Similar controls on calcification under ocean acidification across unrelated coral reef taxa

Abstract

Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Ω), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Ω) is the main driver. We grew corals and coralline algae for 8-21 weeks, under treatments where the seawater parameters Ω, pH, and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Ω , pH , and DIC ). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Ω, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Ω had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resistance of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa.

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

“…An additional factor to consider is the influence of seawater pH on pH cf . Given that the calcifying fluid is not completely isolated from seawater, the external seawater pH can influence pH cf (Comeau, Cornwall, & McCulloch, ; Holcomb et al, ; Hönisch et al, ; Krief et al, ; McCulloch et al, ; Trotter et al, ; Venn et al, ) and rates of calcification (e.g., Comeau et al, ; Jokiel et al, ). Experimentally derived changes in coral pH cf are usually equal to approximately one‐third to one‐half of those in seawater pH (McCulloch et al, ; Trotter et al, ; Venn et al, ).…”

“…While high‐latitude zones are expected to be the first and worst affected by OA due to the already relatively low seawater Ω at high‐latitude (Orr et al, ; van Hooidonk et al, ), our results show that corals from a range of environments (i.e., tropical to temperate surface waters) can regulate their pH cf (and potentially calcification rates), such that corals at high‐latitude show a greater extent of pH upregulation than their tropical counterparts (i.e., ~8.5–8.65 at high‐latitude and ~8.23–8.45 in the tropics). This substantial elevation in pH cf relative to seawater pH (i.e., pH cf : ~0.44–0.55 at high‐latitude and ~0.20–0.44 in the tropics), together with an elevation in DIC cf across multiple coral species, varies with temperature temporally (seasonally) and spatially (with latitude) and is likely to be a critical factor in determining both spatial and species‐specific variability in the sensitivity of corals to OA (Comeau et al, ; McCulloch et al, ).…”

“…In addition to ocean warming, increasing CO 2 is driving declines in seawater pH and thus seawater aragonite saturation state (Ω = [][Ca 2+ ]/ K sp ). 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, ).…”

“…Studies have shown that corals possess mechanisms to modulate the chemical conditions at the site of calcification inside the semi‐isolated “pockets” of calcifying fluid (cf; Clode & Marshall, ; Cohen & McConnaughey, ). This process of physiological control is thought to be predominantly driven by the regulation of pH (McCulloch et al, ; Ries, ; Venn, Tambutté, Holcomb, Allemand, & Tambutté, ) and [Ca 2+ ] (DeCarlo, Comeau, et al, ), as well as the supply of dissolved inorganic carbon (DIC) to the calcifying fluid. These three parameters can all be elevated above seawater levels to maintain high Ω cf (~10–25) and promote skeletal growth.…”

“…The extent to which corals pump H + out of the calcifying fluid to shift the composition of DIC in favour of carbonate () relative to bicarbonate () influences chemical conditions in the calcifying fluid (Cohen & McConnaughey, ; Sinclair, ) and is therefore crucial for determining the sensitivity of coral calcification to OA (Comeau et al, ; McCulloch et al, ; Venn, Tambutté, Holcomb, Laurent, & Allemand, ). At the same time, other factors beyond seawater pH may also influence pH cf and potentially calcification rates.…”