Eco-physiology of the tabletop coral Acropora hyacinthus living in a dynamic reef environment [electronic resource]
Keywords: Biology.

Across Earth's diverse biosphere, organisms match their environment in form, function, and physiology. When exposed to environmental change, organisms may need to make physiological adjustments to maintain homeostasis. The scleractinian corals of coral reef ecosystems are facing new and rapid environmental change due to global climate change and local anthropogenic activity. Through the study of eco-physiology, the sensitivity of corals to environmental change has been demonstrated in diverse reef environments. In this dissertation, I examined two aspects of coral physiology, skeletal linear extension growth and transcriptional regulation, in corals living in a dynamic reef environment. The study site was the back-reef of Ofu Island, American Samoa. Due to shifts in the timing and magnitude of the tide, temperature fluctuates daily. The biogeochemical cycling of dissolved inorganic carbon and oxygen drive daily oscillations in pH and oxygen, which are also impacted by the tide cycle. On days with strong midday and midnight low tides, temperature, pH, and oxygen levels have the largest variation from day to night. The research in Chapter 1 investigated the influence of environmental variability on short-term linear extension growth rates in the tabletop coral Acropora hyacinthus. I developed a technique to measure short-term growth in the field. With this approach, I measured growth during three consecutive 5-day growth periods that had different levels of environmental variability and found that linear extension rates were no higher during more stable environmental periods compared to periods of higher variability. The data suggest that growth of corals living in highly variable back-reef microhabitats is not impacted by large environmental fluctuations over short time scales. In addition to daily environmental variability, corals experience daily variation in food availability and physiological activities such as calcification and photosynthesis by endosymbionts of the genus Symbiodinium. The research in Chapter 2 investigated day-night transcriptional regulation in A. hyacinthus under field conditions. I found that ~ 2% of the transcriptome was differentially regulated from day to night, with the largest fold changes in a set of transcription factors strongly associated with day-night gene regulation in other animals, including cryptochromes, thyrotroph embryonic factor, and D site-binding protein. I also found large daytime increases in expression of a set of genes involved in glucose transport and glycogen storage. Although greater than 40-fold changes in expression occur in important transcription factors, downstream gene regulation seems very stable in corals from day to night compared to other animals studied. Temporary environmental extremes are another form of environmental change that corals are exposed to. During strong midday low tides, corals experience extremes in temperature, pH, and oxygen. The research in Chapter 3 investigated environmentally driven transcriptional regulation in A. hyacinthus. I identified a group of genes with coordinated expression that increased on two sequential days with the strongest midday low tides. The responsive genes are enriched for those encoding proteins localized to the endoplasmic reticulum and involved in the Unfolded Protein Response and calcium ion homeostasis. These findings suggest that the corals were responding to endoplasmic reticulum stress in the absence of any visible signs of stress (i.e., bleaching). In a laboratory temperature stress experiment, I found that the expression of these environmentally responsive genes was higher in bleached corals than in corals exposed to the mild stress of a strong midday low tide. The results suggest that the Unfolded Protein Response is a first line of defense that corals employ when coping with mild stress on the reef. Together, the research described in the three chapters adds to our understanding of the eco-physiology of scleractinian corals at a time critical for coral reef conservation.