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Is Aragonite Saturation State (Ωa) the Best Way to Describe Calcification Rate?

Jokiel PL.  Coral reef calcification: carbonate, bicarbonate and proton flux under conditions of increasing ocean acidification. Proc R Soc B 280: 20130031. 




Studies on the impact of ocean acidification on coral and algae calcification are mainly reported as a factor of the concentration of carbonate ion [CO32-] or its surrogate aragonite saturation state, known as a. If Ωa > 1, seawater is supersaturated with respect to calcium carbonate (CaCO3) and conditions are favorable for CaCO3 precipitation, the process necessary to form corals; conversely, if Ωa < 1, seawater is undersaturated with respect to CaCO3 and the dissolution of CaCO3 is favored.

Ωa = [Ca2+][CO32- ]/Ksp

Comeau et al. points out that both bicarbonate (HCO3-) and carbonate (CO32-) ions are involved in calcification. In a seawater environment (pH≈8.1), bicarbonate is the predominant form of inorganic carbon thus it serves as the most available source of inorganic carbon; meanwhile, organisms’ active intake of CO32- from the seawater remains a mystery. A third component of total inorganic carbon in seawater, aqueous carbon dioxide (CO2), is also untested.

There is an alternative way to correlate calcification rate: the [DIC]/[H+] ratio. [DIC] is the sum of all dissolved inorganic carbon species in the water, which includes HCO3, CO32- and aqueous CO2. [H+], a measure of acidity, is the efflux of calcification behavior. Its concentration has a negative effect on calcification, in which a lower calcification rate would be resulted from a higher concentration of H+. The ratio [DIC] divided by [H+] can be viewed as the relative availability of reactant (inorganic carbon) in relation to concentration of inhibitory calcification waste product (protons) in the bulk water surrounding the calcifying organism



In order to see if the [DIC]/[H+] ratio is a good way to represent the calcification condition, data from an incubation experiment was used to test this hypothesis. There was a set of incubation experiments analyzing impact of ocean acidification on calcifying organisms. Corals and calcifying algae were taken from the coral reefs of Moorea, French Polynesia. Then samples were sent to the laboratory and underwent incubation experiments under different carbonate conditions (e.g. high carbonate and low bicarbonate). Experiment trials were conducted twice on corals Porites rus and alga Hydrolithon onkodes.



Calcification rate was plotted against the [DIC]/[H+] ratio and results indicate that calcification is controlled by the [DIC]/[H+] ratio. This ratio was plotted against carbonate concentration [CO32-] or aragonite saturation, a. It is apparent that the correlation of calcification rate with the [DIC]/[H+] ratio is as good as the one with [CO32-]. The reason is that [CO32-] is correlated with the ratio of [DIC]/[H+]. Likewise, a  is essentially a function of [CO32-], and therefore will also correlate with calcification rate.








a has long been widely used to represent past, present, and future global calcification patterns. However, compared to the [DIC]/[H+] ratio, it does not have any physiological meaning. The [DIC]/[H+] ratio is based on the most important source and sink for precipitated calcium carbonate and thus it helps us to understand the physiological processes behind calcification. It is an essential approach in helping to understand the impact of ocean acidification (which increases [H+]) on corals on an individual level. Some certain kinds of corals survive better under a more acidic environment. Under such circumstances, there can be an increase in both HCO3 and also H+. Therefore, those individuals who are able to take up HCO3and release H+ into the seawater more effectively have better chances of survival.



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