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Precipitation of calcium carbonate takes place in the metabolically controlled calcifying fluid beneath the polyp tissue. The model is adjusted to a state of activity as observed by direct microsensor measurements in the calcifying fluid. As aragonite precipitation removes carbonate ions in equal amount, this process also decreases total alkalinity and dissolved inorganic carbonate (DIC). Simulated CO2 perturbation experiments reveal decreasing calcification rates under elevated pCO2 despite strong metabolic control of the calcifying fluid.

Decreasing coral calcification at elevated seawater pCO2 is the result of increased CO2 diffusion into the calcification site. However, we found that CO2 diffusion alone is not sufficient to sustain observed calcification rates and that an additional mechanism must exist to supplement carbon into the calcifying fluid. The model correctly reproduces the observed changes in calcifying fluid pH and calcium ion concentrations and shows a sustained calcification only when bicarbonate transport in symport with protons is considered. In our model, coral calcification is not directly dependent on the abundance of carbonate ions in seawater.

Related literature:

S. Hohn & A. Merico (2015) Quantifying the relative importance of transcellular and paracellular ion transports to coral polyp calcification, Frontiers in Earth Science, 2(37), 1-11.

S. Hohn & A. Merico (2012) Modelling coral polyp calcification in relation to ocean acidification, Biogeosciences, 9, 4441-4454. 

 

Processes model coral polyp calcificationProcesses model coral polyp calcification

 

Temporal changes carbonate system variablesTemporal changes of carbonate system variables in the calcifying fluid.Red dots show data from microsensor studies, while continuous blue lines represent model results. Light periods from 0 to 420 seconds and from 840 to 1400 seconds. Dark period between 420 and 840 seconds.

 

calcification ratesElevated pCO2 in seawater increases CO2 diffusion into the calcifying fluid thus decreasing carbonate ion concentrations and aragonite saturation in that compartment. This cause a decrease in calcification rates.