A microelectrode study of coral calcification: how ocean acidification affects ion concentrations inside coral polyps

Date
2015
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University of Delaware
Abstract
Coral reefs are a critical building block of the ocean ecosystem, whose health is threatened by ocean acidification (OA) and warming due to increased atmospheric CO2 (Hoegh-Guldberg, 2010; IPCC, 2014). Reliably predicting how coral calcification may respond to OA depends on our understanding of their calcification mechanisms (Ries, 2011; Holcomb et al., 2014; Allison et al., 2014; Gagnon, 2013). But obtaining relevant data on the calcification mechanism is difficult. First, because of coral’s structural arrangement, little is understood about the chemical dynamics inside coral polyps. Second, the speciation, sources, and dynamics of dissolved inorganic carbon (DIC) inside corals remain unresolved because only pH has been measured while a critical second parameter needed to fully characterize the internal carbonate chemistry at the site of coral calcification has been missing (Ries, 2011). Coral calcification processes are affected by changes in ion concentrations due to ocean acidification. Microsensors enable us to measure biological processes in different localities of the coral polyp and we have successfully built pH, CO3 2-, and Ca2+ microelectrodes that are suitable for coral studies with a tip diameter of 10-15 μm. Also this research is the first to combine pH and CO3 2- to calculate DIC inside coral polyp. Two chapters are included in this thesis: chapter 1 focuses on pH and CO3 2-concentrations inside calcifying fluid and chapter 2 focuses on the effects of light on Ca2+ , CO3 2-, and pH dynamics inside coral polyps and different factors that affect the concentration change. In chapter 1, we report the first depth profiles of pH and carbonate ion concentrations ([CO3 2-]) measured inside coral polyps. We observed sharp increases in pH and [CO3 2-] inside the calcifying fluid and very low pH and [CO3 2- ] above it in the coelenteron, supporting the existence of an active process that pumps protons (H+) out of the calcifying fluid. This results in a sharp CO2 gradient from the coelenteron to the calcifying fluid, which draws in enough CO2 to sustain the high calcification rates typically observed in tropical corals (Alison et al, 2014; Furla et al., 2000). However, in contrast to the current view that corals substantially concentrate both DIC and total alkalinity (TA) in their calcifying fluid (Allison et al., 2014), our data and model calculations suggest that corals can achieve a high aragonite saturation state (Ωarag) by maintaining a high pH while at the same time keeping [DIC] and TA relatively low. Such a state requires less H+- pumping for upregulating pH compared to a high [DIC] scenario. In chapter 2, the effects of light on Ca2+ , CO3 2-, and pH dynamics were measured by microelectrodes inside the polyps of two scleractinian corals, Orbicella faveolata and Turbinaria reniformis. In the upper part of the coelenteron solution, pH and CO3 2- both increased in the light and decreased in the dark. Ca2+ concentrations decreased in the light and increased in the dark. pH and Ca2+ dynamics have been studied in many other studies but no one has yet measured CO3 2- concentrations. Now with our CO3 2- data, we can get a better understanding of carbonate system dynamics over light/dark cycles. Based on our pH and CO3 2- data, we calculated the total alkalinity (TA) and dissolved inorganic carbon (DIC) dynamics and set up a numerical simulation model to analyze the effects of different parameters. The model incorporated calcification, photosynthesis, respiration, physical diffusion with seawater, transmembrane ion transport by Ca- ATPase, and paracellular ion fluxes. Our model was based on the model of Nakamura et al., (2013) and our experimental data (e.g., depth, calcification rate, alkalinity and DIC concentrations) were used to replace some tuning parameters. In our experiment, we found that both TA and DIC decreased in the light and increased in the dark. Our model showed that: 1) Most of the TA and DIC increase in dark were due to physical diffusion from overlying seawater; 2) There are unknown TA sources inside coral polyp that provided about 40% TA in dark, about 15% of that come from inorganic sources; 3) TA and DIC decreases in the light were driven by calcification and photosynthesis. The model agreed with the trends in our experimental data and allowed us to constrain the ratio of different parameters.
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