A microelectrode study of coral calcification: how ocean acidification affects ion concentrations inside coral polyps
Date
2015
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
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.