Uranium in larval shells as a barometer of molluscan ocean acidification exposure

As the ocean undergoes acidification, marine organisms will become increasingly exposed to reduced pH, yet variability in many coastal settings complicates our ability to accurately estimate pH exposure for those organisms that are difficult to track. Here we present shell-based geochemical proxies that reflect pH exposure from laboratory and field settings in larvae of the mussels Mytilus californianus and M. galloprovincialis. Laboratory-based proxies were generated from shells precipitated at pH 7.51 to 8.04. U/Ca, Sr/Ca, and multielemental signatures represented as principal components varied with pH for both species. Of these, U/Ca was the best predictor of pH and did not vary with larval size, with semidiurnal pH fluctuations, or with oxygen concentration. Field applications of U/Ca were tested with mussel larvae reared in situ at both known and unknown pH conditions. Larval shells precipitated in a region of greater upwelling had higher U/Ca, and these U/Ca values corresponded well with the laboratory-derived U/Ca-pH proxy. Retention of the larval shell after settlement in molluscs allows use of this geochemical proxy to assess ocean acidification effects on marine populations.

随着海洋酸化进程持续，海洋生物将愈发频繁地暴露于pH值降低的环境中。但多数近岸海域的环境异质性，极大增加了精准估算难以追踪的海洋生物所经历的pH暴露水平的难度。本研究针对加州贻贝（Mytilus californianus）和地中海贻贝（M. galloprovincialis）的幼虫，开发了可反映实验室与野外环境中pH暴露水平的贝壳基地球化学代用指标。实验室代用指标基于pH 7.51至8.04环境下沉积形成的贝壳构建。两种贻贝的U/Ca、Sr/Ca以及以主成分表征的多元素特征均随pH值变化而改变。其中，U/Ca是最佳的pH预测指标，且不受幼虫体长、半日尺度pH波动以及溶氧浓度的影响。研究针对U/Ca的野外应用场景进行了测试，将贻贝幼虫在已知与未知pH条件下进行原位培育。在上升流更强的区域沉积形成的幼虫贝壳，其U/Ca比值更高，且该数值与实验室构建的U/Ca-pH代用指标吻合度良好。软体动物在附着定居后仍保留幼虫贝壳这一特性，使得该地球化学代用指标可用于评估海洋酸化对海洋种群的影响。