Crystallographic and chemical signatures in coral skeletal aragonite

Gabriela A Farfan,Amy Apprill,Anne Cohen,Jeffrey E Post,Colleen M Hansel,Rhian G Waller,Thomas M Decarlo

doi:10.1007/s00338-021-02198-4

Abstract

Corals nucleate and grow aragonite crystals, organizing them into intricate skeletal structures that ultimately build the world’s coral reefs. Crystallography and chemistry have profound influence on the material properties of these skeletal building blocks, yet gaps remain in our knowledge about coral aragonite on the atomic scale. Across a broad diversity of shallow-water and deep-sea scleractinian corals from vastly different environments, coral aragonites are remarkably similar to one another, confirming that corals exert control on the carbonate chemistry of the calcifying space relative to the surrounding seawater. Nuances in coral aragonite structures relate most closely to trace element chemistry and aragonite saturation state, suggesting the primary controls on aragonite structure are ionic strength and trace element chemistry, with growth rate playing a secondary role. We also show how coral aragonites are crystallographically indistinguishable from synthetic abiogenic aragonite analogs precipitated from seawater under conditions mimicking coral calcifying fluid. In contrast, coral aragonites are distinct from geologically formed aragonites, a synthetic aragonite precipitated from a freshwater solution, and mollusk aragonites. Crystallographic signatures have future applications in understanding the material properties of coral aragonite and predicting the persistence of coral reefs in a rapidly changing ocean.

Highlights

Scleractinian coral skeletons form the framework of coral reef ecosystems, supporting a diversity of marine organisms, the livelihoods of hundreds of millions of people, and amassing an economic value of almost one trillion dollars (Carpenter et al 2008; Hoegh-Guldberg et al 2017)
Crystallographic measurements taken by micro-powder X-ray diffraction (XRD) confirm that all of the samples measured are predominantly aragonite (Fig. 1)
We find significant and reproducible similarities and differences among aragonites

Summary

Introduction

Scleractinian coral skeletons form the framework of coral reef ecosystems, supporting a diversity of marine organisms, the livelihoods of hundreds of millions of people, and amassing an economic value of almost one trillion dollars (Carpenter et al 2008; Hoegh-Guldberg et al 2017). This calcifying fluid has been measured in terms of internal pH and internal aragonite saturation state (Xcf), using crystal morphology (Cohen et al 2009; Holcomb et al 2009), pH-sensitive dyes (Venn et al 2011), microsensors (Cai et al 2016; Sevilgen et al 2019), boron isotopes (Allison et al 2010; McCulloch et al 2012a, b), and Raman spectroscopy (DeCarlo et al 2017) These measurements all indicate that corals elevate aragonite saturation conditions well above that of surrounding seawater. Coral skeletal health under climate change is dependent on the corals’ ability to control their calcifying fluid chemistry and biomolecular templates

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Conclusion