The impact of seawater saturation state and bicarbonate ion concentration on calcification by new recruits of two Atlantic corals
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Rising concentrations of atmospheric CO2 are changing the carbonate chemistry of the oceans, a process known as ocean acidification (OA). Absorption of this CO2 by the surface oceans is increasing the amount of total dissolved inorganic carbon (DIC) and bicarbonate ion (HCO3 (-)) available for marine calcification yet is simultaneously lowering the seawater pH and carbonate ion concentration ([CO3 (2-)]), and thus the saturation state of seawater with respect to aragonite (Omega(ar)). We investigated the relative importance of [HCO3 (-)] versus [CO3 (2-)] for early calcification by new recruits (primary polyps settled from zooxanthellate larvae) of two tropical coral species, Favia fragum and Porites astreoides. The polyps were reared over a range of Omega(ar) values, which were manipulated by both acid-addition at constant pCO(2) (decreased total [HCO3 (-)] and [CO3 (2-)]) and by pCO(2) elevation at constant alkalinity (increased [HCO3 (-)], decreased [CO3 (2-)]). Calcification after 2 weeks was quantified by weighing the complete skeleton (corallite) accreted by each polyp over the course of the experiment. Both species exhibited the same negative response to decreasing [CO3 (2-)] whether Omega(ar) was lowered by acid-addition or by pCO(2) elevation-calcification did not follow total DIC or [HCO3 (-)]. Nevertheless, the calcification response to decreasing [CO3 (2-)] was nonlinear. A statistically significant decrease in calcification was only detected between Omega(ar) = < 2.5 and Omega(ar) = 1.1-1.5, where calcification of new recruits was reduced by 22-37% per 1.0 decrease in Omega(ar). Our results differ from many previous studies that report a linear coral calcification response to OA, and from those showing that calcification increases with increasing [HCO3 (-)]. Clearly, the coral calcification response to OA is variable and complex. A deeper understanding of the biomineralization mechanisms and environmental conditions underlying these variable responses is needed to support informed predictions about future OA impacts on corals and coral reefs.