Succession of Ocean Acidification and its Effects on Reef-Building Corals

Since 2008, we have been conducting a series of coral-rearing experiments, mainly at the Sesoko Station of the Tropical Biosphere Research Center at the University of the Ryukyus, under an overarching project, called the Acidification Impact on Calcifiers project (AICAL project). The AICAL project integrates the efforts of several individual research programs, and project members employ a custom-made, high-precision pCO2-adjusted seawater generator (the AICAL apparatus) to study the effects of ocean acidification on marine calcifying organisms. With this system, rearing experiments can be conducted under conditions mimicking those in the preindustrial era, and the future. In this review, we summarize the results of ocean acidification experiments on corals and other organisms, with a focus on studies conducted by the AICAL project members. We examine the response of organisms to ocean acidification in a hierarchical fashion: differences among various groups of calcifying organisms, and interspecific and intraspecific variation in corals. In the case of corals, we consider not only the effects of ocean acidification, but also those caused by rising seawater temperatures and eutrophication. Our major findings are that coral calcification may have already decreased from a preindustrial level and that there are evident interspecific and intraspecific differences in tolerance against ocean acidification. These findings suggest future decrease of coral cover, accompanied by species compositional changes under climate change scenarios.