How do you know Corals remove CO2?

There are two known, well established ways that corals removes CO2. First is by photosynthesis, and the second is by the formation of its Calcium Carbonate skeleton.

Recent estimates have calculated that 26 percent of all CO2 from fossil fuel burning, cement manufacture, and land-use changes over the decade 2002–2011 was absorbed by the oceans. (

Some of it stays as dissolved gas, but most reacts with the water to form carbonic acid or reacts with carbonates already in the water to form bicarbonates.

Photosynthetic CO2 fixation may have contributed to the control of atmospheric pCO2 during past glacial-interglacial cycles (Opdyke and Walker, 1992).

That's amazing ! During previous climate cycles, corals may have had a big part in controlling the CO2 levels. It certainly makes sense since nearly all corals are photosynthetic, and both hard and soft corals use CO2 for Calcium Carbonate support structures (called spicules in soft corals).

Who knew corals can reduce atmospheric CO2! After all, they don't have leaves, or live on land, so how can they pull CO2 from the air?

Much of the CO2 dissolved in the ocean came originally FROM Earth's atmosphere. Through simple diffusion, a higher concentration in the air resulted in CO2 moving into the ocean water. In fact, it's estimated that 26 percent of all the carbon released as CO2 from burning fossil fuels was absorbed by the oceans.

CO2 dissolved in ocean water is constantly equilibrating with atmospheric CO2, so any CO2 removed from seawater also reduces the amount in the atmosphere.

So how can corals possibly remove CO2?

Many plants and animals, including hard and and soft corals, use the bicarbonate to form calcium carbonate shells. 

Corals utilize the carbon dioxide two ways. First via photosynthesis they convert CO2 into glucose in the presence of sunlight. And second, corals calcify their stony skeletons using CO2 that has been converted to bicarbonate.



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.

The coral host plays a major role in supplying carbon for the photosynthesis by the algal symbionts through a system similar to the carbon-concentrating mechanism described in free living algal cells. The details of carbon supply to the calcification process are almost unknown, but metabolic CO2 seems to be a significant source.

Calcium and inorganic carbon are the two major substrates of photosynthesis and calcification. At larger scales of both space and time, the surface mixed layer remains close to equilibrium with the atmospheric CO2 concentration; this equilibrium permits estimation of the overall effects of atmospheric concentration change on marine biomineralization.

Also, surface seawater is super-saturated with respect to both calcite and aragonite, the two major forms of calcium carbonate, down to a depth of a few thousand meters (reviewed in Morse and Mackenzie [1990]).

Photosynthesis and calcification

Dissolved inorganic carbon (DIC) is used by the animal host to deposit skeletal CaCO3 and by the endosymbiont for its photosynthesis.

Photosynthesis, respiration (of the animal and algal components) and calcification can take place simultaneously according to the following simplified equations:

CO2 + H2O

-> CH2O + O2 (photosynthesis)


CH2O + O2

-> CO2 + H2O (respiration)


Ca2+ + 2HCO3-

-> CaCO3 + CO2 + H2O (calcification)


(OK we have to get even more technical for you guys who want the nitty gritty on how corals can do this.)

Photosynthesis and calcification both consume inorganic carbon but the combined processes can also be viewed as mutually supporting because CO2 generated by calcification can be used for photosynthetic carbon fixation.

The mechanisms of calcium transport for calcification in corals are poorly known, as is the case in other invertebrates (Simkiss and Wilbur, 1989).

The preferred DIC substrate for coral photosynthesis is external HCO^ (Land et al, 1975; Goreau, 1977; Al-Moghrabi et

al, 1996; Goiran et al, 1996), although it has been suggested that CO2 is a major source (Taylor, 1983).

Goiranet al., 1996) strongly suggests that a carbon concentrating mechanism (CCM)-like system, which actively absorbs HCOj to sustain photosynthesis, operates in the animal host, as previously hypothesized by Raven (1992).

There are very limited data on the source and transport mechanisms of the inorganic carbon used for coral calcification. Radioisotopic

tracer experiments demonstrate that DIC from seawater can be incorporated into the skeleton (Goreau 1961, 1963; Taylor,1983).

Lucas and Knapp (1997) found that DIDS does not completely inhibit calcification in the non-zooxanthellate octocoral L. virgulata, suggesting that CO2 can reach the site of skeletogenesis by a passive transport pathway and be used as a substrate for calcification. Carbonic anhydrase, an enzyme located in the calicoblastic epithelium (Isa and Yamazato, 1984), catalyzes the conversion of respiratory CO2 into HCO3~,as demonstrated by inhibition experiments (Goreau, 1959; Tambutte et al., 1996). 

The first indication of a possible link between zooxanthellar photosynthesis and coral calcification is probably the finding by Kawaguti and Sakumoto (1948) that calcification is higher in the light than in darkness.The analysis of 108 data compiled from 26 publications provides overwhelming evidence that calcification in the light is significantly higher than calcification in the dark.

Photosynthetic CO2 uptake lowers the extracellular CO2 partial pressure in the coral tissue, which increases carbonate saturation and favors precipitation of CaCO3 (Goreau, 1959, 1961).

Photosynthesis and calcification could also compete for a single DIC pool (Kuile et al, 1989b).

CO2 uptake by net photosynthesis under saturating irradiance is significantly higher than the concurrent CO2 release by calcification (ca. 160 vs. 106 (xmol CO2 (mg Chl-a)"' hr1;Fig. 5). An additional source of inorganic carbon (from external seawater) is therefore required to sustain photosynthetic CO2 fixation. CO2 generated by CaCO3 deposition (0.6 times net calcification) could potentially supply 78% of the inorganic carbon required for zooxanthellar photosynthesis. The large decrease in pCO2 measured in such systems during the day (down to ca. 250 u.atm; Frankignoulle et al, 1996) demonstrates that DIC is drawn from the seawater reservoir and, to a much lesser extent, from the invasion of atmospheric CO2.

Smith and Buddemeier (1992) identified the following major parameters that may affect the structure and function of coral reefs: sea level, temperature, CO2, ultraviolet (UV) radiation, hydrodynamics, sedimentation, salinity and nutrients. Photosynthesis of marine phototrophs is generally not considered as carbon-limited due to the large pool of total inorganic carbon in the form of bicarbonate (Raven, 1997).

And in case you are wondering, the processes of calcification and photosynthesis are spatially separated (Vandermeulen and Muscatine, 1974). Skeletogenesis is performed by the ectodermal cells of the aboral layers, the calicoblastic epithelium, whereas photosynthesis is carried out by zooxanthellae which are mainly located in the endodermal cells of the oral layers.

It was assumed that the surface seawater and the atmosphere are in equilibrium with respect to CO2. The pCO2 of coral reef waters will generally be equilibrated with the higher atmospheric CO2 levels, but non-atmospheric inputs will (1) diminish. The responses of scleractinian corals to short-term changes in a single environmental parameter are reasonably well known through experimental and ecological observations, but synergistic effects are extremely difficult to predict.

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