Boron Isotopes pp 145-162 | Cite as

Boron Isotopic Systematics in Scleractinian Corals and the Role of pH Up-regulation

  • Malcolm T. McCulloch
  • Juan P. D’Olivo
  • James Falter
  • Lucy Georgiou
  • Michael Holcomb
  • Paolo Montagna
  • Julie A. Trotter
Part of the Advances in Isotope Geochemistry book series (ADISOTOPE)


The boron isotopic composition (δ11B) of scleractinian corals has been used to track changes in seawater pH and more recently as a probe into the processes controlling bio-calcification. For corals that precipitate aragonite skeletons, up-regulation of pH appears to be a general characteristic, typically being ~0.3 to ~0.6 pH units higher than ambient seawater. The relationship between the pH of the corals calcifying-fluid (pHcf) and seawater pHT (total scale) is shown to be dependent on both physiological as well environmental factors. In laboratory experiments conducted on symbiont-bearing (zooxanthellate) corals under conditions of constant temperature and seawater pH, changes in the δ11B derived calcifying fluid pHcf is typically 1/3 to 1/2 of that of ambient seawater. Similar linear relationships are found for cold water corals that live in relatively stable, cold, deep-water environments but at significantly elevated levels of pHcf (~0.5–1 pH units above seawater), a likely response to the lower pH of their deep-sea environments. In contrast, zooxanthellae-bearing corals living in shallow-water reef environments that experience significant natural variations in temperature, light, nutrients and seawater pH, show different types of responses. For example, over seasonal time-scales Porites corals from the Great Barrier Reef (GBR) have a large range in pHcf of ~8.3 to ~8.5, significantly greater (~×2 to ~×3) than that of reef-water (pHT ~8.01 to ~8.08), and an order of magnitude greater than that expected from ‘static’ laboratory experiments. Strong physiological controls, but of a different character, are found in corals grown in a Free Ocean Carbon Enrichment Experiment (FOCE) conducted in situ within the Heron Island lagoon (GBR). These corals exhibit near constant pHcf values regardless of external changes in temperature and seawater pH. This pattern of strong physiologically controlled ‘pH-homeostasis’, with elevated but constant pHcf has been found despite large natural seasonal variations in the pH (±0.15 pH units) of the lagoon waters, as well as the even larger super-imposed decreases in seawater pH (~0.25 pH units) designed to simulate year 2100 conditions. In natural reef environments we thus find that the processes influencing the up-regulation of pHcf in symbiont-bearing corals are subject to strong physiological controls, behaviour that is not well simulated in the current generation of aquaria-based experiments with fixed seawater pH and temperature. Conversely, cold-water corals that lack symbionts and inhabit the relatively stable deep-sea environments hold the best prospects for providing reliable reconstructions of seawater pH. Clearly, further studies utilising the δ11B-pHcf proxy combined with other DIC/carbonate-ion proxies (e.g. B/Ca), but conducted under realistic ‘natural’ conditions, are required to elucidate the processes controlling coral bio-calcification and to better understand the vulnerability of scleractinian corals to anthropogenic driven warming and ocean acidification.


pH up-regulation Boron isotopes Corals 



This research was supported by funding from an ARC Laureate Fellowship awarded to Professor Malcolm McCulloch and the ARC Centre of Excellence for Coral Reef Studies. Dr Julie Trotter was supported by an ARC Future Fellowship and Dr Michael Holcomb by an ARC Super Science Fellowship. Dr Paolo Montagna is grateful for ongoing support by the Institute of Marine Sciences, CNR, Italy and this is ISMAR-CNR Bologna scientific contribution n. 1915. We thank Anne-Marin Comeau and Dr Kai Rankenburg for their technical assistance with measurements of the δ11B isotopic and B/Ca elemental ratios being conducted at The University of Western Australia’s Advanced Geochemical Facility for Indian Ocean Research (AGFIOR).


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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Malcolm T. McCulloch
    • 1
    • 2
  • Juan P. D’Olivo
    • 1
    • 2
  • James Falter
    • 1
    • 2
  • Lucy Georgiou
    • 1
    • 2
  • Michael Holcomb
    • 1
  • Paolo Montagna
    • 3
  • Julie A. Trotter
    • 1
  1. 1.Oceans Institute Graduate School and School of Earth SciencesThe University of Western AustraliaCrawleyAustralia
  2. 2.ARC Centre of Excellence for Coral Reef StudiesThe University of Western AustraliaCrawleyAustralia
  3. 3.Institute of Marine SciencesCNRBolognaItaly

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