Role of Symbiosis in Calcification

Zooxanthellae accelerate skeletal formation in reef-building corals by a (poorly understood) phenomenon called "light enhanced calcification". Zooxanthellae therefore play a dual role in explaining the success of reef corals - first, by contributing to the coral's energy budget; and second, by accelerating their rate of calcification and hence their growth rate in competition with other benthic organisms.

Exactly how calcification is accelerated is not fully understood. Some possible mechanisms include:

1. Alteration of physicochemical conditions so as to favor calcification:

Ca2+ + 2HCO3- <->Ca(HCO3)2 <->CaCO3 + H2CO3 <-> CaCO3 + H2O + CO2

Removal of water and carbon dioxide by photosynthesis (H2O + CO2 ->CH2O + O2) pushes the equation to the right and favors formation of calcium carbonate.

2. Contribution of energy gained from breakdown of translocated photosynthetic product to the process of calcification.

3. Photosynthesis increases pH (i.e., makes more alkaline), which increases the concentration of available carbonate ions.

4. Photosynthesis removes inorganic nutrients (e.g., phosphate), which inhibit the formation of aragonite (calcium carbonate) crytals.