Insights into coral recovery based on symbiont state and environmental conditions in the temperate, facultatively symbiotic coral Astrangia poculata
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Coral reefs are declining globally, calling for better ways to quantify coral health and predict resilience to future stress. The relationship between bleaching and fitness is key, as is reserve capacity to deal with physical trauma. This dissertation is an integrative study of the coral-algal symbiosis, holobiont performance under varied environmental conditions, and interactions between holobiont and environment on coral colony health and ability to recover from routine partial damage. I utilize the facultatively symbiotic, temperate coral Astrangia poculata as a natural model to explore the dynamics of colony health, performance, and the influence of environmental and nutritional stress under stable aposymbiotic and symbiotic states. Unlike most tropical hermatypic corals that rely heavily upon photosynthetic symbionts for energy, A. poculata can (1) flexibly use both heterotrophic and autotrophic nutritional pathways and (2) exist in naturally occurring, stable, and measurable aposymbiotic and symbiotic states. I begin by describing the impacts of environmentally relevant (winter, summer, and above range) temperatures on small-scale wound healing and recovery. Next, I explore the effects of nutritional and symbiotic states by comparing wound recovery, total colony health, host behavior, and symbiont performance in fed and starved colonies. Finally, I generate a novel reference transcriptome for A. poculata, and use computational approaches to characterize variation in gene expression between the symbiotic and aposymbiotic states. This analysis reveals that regardless of temperature, and with or without the potential for heterotrophic nutritional sources, a relationship with Symbiodinium enhances wound recovery and resilience to stress. Compromised healing ability and tissue cover at low temperatures suggest that in temperate stony corals, recovery and survival are more impacted by winter conditions than by exposure to high summer temperatures. Differential expression analysis revealed predictable enhancements to photosynthesis-related gene expression in symbiotic colonies. Together these results illuminate the complex interactions among symbiotic state, stress, recovery, and performance. We propose that studies like ours that examine the effects of combined stressors, as opposed to a monotonic focus on coral bleaching per se, are essential to clinical diagnosis and stewardship for coral reefs subjected to intense, cumulative human impacts.
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