Climate Change Effects on Coral Symbioses
Open Access
- Author:
- Chan, Andrea
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 04, 2018
- Committee Members:
- Iliana Brigitta Baums, Dissertation Advisor/Co-Advisor
Iliana Brigitta Baums, Committee Chair/Co-Chair
Todd C LaJeunesse, Committee Member
Roberto Iglesias-Prieto, Committee Member
Lee Kump, Outside Member - Keywords:
- coral reef
population genetics
climate change
Symbiodiniaceae
gene expession
symbiosis
bioerosion
physiology - Abstract:
- Coral reefs are already suffering the impacts of global climate change, including mass coral bleaching, unprecedented disease outbreaks, and increased damage from more intense tropical storms. The loss of reefs would be devastating because these ecosystems support a diversity of fishes and invertebrates, as well as ecosystem services like commercially important fisheries, tourism revenue, and coastal protection. The framework of coral reefs is built by scleractinian corals, which form a symbiotic relationship with intracellular dinoflagellates in the family Symbiodiniaceae. Understanding how a multi-faceted stressor like climate change will impact coral symbioses requires research conducted at multiple levels of organization, including gene expression and physiology, population connectivity, and interactions between species. For my thesis, I studied these impacts of climate change using three different scleractinian coral species. To increase our understanding of the cellular mechanisms resulting in coral bleaching, we conducted a chronic heat stress experiment using the facultatively symbiotic northern star coral, Astrangia poculata, which naturally occurs with (symbiotic) and without (aposymbiotic) its algal symbiont Breviolum psygmophilum – sometimes on the same coral colony. With replicate symbiotic and aposymbiotic ramets of A. poculata, we could separate the heat stress response of the coral host from the coral in symbiosis with its symbiont, while also characterizing the response of the symbiont. Sustained high temperature stress resulted in photosynthetic dysfunction of the symbiont, including a drop in maximum photosynthesis rate, maximum photochemical efficiency, and the absorbance peak of chlorophyll a. Interestingly, the metabolic rates of symbiotic and aposymbiotic coral hosts were differentially impacted. RNAseq analysis revealed more differentially expressed genes between heat-stressed and control aposymbiotic colonies than heat-stressed and control symbiotic colonies. Notably, aposymbiotic colonies increased the expression of inflammation-associated genes such as nitric oxide synthases. Unexpectedly, the largest transcriptional response was observed between heat-stressed and control B. psygmophilum, including genes involved in photosynthesis, response to oxidative stress, and meiosis. Thus, in contrast with previous studies, the algal symbiont responded more strongly to high temperatures than the coral host, possibly resulting in suppressed immune function of the coral. In a separate study, I developed novel microsatellite markers to assess population and clonal structure in the threatened pillar coral, Dendrogyra cylindrus, and its specific symbiont, Breviolum dendrogyrum. Patterns of population structure differed between host and symbiont, with more restricted gene flow for the symbiont along the Florida Reef Tract. Sites with multiple colonies of D. cylindrus were found to be clonal, with the same genotype of the coral host often associating with the same strain of the algal symbiont. High clonality in Florida may have increased the vulnerability of D. cylindrus to a recent thermal stress-associated disease outbreak, resulting in a precipitous population decline. Lastly, I investigated clonal structure in the lobe coral, Porites lobata, in two regions with similar gradients of abiotic variables and bioeroding mussel densities. While genotypic diversity was lower at Galapagos sites that are more acidic, similar sites in Palau had relatively equal levels of genotypic diversity across an acidification gradient. These contrasting results are likely due to differences in biotic interactions between the two regions, such as the presence of coral-biting triggerfish in the Galapagos that prey on bioeroding mussels. This points to the importance of considering these interactions when predicting how climate change could impact asexual reproduction in foundational species like corals. Overall, the work presented in this thesis highlights the variability of biological responses at different levels of organization in coral reef environments that will continue to be impacted by climate change.