Effects of seawater pH on growth and skeletal U/Ca ratios of Acropora digitifera coral polyps

The impact of ocean acidification caused by the increasing atmospheric CO₂ has been studied in marine calcifiers, including hermatypic corals. However, the effect of elevated pCO₂ on the early developmental life-cycle stage of corals has been little studied. In this study, we reared polyps of Acropora digitifera in seawater at pH_T 6.55, 7.31, 7.64, 7.77, and 8.03, controlled by CO₂ bubbling. We measured the dry weights of polyp skeletons after the 40-d experiment to investigate the relationship between the seawater aragonite saturation state and polyp growth. In addition, we measured skeletal U/Ca ratio to estimate their pH dependence. Skeletal weights of coral polyps increased with the aragonite saturation state and reached an apparent saturation plateau above pH 7.77. U/Ca ratios had a strong inverse relationship with pH and a negligible relationship with skeletal growth rate (polyp weight), suggesting that skeletal U/Ca could be useful for reconstructing paleo-pH.