Chair: Janice Lough

Malcolm T. McCulloch (^1)*, Michael Holcomb (¹⁾, James Falter (¹⁾, Paolo Montagna (²⁾, Juan Pablo D’Olivo Cordero (¹⁾, Marco Taviani (²⁾ and Julie A. Trotter (¹⁾

1 School of Earth and Environment, UWA Oceans Institute and ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia, Crawley, WA, 6009, Australia
2 Institute of Marine Sciences, National Research Council, 40129 Bologna, Italy

Background
Scleractinian corals have been spectacularly successful in building reef edifices that harbour ~¹/₃ of the oceans biodiversity, as well as occupying many cold-water, deep-sea habitats. This success can be largely attributed to their ability to biologically mediate the composition of their calcifying fluid (cf), and thereby increase aragonite saturation state (cf) to enhance the kinetics of calcification. Manipulation of calcifying fluidcomposition is thought to occur via Ca-ATPase controlled up-regulation of pH and still poorly understood transport mechanisms for DIC. How the biologically mediated process of pHcf up-regulation will respond to the combined pressures of both extremely rapid CO₂-driven warming and ocean acidification, is thus critical to the future of scleractinian corals.

Methods
We have undertaken a global survey of hermatypic zooxanthellate corals sampled from the tropical Indo-Pacific and Caribbean, as well as azooxanthellate corals from both shallow-water tropical and deep-sea environments. Combined boron isotopic (¹¹B), B/Ca, and Sr/Ca systematics in their carbonate skeletons have been used to infer pHcf, and DICcf and hence the cf required to sustain biomineralisation across these diverse environments.

Findings
Regardless of location, we find that the dominant aragonitic zooxanthellate reef-building corals (e.g. Acropora, Favia, Porites, Pocillopora, Galaxea, Turbinaria, Fungia, Montipora, Montastraea, Stylophora, Trachyphyllia, Lobophyllia) have a similar range of pHcf of 8.4 to 8.6, ~0.3 to ~0.5 pH units higher than ambient seawater. Interestingly, we find that the azooxanthellate tropical coral Tubastrea exhibits high pH_cf up-regulation (+0.5 to 0.6 pH units), similar to that found in deep-sea cold-water corals (e.g. Desmophyllum). The DICcf characteristics are found to be systematically enriched (x1.5 to x2 relative to seawater) with tropical corals having significantly elevated cf (14 to 20).

Conclusions
Up-regulation of calcifying fluid pHcf and hence cf is a ubiquitous feature of scleratinian corals and crucial to understanding their future in a high CO₂ World.