Date

Fall 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Galapagos corals in the Eastern Tropical Pacific inhabit marginal, suboptimal environments and experience frequent, intense thermal stress-increasingly common conditions for reefs under recurrent climate anomalies. Marginal populations may host resilient individuals, providing insight into climate-driven acclimatization and adaptation. In the Galapagos, coral community resilience has been highest in the far northern island sites of Darwin and Wolf, and experimental thermal assays show that Pocillopora colonies from these sites are more heat- and cold-tolerant than colonies from the southwestern island of Isabela.

To identify drivers of this variation, we sequenced the mitochondrial open reading frame (mtORF) to determine host haplotype and used actin-targeted quantitative PCR to characterize symbiont community composition across seven sites spanning the thermal range experienced by corals across the archipelago. We also conducted a preliminary RNA-seq analysis of heat and cold stress responses in colonies from Isabela, which will be expanded in future work to incorporate TagSeq data from all seven sites.

Pocillopora from resilient far northern sites were mtORF Haplotype 1 and hosted the highest relative abundances of stress-tolerant Durusdinium symbionts, while thermally sensitive colonies from Isabela were clonal Haplotype 3 individuals hosting exclusively stress-sensitive Cladocopium symbionts. Across all seven sites, host haplotype was the strongest predictor of thermal sensitivity. Preliminary gene expression analyses from Isabela revealed that corals induced many of the same core biochemical pathways under acute heat and cold stress. However, some regulatory patterns diverged depending on the stressor. Heat stress broadly activated molecular chaperones, antioxidants, and immunity and cell-death regulators, while suppressing metabolic functions. Cold stress elicited widespread repression of these same pathways, with targeted induction of genes stabilizing membranes, proteins, and cytoskeletal structures.

Together, these results demonstrate that geographic variation in host lineage and symbiont identity, combined with conserved and stressor-specific molecular responses, underlies the observed differences in thermal tolerance across the archipelago. This work improves understanding of how corals persist in highly variable environments and provides a framework for predicting resilience in vulnerable reef systems worldwide.

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