Date of Award
Doctor of Philosophy in Biological and Environmental Sciences
Evolution and Marine Biology
Hollie M. Putnam
Coral reefs worldwide are threatened by climate change effects like increasing ocean warming and ocean acidification. These increased pressures cause a dysbiosis between the coral host, algal endosymbionts, and associated coral microbiome that results in the coral host expelling algal endosymbionts, leaving the coral host with a stark white ‘bleached’ appearance. Without their endosymbionts, coral hosts are forced to sustain themselves energetically with heterotrophy instead of relying on the autotrophic carbon and energy sources that once came from the algal endosymbionts. When this response, termed ‘coral bleaching’, happens reef-wide during an extreme wave of increased ocean temperatures, this is called a mass Coral Bleaching Event. The frequency and intensity of mass Coral Bleaching events are increasing around the world, forcing corals to acclimatize to survive. This dissertation investigates the physiological and genomic mechanisms underlying acclimatization and increased stress tolerance in two common, reef-building corals: Montipora capitata and Pocillopora acuta. In three chapters, I present findings that support phenotypic plasticity and increased stress tolerance in M. capitata and hypothesize the mechanisms contributing to this. In Chapter 1, I conducted an ex-situ experiment that mimicked an environmentally realistic, extended heatwave and ocean acidification scenario in a factorial design of increased temperature and increased pCO2 conditions for a two-month stress period and a two-month recovery period. Both species’ physiological states were significantly challenged but M. capitata displayed a more favorable photosynthetic rate to antioxidant capacity ratio and associated with more thermally tolerant symbionts. Although M. capitata survived at higher rates than P. acuta, physiological state was still significantly impacted after two months of recovery, suggesting that marine heatwaves likely induce physiological legacies that may impact performance during the next, inevitable heatwave. In Chapter 2, I further investigated P. acuta’s stress response from Chapter 1 at a genomic level. We sought to test the effects of environmental stressors on gene body DNA methylation patterns to elucidate how environmentally sensitive and dynamic DNA methylation changes are in invertebrates. However, when analyzing gene expression data, our team found that polyploidy was prevalent in our samples, which convoluted our ability to test environmental effect in addition to polyploidy structure. We found that DNA methylation patterns followed polyploidy genetic lineage with diploid corals exhibiting the highest levels of DNA methylation despite lower gene expression levels of epigenetic machinery proteins. Despite significant DNA methylation pattern differences between polyploidies, P. acuta populations still severely declined in increased stress conditions (outlined in Chapter 1), suggesting that regardless of differential gene body methylation and ploidy status, this species may be ultimately too sensitive to future ocean conditions. In Chapter 3, I further investigated the genomic mechanisms underlying stress response in Montipora capitata, by directly comparing bleached (‘Susceptible’) and non-bleached (‘Resistant’) phenotypes of conspecific pairs. We found very little genetic diversity among our samples suggesting there is no effect of genetic structure on phenotypic variation in this context. ‘Resistant’ corals were characterized by association with more thermally tolerant symbionts, lower gene expression variability, higher gene body methylation levels on genes involved in death and stress response, and a more robust cellular stress response. The results of all three chapters suggest that both physiological and genomic stats impact bleaching susceptibility and phenotype and that not one mechanism may act alone to produce a particular phenotype. This dissertation aids in elucidating the mechanisms of stress response in reef-building corals, ultimately guiding our current knowledge of phenotypic variation in the face of climate change.
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Strand, Emma, "ELUCIDATING THE MECHANISMS OF STRESS TOLERANCE IN REEF-BUILDING CORAL HOLOBIONTS" (2023). Open Access Dissertations. Paper 1568.