The link between the skeleton of reef-building corals and past climatic ocean conditions
Scleractinian corals and reef formation
Coral reefs’ structures are built thanks to the skeleton of Scleractinian corals. These corals begin their life cycle as motile larvae in the water column, to then settle in a hard substrate, where they turn into an immobile polyp with calcium carbonate (CaCO3) skeleton. Each coral is made up of a multitude of polyps. Coral reefs are formed by a multitude of corals of different species. The reefs they create host more than 25% of marine species, which makes them biodiversity hotspots.
Paleoclimatic ocean conditions have shaped the evolution of corals and their skeletons through time
The skeletons of corals are the subject of many scientific studies because they can provide valuable information about the environment in the geological past (paleoclimatic research). In fact, during the formation of their skeletons, corals incorporate different chemical elements present in the environment, such as carbon, oxygen and trace elements (boron, magnesium). Thus, by analysing, the chemical composition of coral skeletons, it is possible to reconstruct some environmental parameters of the past . Also, investigating the evolutionary history of corals can also provide insight into how past climate and geochemical changes shaped the evolution of coral species, and thus help predict the fate of coral reefs and the ecosystem of which they are a part .
The formation of coral skeleton: biomineralization
During their evolution, some species have developed the ability to form rigid mineralized structures by combining minerals and organic molecules. This mechanism is called biomineralization.
This is the case of reef building corals, or Scleractinian corals, which form their skeleton by precipitating calcium carbonate (CaCO3) present in the water (Figure 1). In other words, calcium carbonate dissolved in the ocean will become a solid material. This building material is formed mainly from Calcium (Ca2+) and carbonate ions (CO32-).
Unfortunately, changes in the environment can disrupt the biomineralization process in living organisms, and corals are particularly concerned.
Figure 1 : CaCO3 crystal bundle growing surrounding by Stylophora pistillata cells aggregated into protopolyps. Source : 
Ocean acidification – an example of climate change disrupting the biomineralization of Scleractinian corals
What is ocean acidification? H+ ions are acidic ions. If the quantity of H+ ions increases in the ocean, its pH will decrease (the lower the pH, the more acidic it is), this is called ocean acidification. But where do these H+ come from? In general terms, their appearance is linked to increased levels of carbon dioxide dissolved in water (CO2). Since the 19th century, CO2 concentrations in the atmosphere have increased due to human activities. The ocean absorbs one part of the CO2, which then reacts with the water (H2O) creating carbonic acids (H2CO3). The chemical reactions result in the release of more H+ in the ocean, and acidification therefore increases (Figure 2).
Figure 2 : Scheme of the ocean acidification process. Source: Ocean and Climate platform.
The problem regarding corals in an acidic environment is that the availability of carbonate ions (CO32-) in the water decreases, which is part of the building material of coral skeletons. At the same time, calcium carbonate will react with carbonic acids (H2CO3) and turn into calcium bicarbonate Ca(HCO3)2 which is not usable for skeleton building.
So, to sum up, if there is more CO2 in the water, the calcium carbonate will no longer be available for hard-skeleton corals. Worse, in some cases, organisms with an already formed skeleton can dissolve.
The diversification of coral species over time in relation to past ocean conditions
As the explanation of ocean acidification shows, fluctuating environmental conditions impact the survival of corals. Global warming and ocean acidification are the causes of at least two of the five mass extinctions and two other reef crises . However, the resilience to environmental changes might change depending on the species.
A study conducted by Quattrini et al.  showed that the evolutionary diversification of coral species took place in response to the paleoclimatic conditions, linked to their skeleton structure. This means that the coral species that exist today have diversified over time, responding to the fluctuating conditions of the oceans. This includes variations in temperature, atmospheric CO2 levels, and geochemical set up, which can be more or less favourable to the different species. For instance, the ratio between magnesium and calcium (Mg2+ / Ca2+) present in the water has an impact on the species of coral communities . In what way? Calcium carbonate naturally crystallizes into two main forms: aragonite and calcite. Depending on ocean conditions, the ocean has experienced cycles of aragonite seas and calcite seas . In fact, high Mg2+/Ca2+ ratios favour the precipitation of aragonite , (aragonite seas). While low Mg2+/Ca2+ ratios favour the precipitation of calcite (calcite sea) .
Scleractinian corals have an aragonite skeleton, so these species must have appeared and diversified in the conditions of aragonite seas . While other Anthozoa species with a different skeleton (for example, octocorals and sea anemones with non calcifiying or with calcitic skeleton) would have evolved during periods of calcite seas, and high levels of atmospheric CO2 .
Although these evolutionary processes have occurred over long periods of time (thousands of years), it teaches us something about the current ocean condition. The decrease in the presence of reef-building corals due to the acidification of the ocean, can favour the survival of non-aragonite species (octocorals, anemones), to which acidic conditions are favourable. Therefore, these species can colonize areas where coral reefs formed by hard-skeleton corals are degraded.
Certain species have resisted better during and immediately after certain events (mass extinction, reef crises…) compared to hard-skeleton corals (reef builders). One of the reasons would be a higher adaptive capacity in response to environmental changes (no need for calcium carbonate to build their skeleton for example) coupled with new ecological opportunities.
By studying past environmental conditions, it is possible on one hand to understand the evolutionary appearance of new species, and on the other hand to predict the dynamics of coral communities over time, which can maybe help to change some human behaviours.
 Drake, J. L., Mass, T., Stolarski, J., Von Euw, S., van de Schootbrugge, B., & Falkowski, P. G. (2020). How corals made rocks through the ages. Global change biology, 26 (1), 31-53.
 Quattrini, A. M., Rodríguez, E., Faircloth, B. C., Cowman, P. F., Brugler, M. R., Farfan, G. A., … & Reimer, J. D. (2020). Palaeoclimate ocean conditions shaped the evolution of corals and their skeletons through deep time. Nature Ecology & Evolution, 4 (11), 1531-1538.
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