooxanthellae

Zooxanthellae are dinoflagellate algae of the Zooxanthellae are dinoflagellate algae of the genus genus Symbiodinium, Symbiodinium, and live in coral tissue.and live in coral tissue.

They provide nutrients such as sugars and They provide nutrients such as sugars and oxygen that are essential for the production of oxygen that are essential for the production of calcium carbonate (coral reef skeleton).calcium carbonate (coral reef skeleton).

Provide coral with beautiful coloration (without Provide coral with beautiful coloration (without them corals are clear or white; phenomenon them corals are clear or white; phenomenon known as coral bleaching).known as coral bleaching).

More than 50% of the corals’ nutrients are More than 50% of the corals’ nutrients are derived from photosynthetic products produced derived from photosynthetic products produced by zooxanthellae.by zooxanthellae.

Coral ReefsCoral Reefs

o Coral Reefs are most diverse and productive Coral Reefs are most diverse and productive communities on Earth.communities on Earth.

o They are referred to as the “Tropical Rainforest of They are referred to as the “Tropical Rainforest of the Ocean” the Ocean”

o Found in warm, clear, shallow waters.Found in warm, clear, shallow waters.o Provide zooxanthellae with a protected Provide zooxanthellae with a protected

environment, steady supply of carbon dioxide, environment, steady supply of carbon dioxide, and nitrogen. and nitrogen.

Factors Affecting Coral BleachingFactors Affecting Coral Bleaching

Temperature:Temperature: Fluctuation of low and high sea temperature Fluctuation of low and high sea temperature accompanying intense upwelling.accompanying intense upwelling.

Winter: Change from -3 C to -5 C for 5 to 10 daysWinter: 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 Summer: Change from 1 to 2 C for 5 C to 10 days

(more common)(more common)

SalinitySalinity:: Zooxanthellae are sensitive to low salinity due to Zooxanthellae are sensitive to low salinity due to precipitation and runoff. Therefore, they tend to live near precipitation and runoff. Therefore, they tend to live near shallow, clear waters (no deeper than 100m) with plenty of shallow, clear waters (no deeper than 100m) with plenty of sunlight. sunlight.

UV radiationUV radiation: High levels of photosynthesis leads to high : High levels of photosynthesis leads to high amounts of nitric oxide production by zooxanthellae. amounts of nitric oxide production by zooxanthellae.

More FactorsMore FactorsNatural PhenomenonNatural Phenomenon: : Violent storms, flooding, ENSO (El Violent storms, flooding, ENSO (El

Niño Southern Oscillation), predatory outbreaks, and Niño Southern Oscillation), predatory outbreaks, and tsunami are devastating to coral reefs.tsunami are devastating to coral reefs.

Anthropogenic PhenomenonAnthropogenic Phenomenon: Overexploitation, : Overexploitation, overfishing, increased sedimentation, and nutrient overfishing, increased sedimentation, and nutrient overloading.overloading.

Map of coral bleaching over the past 15 years, and major coral Map of coral bleaching over the past 15 years, and major coral bleaching events (yellow dots).bleaching events (yellow dots).

www.iper1.it/.../ index/cerca/reefwww.iper1.it/.../ index/cerca/reef

ResearchResearch

Study done by Rosenberg and Ben-Haim (2002):Study done by Rosenberg and Ben-Haim (2002):o The actual agent leading to bleaching of hard coral, The actual agent leading to bleaching of hard coral, Oculina Oculina

patagonia,patagonia, is caused by an infectious bacteria called is caused by an infectious bacteria called Vibrio Vibrio shiloi.shiloi.

o Purpose of the experiment was to test the growth of Purpose of the experiment was to test the growth of bacteria within two different sea water temperatures in an bacteria within two different sea water temperatures in an aquaria environment.aquaria environment.

o Results showed bacteria produced an extracellular toxin Results showed bacteria produced an extracellular toxin that blocked photosynthesis within the zooxanthellae that blocked photosynthesis within the zooxanthellae

o The toxin would bind to the algal membranes and prevent The toxin would bind to the algal membranes and prevent the ammonia and lead to the destruction of pH gradient the ammonia and lead to the destruction of pH gradient across the thylakoid membrane. across the thylakoid membrane.

Research 2Research 2

Study done by Rosenberg and Falkovitz, 2004:Study done by Rosenberg and Falkovitz, 2004:• Similar study done with Similar study done with Oculina patagonica, Oculina patagonica, hard coral and hard coral and

Vibrio shiloiVibrio shiloi, bacteria., bacteria.• Results showed the inability of the bacteria to bleach the Results showed the inability of the bacteria to bleach the

coral at cold temperatures due to increased resistance of coral at cold temperatures due to increased resistance of the coral and in hot temperatures the bacteria was able to the coral and in hot temperatures the bacteria was able to produce.produce.

• Laboratory experiment demonstrated high temperature Laboratory experiment demonstrated high temperature would cause bleaching by the bacteria.would cause bleaching by the bacteria.• Temperature at 29 C there was rapid and complete bleachingTemperature at 29 C there was rapid and complete bleaching• At 25 C bleaching was slower and incompleteAt 25 C bleaching was slower and incomplete• Below 20 C there was no bleaching, even with large amounts Below 20 C there was no bleaching, even with large amounts

of the bacteria exposed. of the bacteria exposed.

Conclusion Conclusion Both studies showed the bacteria was unable to grow in Both studies showed the bacteria was unable to grow in

cold temperatures.cold temperatures. A theory was the zooxanthellae provided protection to the A theory was the zooxanthellae provided protection to the

coral from the intracellular bacteria. coral from the intracellular bacteria. • Based on an alternative explanation of the zooxanthellae Based on an alternative explanation of the zooxanthellae

production and concentration of oxygen to the coral was toxic production and concentration of oxygen to the coral was toxic to the bacteria.to the bacteria.

Results suggested the zooxanthellae are essential in Results suggested the zooxanthellae are essential in providing nutrients to coral, as well as, providing protection providing nutrients to coral, as well as, providing protection to coral from the bacteria, to coral from the bacteria, V. shiloi.V. shiloi.

Future studies are to be done on the symbiotic relationship Future studies are to be done on the symbiotic relationship between zooxanthellae and coral reefs.between zooxanthellae and coral reefs.

Human interaction should also be taken in consideration.Human interaction should also be taken in consideration.

Global WarmingGlobal Warming

Global warming is a major concern:Global warming is a major concern:• If temperatures continue to increase to 1-2 C, for the If temperatures continue to increase to 1-2 C, for the

next 20 years there will be mass coral bleaching next 20 years there will be mass coral bleaching worldwide.worldwide.

• The high seawater temperature elevation will affect over The high seawater temperature elevation will affect over 95% of the species living within the coral and lead 95% of the species living within the coral and lead species to become extirpated or extinct.species to become extirpated or extinct.