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.
Unbleached coral Bleached coral
CaCO3 → CaO + CO2
Coral Bleaching is a stress condition in coral reefs that involves the breakdown of zooxanthellae.
How is atmospheric CO2 responsible for ocean acidification?
When CO2 dissolves in seawater, carbonic acid is produced via the reaction:
This carbonic acid dissociates in the water, releasing hydrogen ions and bicarbonate:
One result of the release of hydrogen ions is that they combine with any carbonate ions in the water to form bicarbonate:
The increase in the hydrogen ion concentration causes an increase in acidity, since acidity is defined by the pH scale, where pH = -log [H+] (so as hydrogen increases, the pH decreases). This log scale means that for every unit decrease on the pH scale, the hydrogen ion concentration has increased 10-fold.
This removes carbonate ions from the water, making it more difficult for organisms to form the CaCO3 they need for their shells.
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
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.
More FactorsNatural 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).
ResearchStudy done by Rosenberg and Ben-Haim (2002):o The actual agent leading to bleaching of hard coral, Oculina
patagonia, is caused by an infectious bacteria called Vibrio shiloi.o Purpose of the experiment was to test the growth of bacteria within
two different sea water temperatures in an aquaria environment.o Results showed bacteria produced an extracellular toxin that
blocked photosynthesis within the zooxanthellae o The toxin would bind to the algal membranes and prevent the
ammonia and lead to the destruction of pH gradient across the thylakoid membrane.
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.