Mechanisms of Coral Calcification: a Geochemical and Computational Approach to Understanding Biomineralisation

Coral reefs are biodiversity hotspots, vital to the health of the entire marine ecosystem and play a critical part in the carbon cycle and climate feedbacks. In the face of a rapidly changing climate, the future of these reefs is uncertain. Corals also provide us with an invaluable archive of past ocean conditions through the impurities incorporated into their aragonite skeletons as they grow. The more robust our understanding of coral calcification, the better we will be able to constrain our estimates of past environmental change and predict how coral calcification will be impacted in the future. This thesis uses geochemical and computational approaches to understand the dynamics of the micron-sized space where coral biomineralisation occurs, with the aim of building a complete mechanistic model of how corals transform dissolved ions in seawater into an intricate mineralised skeleton. Analysis of skeletal chemistry following controlled culturing experiments enables a non-invasive, albeit indirect, investigation into transport processes, steady-state dynamics and kinetic effects. I present the results of two experiments examining the response of coral calcification to modified seawater chemistry. The first varies the concentration of calcium in seawater [Ca]²⁺, and the second varies seawater carbon chemistry (pH, DIC, CO₃²⁻ ). In both experiments, I consider the impact of these parameters on the incorporation of trace elemental impurities into the skeletons of six species of reef-building coral, from the genera Acropora and Pocillopora. I then interpret these data using quantitative models of the mechanisms of biomineralisation, which allow me to make inferences about the processes corals employ to build their skeletons. The concentrations of the cations Mg ad Sr indicate that, in a carbon-limited system, El/Ca concentrations in the coral skeleton are primarily set by a seawater-derived fluid with Ca upregulation of 1.3 times the seawater concentration, in agreement with independent microsensor data. This result supports the accepted theory that the calcifying fluid is a semi-open space modified by both passive seawater transport, and active ion-specific modification. When the dissolved inorganic carbon concentration of the SW is increased trace element relationships become more consistent with Rayleigh fractionation processes. Both end-member situations are consistent with and support a model of biomineralisation that involves a recently proposed process, ‘macropinocytosis’, playing a key role in coral biomineralisation. To test the importance of the macropinocytosis mechanism, I developed a novel dynamic model of calcifying fluid chemistry - the first to explicitly integrate macropinocytosis. This model successfully reproduces the direction and magnitude of trace element trends observed under both variable Ca and DIC conditions, and offers a mechanistic explanation for the distinct chemical signatures associated with two calcification modes: ion-by-ion precipitation and particle attachment. My new model reiterates that the calcifying space remains relatively open to seawater exchange, leaving corals potentially vulnerable to future ocean chemistry changes. Together, the experimental and modelling results bring us closer to a complete mechanistic understanding of coral biomineralisation. This work lays the foundation for refining coral-based paleo-proxies that will enhance our ability to interpret environmental signals recorded in coral skeletons.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• East, Madison

Advisor dc:contributor.advisor

• Branson, Oscar

• Geochemistry
• Calcification
• Biomineral
• Coral

Licence

• https://www.repository.cam.ac.uk/bitstreams/c6ebdcd3-37d2-4012-bc32-0ab3460afcc9/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.127215

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/398339