An Experimental Investigation of the Impact of Ocean Acidification on Coral Calcification

Anne L. Cohen, Daniel C. McCorkle, Nobuchimi Shimizu, Justin B. Ries and Michael C. Holcomb
      -- All Department of Geology and Geophysics, Woods Hole Oceanographic Institution


Rising concentrations of CO2 in Earth’s atmosphere are changing the carbonate chemistry of the ocean, with potentially serious consequences for marine organisms and ecosystems. In laboratory studies, calcification rates of tropical corals decline significantly with decreasing seawater saturation state (Ω), long before undersaturation (Ω<1) is reached. Extrapolation of the laboratory data predicts that by 2100 AD, calcification rates of tropical corals could decline by up to 60% under the “business as usual” CO2 emissions scenario. These predictions have major implications for the sustainability of modern tropical coral reef ecosystems.

The goal of our proposed work is to identify the mechanism(s) by which changes in the carbonate chemistry of seawater affect the calcification rates of scleractinian corals. Production of CaCO3 by corals does not occur directly from seawater, but from a calcifying fluid whose composition is tightly regulated by the organism. Our key question is whether the composition of that fluid is sensitive to changes in seawater chemistry. Direct monitoring is difficult, if not impossible, due to the small size and inaccessibility of the calcifying space, which lies between the living coral tissue and the growing carbonate skeleton. In this study, we will use the morphology and chemical composition of the crystals themselves to inform us of the conditions within the calcifying region of corals. The shapes of crystals, their nanoscale morphology and boron isotope ratios will be used to quantify the saturation state and pH of the calcifying fluid as seawater pH and saturation state are independently manipulated in our laboratory aquaria.