Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

Thomas M Decarlo,Taryn Foster,Juan P D'Olivo,Malcolm T Mcculloch,Michael Holcomb,Thomas Becker

doi:10.5194/bg-14-5253-2017

Abstract

Abstract. Quantifying the saturation state of aragonite (ΩAr) within the calcifying fluid of corals is critical for understanding their biomineralization process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry enable the determination of calcifying fluid pH and [CO32−], but direct quantification of ΩAr (where ΩAr = [CO32−][Ca2+]∕Ksp) has proved elusive. Here we test a new technique for deriving ΩAr based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of ΩAr from 10 to 34, and we found a strong dependence of Raman peak width on ΩAr with no significant effects of other factors including pH, Mg∕Ca partitioning, and temperature. Validation of our Raman technique for corals is difficult because there are presently no direct measurements of calcifying fluid ΩAr available for comparison. However, Raman analysis of the international coral standard JCp-1 produced ΩAr of 12.3 ± 0.3, which we demonstrate is consistent with published skeletal Mg∕Ca, Sr∕Ca, B∕Ca, δ11B, and δ44Ca data. Raman measurements are rapid ( ≤ 1 s), high-resolution ( ≤ 1 µm), precise (derived ΩAr ± 1 to 2 per spectrum depending on instrument configuration), accurate ( ±2 if ΩAr < 20), and require minimal sample preparation, making the technique well suited for testing the sensitivity of coral calcifying fluid ΩAr to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of ΩAr over multiple years of growth in a Porites skeleton from the Great Barrier Reef, and we evaluate the response of ΩAr in juvenile Acropora cultured under elevated CO2 and temperature.

Highlights

The calcium carbonate (CaCO3) skeletons built by coral polyps are the building blocks of massive coral reef structures that protect shorelines, bolster tourism, and host some of the greatest concentrations of biodiversity on the planet (Knowlton et al, 2010; Costanza et al, 2014)
In the abiogenic aragonites analysed in this study, ν1 FWHM was strongly correlated with seawater Ar (r2 = 0.70, p < 0.001; Fig. 2 and Tables 1–2)
In contrast to the strong dependence of ν1 FWHM on Ar, there was no significant correlation between ν1 FWHM and G (Fig. 3; Table 1)

Summary

Introduction

The calcium carbonate (CaCO3) skeletons built by coral polyps are the building blocks of massive coral reef structures that protect shorelines, bolster tourism, and host some of the greatest concentrations of biodiversity on the planet (Knowlton et al, 2010; Costanza et al, 2014). DeCarlo et al.: Raman proxy for aragonite saturation state sions continue unabated, by the end of the 21st century the [CO23−] of surface seawater is projected to decline to ∼ 50 % of pre-industrial levels (Hoegh-Guldberg et al, 2014). This rapid change in ocean carbonate chemistry, likely unprecedented for hundreds of millions of years (Hönisch et al, 2012; Zeebe et al, 2016), has sparked concerns for coral growth. Laboratory experiments repeatedly demonstrate that coral calcification rates decrease in response to lower [CO23−] or [Ca2+] (Gattuso et al, 1998; Chan and Connolly, 2013; Comeau et al, 2017), leading to projections that as CO2 levels continue to rise calcification will decline to unsustainable levels, such that there is net reef erosion (HoeghGuldberg et al, 2007; Pandolfi et al, 2011)

Methods

Results

Discussion

Conclusion