Rising concentrations of carbon dioxide (CO2) in Earth's atmosphere are changing the carbonate chemistry of the ocean. Models indicate that as CO2 levels continue to rise over the next century, surface water carbonate ion concentration may decrease by almost 50% relative to pre-industrial levels. In laboratory studies, multiple taxa of marine calcifying organisms exhibit high sensitivity to changes in seawater carbonate concentration. The response of scleractinian corals is particularly well documented: under controlled experimental conditions, coral calcification rates show a measurable decline with decreasing seawater carbonate ion concentration. These observations have significant implications for the health and survivability of coral reef ecosystems in the face of global change. Predictions of the response of coral calcification to changes in seawater saturation state expected over the next century are limited however, by how little we know of the fundamental mechanisms of coral calcification. For instance, that calcification displays any sensitivity at all to changes in external carbonate ion concentration is puzzling, given current models of coral biomineralization.
To answer this puzzle, scientists from Woods Hole Oceanographic Institution and Rosenstiel School of Marine and Atmospheric Science at the University of Miami will address two key questions: (1) What is the saturation state(s) of the internal calcifying fluid in which crystal growth occurs and how does it respond to changes in external seawater concentrations of carbonate? (2) Are algal symbionts implicated in the sensitivity of coral calcification to seawater carbonate? They will also tests the hypothesis that aragonite nucleation, catalyzed by proteins, and occurring at near ambient saturation levels, is the pH-sensitive step in the coral mineralization cycle. The PIs will conduct a series of laboratory precipitation and coral culturing experiments designed to increase our understanding of how corals calcify, and to lay the groundwork for predictions of the coral response to ocean acidification. They will utilize micro- and nanoscale crystal morphology, and geochemical signatures of aragonite crystals accreted by zooxanthellate and azooxanthellate corals, to interpret conditions within the calcifying space.
Among the broader impacts, the interdisciplinary research will benefit society by elucidating the fundamental mechanisms by which coral reefs are built and the potential impacts of future climate changes on ecosystems of enormous economic and scientific importance. Their findings also will have broader implications for interpretation of stable isotope proxies (d18O, d11B) in coral and other biogenic skeletons. The PIs will also mentor students and continue their extensive outreach programs
