Coral calcification mechanisms facilitate adaptive responses to ocean acidification

Abstract

Ocean acidification (OA) is a pressing threat to reef-building corals, but it remains poorly understood how coral calcification is inhibited by OA and whether corals could acclimatize and/or adapt to OA. Using a novel geochemical approach, we reconstructed the carbonate chemistry of the calcifying fluid in two coral species using both a pH and dissolved inorganic carbon (DIC) proxy (δB and B/Ca, respectively). To address the potential for adaptive responses, both species were collected from two sites spanning a natural gradient in seawater pH and temperature, and then subjected to three pH levels (8.04, 7.88, 7.71) crossed by two temperatures (control, +1.5°C) for 14 weeks. Corals from the site with naturally lower seawater pH calcified faster and maintained growth better under simulated OA than corals from the higher-pH site. This ability was consistently linked to higher pH yet lower DIC values in the calcifying fluid, suggesting that these differences are the result of long-term acclimatization and/or local adaptation to naturally lower seawater pH. Nevertheless, all corals elevated both pH and DIC significantly over seawater values, even under OA. This implies that high pH upregulation combined with moderate levels of DIC upregulation promote resistance and adaptive responses of coral calcification to OA.

Environmental and physiochemical controls on coral calcification along a latitudinal temperature gradient in Western Australia

“…This results in a relatively “stable” yet elevated Ω cf that falls within a relatively narrow range (11 ± 1.5 across all sites and species; Figure b) and a decoupling between Ω cf and rates of calcification can occur (Figure f). This is consistent with previous studies showing that a relatively “stable” yet elevated threshold Ω cf inside the coral calcifying fluid is likely a pre‐requisite for calcification (D'Olivo & McCulloch, ; McCulloch et al, ; Schoepf et al, ). During periods of lower calcification rates, there is an increase in [Ca 2+ ] cf that is consistent with less Ca 2+ being depleted from the calcifying fluid during the calcification process (Figure e) indicating that [Ca 2+ ] cf is not a limiting component of the calcification process.…”

“…Consistent with previous studies, pH cf was generally counter‐regulated with DIC cf (Cornwall et al, ; D'Olivo & McCulloch, ; McCulloch et al, ; Ross et al, ; Schoepf et al, ) thereby maintaining a tightly controlled chemical composition in the calcifying fluid at species‐dependent levels (McCulloch et al, ). Accordingly, changes in coral pH cf upregulation could be a mechanism aimed at maximizing and stabilizing both Ω cf and calcification rates.…”

“…OA can result in morphological deformities in juvenile corals (Cohen, McCorkle, de Putron, Gaetani, & Rose, ; Foster, Falter, McCulloch, & Clode, ) and a reduction in coral calcification rates for some adult corals (e.g., Crook, Cohen, Rebolledo‐Vieyra, Hernandez, & Paytan, ; Jokiel et al, ; Marubini et al, ; Marubini, Ferrier‐Pagès, & Cuif, ; Mollica et al, ), although this is not always the case (e.g., Comeau et al, ; Comeau, Cornwall, DeCarlo, Krieger, & McCulloch, ; Jury, Whitehead, & Szmant, ; Schoepf et al, ). Variation in the responses of coral calcification to OA can be explained by differences in species, life stage, food availability, growth form (e.g., Albright & Langdon, ; Cohen & Holcomb, ; Kornder, Riegl, & Figueiredo, ), and bio‐calcification mechanisms (e.g., Comeau et al, ; DeCarlo, Comeau, Cornwall, & McCulloch, ; Georgiou et al, ; Schoepf, Jury, Toonen, & McCulloch, ).…”

Reviews and syntheses: Revisiting the boron systematics of aragonite and their application to coral calcification

“…CC BY 4.0 License. Ross et al, 2017;Schoepf et al, 2017). Here, we investigate the differences in derived coral calcifying fluid [CO DIC cf are both highest in summer and lowest in winter over a multi-year time series.…”

“…In recent years, boron systematics (including δ 11 B and B/Ca) have become one of the most commonly applied proxies for the carbonate chemistry of coral calcifying fluid (cf) (Hönisch et al, 2004;Trotter et al, 2011;McCulloch et al, 2012bMcCulloch et al, , a, 2017Allison et al, 2014;DeCarlo et al, 2016;Stewart et al, 2016;Comeau et al, 2017;Wu et al, 2017;D'Olivo and McCulloch, 2017;Kubota et al, 2017;Ross et al, 2017;Schoepf et al, 2017). The sensitivity of boron isotopes to seawater pH arises from the borate versus boric acid speciation being pH-dependent and the isotopic fractionation between these species being constant 15 (Klochko et al, 2006).…”