Climate Regulation: Coral Reefs are not Rainforests of the Sea

When it comes to regulating climate, there is no argument about the huge role that forests play by acting as sinks for carbon dioxide and releasing oxygen to further purify the atmosphere. This is why forests are referred to as the lungs of the earth by some folks. We all know the roles that lungs play in the body of mammals, right?

The lung is an organ in which gaseous exchange takes place. Oxygen is taken in while carbon dioxide is taken out or both gases are simply exchanged for each other. A similar thing also happens with forests during respiration. Forest species take in oxygen for respiration and release carbon dioxide. The huge difference is that trees, and green plants generally, are able to utilize carbon dioxide to produce foods that are broken down during respiration. They also generate oxygen in the process. Thus, they are much more than just lungs.

Among the world's forest layers, the rainforest stands out. Not just because of its carbon sequestration ability but due to its high-level diversity of organisms ranging from plants, animals, to microbes. That is as far as terrestrial ecosystems are concerned. When it comes to aquatic ecosystems, the high diversity of organisms in coral reefs has led to the structures being nicknamed the rainforest of the seas. While the biodiversity comparison might not be debatable for now, there have been huge arguments about whether coral reefs are or can be as important as rainforests in terms of carbon sinks/oxygen production and general regulation of climates.

Coral reefs are formed from the clustered exoskeletons of reef-forming polyps, otherwise known as hard corals. The tiny polyps build exoskeletons by a process known as calcification and are able to survive through a symbiotic association with another organism - the zooxanthellae. The latter are able to photosynthesize and provide food to the hard corals while the calcified exoskeletons of the corals provide secured homes for the zooxanthellae.

Thus, on the surface, the same process of carbon fixation through photosynthesis and carbon emission through respiration happens in both rainforests and coral reefs. Ipso facto, one would deem it logical that coral reefs are being tagged rainforests of the sea, especially when the biodiversity level of coral reefs is also considered.

The process leading to the formation of the exoskeletons of polyps, however, emits carbon dioxide. The calcium carbonate exoskeleton is formed from the salt water of the sea in a process known as calcification. The entire process can be summarized according to the chemical equation below:

Ca²⁺ + 2HCO₃⁻ ---> CaCO₃ + H₂O + CO₂

It is now left to determine whether the rate of emission is greater than the rate of sequestration or vice versa as far as the coral reef ecosystems are concerned. Usually, for most photosynthetic organisms, positive net primary production is a thing. In other words, the rate at which carbon dioxide is fixed into organic products outweighs the rate at which the organic product is broken down for energy and carbon dioxide.

This means that what differentiates rainforest and coral reefs as far as the metabolism of carbon dioxide is concerned is the extra emission that comes from calcification leading to the formation of the exoskeletons of coral polyps. Whether the extra emission is enough to throw the net primary production of coral reefs to negative remains a subject of research among ecologists. This will effectively indicate whether coral reefs are carbon sources or sinks.

Early findings currently report that coral reefs sequestrate as much carbon dioxide as they give off from respiration and calcification if both the reef flat and the lagoon sides of the geological feature are taken into consideration. If the reef flats are considered in isolation, the net primary productivity is negative while it is positive for the lagoon sides of the coral reefs.

Another thing to consider when comparing rainforests and coral reefs is that coral reefs may store carbon much longer than forests if we consider the rate at which forest trees die, rot/decompose. All in all, everything just points to the fact that coral reefs have their unique characteristics when it comes to carbon economy and their roles in climate regulation might have been misunderstood.

Thank you all for your audience.