Seawater carbonate chemistry, Acropora digitifera growth and skeletal U/Ca ratios during experiments, 2011

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 pHT 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.

大气二氧化碳浓度升高引发的海洋酸化（ocean acidification）对海洋钙化生物（marine calcifiers）的影响已在包括造礁珊瑚（hermatypic corals）在内的类群中得到广泛研究。然而，pCO₂升高对珊瑚早期发育生命周期阶段的影响却鲜有报道。本研究通过二氧化碳鼓泡法调控海水pHT分别为6.55、7.31、7.64、7.77及8.03，在此条件下培养指形鹿角珊瑚（Acropora digitifera）的珊瑚虫。实验持续40天后，我们测定了珊瑚虫骨骼的干重，以探究海水文石饱和状态与珊瑚虫生长之间的关联。此外，我们还测定了骨骼的铀钙比（U/Ca ratio），以评估其与pH的依赖性。结果显示，珊瑚虫骨骼干重随文石饱和状态升高而增加，并在pHT高于7.77时达到明显的饱和平台。铀钙比与pH呈显著负相关，而与骨骼生长速率（珊瑚虫干重）的相关性可忽略不计，这表明骨骼铀钙比可用于古海水pH（paleo-pH）的重建。