Coral calcifying fluid pH dictates response to ocean acidification

Ocean acidification driven by rising levels of CO2 impairs calcification, threatening coral reef growth. Predicting how corals respond to CO2 requires a better understanding of how calcification is controlled. Here we show how spatial variations in the pH of the internal calcifying fluid (pH(cf)) in coral (Stylophora pistillata) colonies correlates with differential sensitivity of calcification to acidification. Coral apexes had the highest pH(cf) and experienced the smallest changes in pH(cf) in response to acidification. Lateral growth was associated with lower pH(cf) and greater changes with acidification. Calcification showed a pattern similar to pH(cf), with lateral growth being more strongly affected by acidification than apical. Regulation of pH(cf) is therefore spatially variable within a coral and critical to determining the sensitivity of calcification to ocean acidification.