Date of Award

Summer 8-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

Program/Concentration

Ecological Sciences

Committee Director

Daniel J. Barshis

Committee Member

Richard Zimmerman

Committee Member

Ross Cunning

Abstract

Coral reefs are increasingly threatened by rising ocean temperatures, yet coral thermal tolerance varies widely across species, populations, and environments. This dissertation investigates the biological and environmental mechanisms underlying coral thermal resilience, with a focus on symbiosis dynamics, photoacclimation, and the interaction between light and heat stress across environmental gradients. Specifically, it examines how coral-Symbiodiniaceae partnerships respond to thermal stress using the Coral Bleaching Automated Stress System (CBASS) across contrasting thermal and light environments, spanning population-level differences, depth-associated variation, and experimental manipulation of light during acute heat stress. First, thermal thresholds and transcriptomic dynamics were examined in two wild Porites astreoides populations from thermally contrasting habitats with documented differences in bleaching susceptibility: a more tolerant Inshore population vs. a more sensitive Offshore population. Inshore corals exhibited higher thermal thresholds, and mRNA-Sequencing analysis revealed a muted, rapidly recovering gene expression response compared to Offshore corals, which showed larger and more prolonged transcriptional activation. Second, environmental, physiological, and isotopic analyses of the depth-generalist coral Montipora grisea across a 5–45 m depth gradient in American Samoa demonstrated depth-related shifts in light acclimation, trophic strategy, and minor shifts in Symbiodiniaceae community composition. Mesophotic corals displayed increased reliance on heterotrophy, a higher presence of Durusdinium symbionts, and higher thermal tolerance than shallow conspecifics, indicating physiological and symbiotic adjustment across depth. Finally, CBASS assays were performed under varying light intensity to examine how experimental light intensity modifies acute heat stress responses in shallow and mesophotic M. grisea colonies. Linear and dose response curve models of temperature vs. photochemical parameters revealed that light environment is a dominant driver of thermal response in the CBASS assay. Reduced light significantly increased thermal thresholds and reduced photosystem stress, while prior light-related photophysiological response had limited influence under acute stress. Together, these findings demonstrate that coral thermal resilience is regulated by interacting transcriptional, symbiotic, and environmental processes influenced by in situ conditions.

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DOI

10.25777/wa21-6k78

ISBN

9798193214182

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