Do corals store carbon?

Carbon storage in ecosystems is increasingly presented as a way to fight climate change. But the storage and release of carbon by coral skeletons is a complex subject. We will publish several articles in the coming months to explain the debate in detail, starting with this one which is an introduction to the biological process behind the formation of a coral skeleton.

The three-dimensional architecture of coral skeletons allows coral ecosystems to provide habitat, refuge and food for marine species. It also protects coastlines from erosion by mitigating the force of waves, to the benefit of human communities around the world. Thus, by the very shape of their skeleton, corals are invaluable to the marine ecosystem, but also to coastal communities.

 

Coral skeletons’ formation

Hermatypic corals (also known as reef-building corals) are at the origin of the reefs’ limestone framework, the biomineral structure on which the entire ecosystem rests. Unlike soft corals (Gorgonians, Antipatharians, Alcyonacea corals) whose calcium carbonate skeleton is in the form of calcite, the rigidity of the aragonite-shape limestone skeleton of reef-building corals makes them perfect reef engineers (Allemand et al., 2004).

 

The limestone skeleton is responsible for the various shapes of corals. It also contributes to the spread and absorption of light within corals that live in symbiosis with microalgae (also called zooxanthellae) (Enriquez et al., 2005). Calcification, or biomineralization, is the rate at which corals produce their skeleton. Through this process, the calcium carbonate skeleton (CaCO3) is crystallized in the form of aragonite (Courtial et al., 2021). This, thanks to the accumulation and transformation of calcium and carbon.

• The carbon can have two origins : from carbon dioxide (CO2) produced with coral respiration, and bicarbonate ions (HCO3–) present in seawater. 
• Calcium ions (Ca2+) and bicarbonate ions (HCO3–), that are available in seawater, are captured by the coral’s polyps and carried through four layers of cells to reach the calicoblastic cells, which is the last layer before the limestone skeleton (Tambutté et al., 2011). It is also here that the carbon dioxide is transformed into bicarbonate ions as shown in the following reaction (1) :

CO2 + H2O ⇔ HCO3– + H+ (1)
Carbon dioxide + water ⇔ Bicarbonate ions + hydrogen ions

Once settled, bicarbonate and calcium ions are transformed into calcium carbonate, while, at the same time, a hydrogen ion is released following reaction (2). These hydrogen ions will react with bicarbonate ions to produce CO2, following reaction (1).

Ca2+ + HCO3− ⇔ CaCO3 + H+ (2)
Calcium ions + Carbonate ions ⇔ Calcium carbonate + Hydrogen ions

Finally, calcium carbonate formation occurs as shown in reaction (3) :

Ca2+ + 2 HCO3−⇔ CaCO3 + CO2 + H2O (3)
Calcium ions + Carbonate ions ⇔ Calcium carbonate + Carbon dioxide + water

 

The calcium carbonate (CaCO3) is precipitated in aragonite crystals, bricks of the coral skeleton. In fact, the biomineralization process repeats to form an aragonite chain that constitutes the skeleton. However, it is the organic matrix that defines the structure of these crystals’ succession imposing a specific morphology to the coral. It is composed of proteins, lipids and carbohydrates that guide the limestone skeleton formation (Reyes-Bermudez et al., 2009). 

Therefore, the skeleton formation, or biomineralization, is a process that requires both the transport of ions to the cells and the supervision of the construction by the coral organic matrix at the same time.

 

 

Figure 1: simplified layout of carbon transport for the coral skeleton’s formation, inside coral layers. Source: self elaboration based on Furla et al. (2000), Allemand et al. (2004) and Tambutté et al. (2011).

 

Carbon capture and release by corals

 

Through biomineralization and photosynthesis by the zooxanthellae, corals are trading platforms that are both sinks and sources of inorganic carbon. Via photosynthesis, symbiotic zooxanthellae present in corals’ tissues consume CO2 and provide corals with oxygen (Courtial et al., 2021). Regarding biomineralization, the coral captures carbon dioxide and bicarbonate ions to build its limestone skeleton.

However, during calcification, the hydrogen ions (H+), produced in the reactions  (1) and (2), react with the bicarbonate ions (HCO3–) to produce CO2, in the reverse way of reaction (1). This means that the production and liberation of CO2 is induced in the coral tissue, using the excess of H+ after the skeleton formation, in order to re-establish the equilibrium (Allemand et al., 2004). Biomineralization in corals becomes a source of carbon through the release of CO2 occurring during the transformation of hydrogen ions. In fact, in the marine environment, for each unit (mol) of CaCO3 precipitated, 0.6 units (mol) of CO2 are released (Ware, 1991). 

On the other hand, coral respiration releases carbon dioxide. Although some of it is used by zooxanthellae, another part of the carbon ends up in the sea.

 

Coral and carbon debate

 

There is a large debate around the role of corals in carbon capture and release, on both an individual scale and on the scale of the whole ecosystem. The scientific community is often confronted with the complexity of the processes related to the different scales, as well as the variations present within reefs in natural environments.

What would be the impact of this inorganic carbon ratio (stored/released) on the scale of an entire reef? In light of current climate challenges, the storage and release of inorganic carbon within the oceans raises questions about the function of coral reefs in the carbon balance and the threats they face, especially regarding the formation of their skeleton and the support of the ecosystem.

We will be writing short articles over the next few months to introduce you to this fascinating debate, and to try to shed some light on the subject!

 

Bibliography :

Allemand, D., Ferrier-Pagès, C., Furla, P., Houlbrèque, F., Puverel, S., Reynaud, S., Tambutté, É., Tambutté, S., & Zoccola, D. (2004). Biomineralisation in reef-building corals: From molecular mechanisms to environmental control. Comptes Rendus Palevol, 3(6), 453–467. https://doi.org/10.1016/j.crpv.2004.07.011

COURTIAL Lucile – ALLEMAND Denis – FURLA Paola (2021), Coraux : les ingénieurs des océans sont menacés, Encyclopédie de l’Environnement, [en ligne ISSN 2555-0950] url : http://www.encyclopedie-environnement.org/?p=4737

Enriquez, S., Méndez, E.R., Iglesias-Prieto, R., (2005). Multiple scattering on coral skeletons enhances light absorption by symbiotic algae. Limnol. Oceanogr. 50 (4),1025–103.

Furla, P. et al. (2000) ‘Sources and mechanisms of inorganic carbon transport for coral calcification and photosynthesis’, Journal of Experimental Biology, 203(22), pp. 3445–3457. doi:10.1242/jeb.203.22.3445.

Gattuso, J., Pichon, M., Delesalle, B., Canon, C., & Frankignoulle, M. (1996). Carbon fluxes in coral reefs. I. Lagrangian measurement of community metabolism and resulting air-sea CO2 disequilibrium. Marine Ecology Progress Series, 145, 109–121. https://doi.org/10.3354/meps145109 

Reyes-Bermudez, A., Lin, Z., Hayward, D.C. et al, (2009). Differential expression of three galaxin-related genes during settlement and metamorphosis in the scleractinian coral Acropora millepora. BMC Evol Biol 9, 178. https://doi.org/10.1186/1471-2148-9-178

Tambutté, S., Holcomb, M., Ferrier-Pagès, C., Reynaud, S., Tambutté, É., Zoccola, D., & Allemand, D. (2011). Coral biomineralization: From the gene to the environment. Journal of Experimental Marine Biology and Ecology, 408(1), 58–78. https://doi.org/10.1016/j.jembe.2011.07.026

Ware, J.R., 1991. Coral reefs: sources or sinks of atmospheric CO2? Coral Reefs 11, 127–130. https://doi.org/10.1007/BF00255465