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How do corals make their skeletons? Insights from boron geochemistry


   Ocean and Earth Science


Southampton United Kingdom Analytical Chemistry Biochemistry Climate Science Environmental Biology Environmental Chemistry Geochemistry Marine Biology Geology

About the Project

Project Rationale
Tropical coral reefs are diversity hotspots, offer coastal protection, and sustain important economic activities like fisheries and tourism. All of these ecosystem functions depend on the 3D framework of the reef that is constructed by stony corals within the Extracellular Calcifying Medium (ECM) – a micron-sized space sandwiched between each coral animal and its existing skeleton. Stony corals are, however, under threat from anthropogenic activities that disturb key physiological processes such as biomineralisation, leading to decreased fitness and mortality and ultimately a loss of ecosystem function.

In order to better predict the fate of coral reef ecosystems and to develop effective mitigation strategies we need a mechanistic understanding of how corals build their skeletons and how changes in the environment effect this process. Such an understanding is currently lacking, in part because of the difficulty of studying the processes that occur within the micron-sized ECM. It has recently become evident that the boron content (expressed as B/Ca) and boron isotopic composition of the deposited coral skeleton tracks the nature of the carbonate system in the calcifying space, suggesting that the aragonite saturation state is the most important variable in controlling skeleton growth [1]. It is the aim of this PhD to use a series of novel experiments to improve our understanding of how boron tracks the carbonate system in the ECM, to allow increasingly robust models of coral mineralization to be constructed.

Methodology
Whilst the boron isotopic composition of coral skeletons is a well-constrained function of the pH of the ECM [1 & 2], the link between B/Ca ratios and the ECM carbonate ion (CO32-) content is largely empirical and several parameterisations exist [3]. These assumptions currently limit the accuracy of reconstructions of the aragonite saturation state using the boron system, clouding our understanding of the relationship of external environmental change and ECM chemistry.

This project aims to improve this situation by:
1. Performing a suite of well-constrained carbonate precipitation experiments, more fully exploring the influence of precipitation rate, pH, CO32-, and temperature on 11B and B/Ca of inorganic aragonite (following [4])
2. In order to more closely approximate the calcifying environment of coral aragonite we will also carry out precipitation experiments using: (a) coarsely crushed coral skeleton as seed material; (b) soluble skeletal organic matrix isolated from a number of species; (c) aspartic acid and carbonic anhydrase additives; (d) coral acidic proteins isolated from several coral species.
3. Testing the improved understanding that objectives 1 and 2 provide using micro-electrodes and pH sensitive dyes to visualize and directly probe the carbonate chemistry of coral ECM. There is little data of this kind (e.g. [2]) and more is needed to validate the indirect geochemical methods. This will be done in Israel under the supervison of Prof. Erez.

Training
All doctoral candidates will enrole in the Graduate School of NOCS (GSNOCS), where they will receive specialist training in oral and written presentation skills, have the opportunity to participate in teaching activities, and have access to a full range of research and generic training opportunities. GSNOCS attracts students from all over the world and from all science and engineering backgrounds. There are currently around 200 full- and part-time PhD students enrolled (~60% UK and 40% EU & overseas). Specific training will include:

• The operation of a pH-stat system for the precipitation of aragonite under tightly controlled conditions, including the monitoring of the carbonate system and other environmental conditions.
• Inductively coupled plasma mass spectrometry (ICP-MS) to determine the chemical composition of the growth medium and precipitated CaCO3.
• Multicollector ICP-MS for the determination of the boron isotopic composition of growth medium and precipitated CaCO3.
• Raman spectroscopy for the mineralogical characterization of precipitated CaCO3
• Imaging of the pH and measurement of the carbonate system within the ECM of coral using confocal microscopy and micro-electrodes.

The student will be part of a large team, part of the PI Foster’s €3.5million focusing on coral biomineralisation (Microns2Reefs) using a variety of techniques and funded by the European Research Council. This is truly interdisciplinary research that will expose the student to a range of approaches from coral genomics to geochemistry. See this link for more details: http://www.thefosterlab.org/microns2reefs



Funding Notes


References

[1] McCulloch, M. T., J. Falter, J. Trotter, and P. Montagna (2012), Coral resilience to ocean acidification and global warming through pH up-regulation, Nature Climate Change, 2, 623-627, doi: 10.1038/NCLIMATE1473.
[2] Holcomb, M., A. A. Venn, E. Tambutte, D. Allemand, J. Trotter, and M. T. McCulloch (2014), Coral calcifying fluid pH dictates response to ocean acidification, Scientific Reports, 4(5207), 1-4, doi: 10.1038/srep05207
[3] DeCarlo, T. M., M. Holcomb, and M. T. McCulloch (2018), Reviews and syntheses: Revisiting the boron systematics of aragonite and their application to coral calcification, Biogeosciences, 15(9), 2819-2834, doi: 10.5194/bg-15-2819-2018.
[4] Mavromatis, V, Montouillout, V, Noireaux, J., Gaillardet, J., Schott, J. (2015) Characterization of boron incorporation and speciation in calcite and aragonite from co-precipitation experiments under controlled pH, temperature and precipitation rate, Geochimica et Cosmochimica Acta, 150, 299-313.

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