Two ways corals reduce atmospheric CO2
First way - Coral photosynthesis removes CO2.
Few people know that corals are an animal with a plant living inside of each coral polyp in a mutually beneficial relationship. Isn't nature brilliant ?!
This plant inside each coral polyp is an algae (zooxanthellae) that removes CO2 from the seawater through simple photosynthesis, just like trees do.
Coral reefs require sunlight to grow, because photosynthesis is their energy source for growing. That's why they live in the tropics, where the sunlight is intense.
Second way - Building the coral's stony skeleton also removes CO2
But there's actually a second way that growing corals reduces CO2. When they build their stony skeletons, they remove CO2 from the water and convert it first to bicarbonate, and then to CaCO3 (Calcium carbonate) which becomes limestone after natural pressure is applied to it. So the corals produce limestone rock, which was created from CO2 and other minerals dissolved in the water.
This effectively ties up the CO2 permanently.
CO2 from the atmosphere is dissolved in seawater by simple diffusion. When CO2 is dissolved in seawater, it forms bicarbonate. Corals use this bicarbonate plus free Calcium ions to form aragonite which is the “concrete” of their skeletons.
This is the chemical reaction where corals use CO2 dissolved in seawater, to build their stony skeletons:
CO2 + H2O ---> HCO3- (bicarbonate)
The corals then use that free bicarbonate, along with free Calcium ions to form their skeleton:
HCO3- + Ca2+ ---> CO32- + H+ + Ca2+ --> CaCO3 (aragonite rock)
So in simple terms, the CO2 (dissolved in seawater) + H2O (seawater) react and yields HCO3- bicarbonate ions. These bicarbonate ions are then combined with Calcium ions to yield aragonite.
Coral skeletons are made of aragonite, a form of calcium carbonate. To grow up towards sunlight, corals construct a framework of aragonite crystals.
The stony skeleton is that part of a coral that remains even after the coral dies. The CO2 stays locked up essentially permanently, unlike trees which re-release their CO2 into the atmosphere when they burn or die.
This is an important advantage planting corals has over planting trees.
Just like with tree planting, planting corals that otherwise would not have been planted, will reduce your carbon footprint by the same amount as the C offsets you have purchased.