The role of symbiotic algae in the acclimatization of Oculina arbuscula to ocean acidification

Ocean acidification (OA) caused by CO2 emissions is projected to decrease seawater pH to 7.6 by 2100. Scleractinian corals are at risk because excess H+ in seawater binds to carbonate (CO3 2-), reducing its availability for CaCO3 skeletons. The energy demand for skeletal growth increases as pH decreases because corals must actively purge excess H+ from their seawater sourced calcifying fluid to maintain high calcification rates. In scleractinian corals it is hypothesized that photosynthesis by symbiotic algae is critical to meet this increased energy demand. To test this hypothesis, I conducted laboratory and field studies with Oculina arbuscula, a facultatively symbiotic coral common in the southeastern U.S.A., which exhibits resilience to seasonal fluctuations in pCO2 that drive pH to as low as 7.8 in the summer. In the lab, aposymbiotic and symbiotic O. arbuscula colonies were exposed to a pH of 8.0 or 7.6 for 51 days to test the role of the algal symbiosis in maintaining energy reserves, calcifying fluid pH, skeletal organic matrix and calcification rates during OA. To supplement this controlled laboratory experiment, I transplanted 20 coral colonies to a seafloor CO2 monitoring platform, exploiting the natural variation in pCO2 that occurs in Georgia coastal waters. The relationship between calcifying fluid pH and seawater pH was tested for in these samples using boron stable isotopes in coral skeletons. Contrary to the hypothesis, both aposymbiotic and symbiotic O. arbuscula colonies maintained similar calcification rates when exposed to OA. Upregulation of calcifying fluid pH, likely fueled by metabolic energy derived from heterotrophy, was the primary acclimatory mechanism detected. Symbiotic algae were associated with higher coral lipid reserves and denser skeletons, but neither of these variables were affected by seawater pH. Corals growing offshore maintained a consistent calcifying fluid pH in the face of seasonal fluctuations in seawater pH and other environmental variables such as temperature and turbidity. The results of my study provide valuable insight into how O. arbuscula has evolved to survive harsh conditions of seasonally low pH levels characteristic of southeastern U.S.A. coastal waters and the mechanisms that may contribute to its future resilience to increasing OA.

Arneson E. M., 2021. The role of symbiotic algae in the acclimatization of Oculina arbuscula to ocean acidification. MSc thesis, Georgia Southern University, 76 p. Thesis.

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