CO2-driven ocean acidification alters and weakens integrity of the calcareous tubes produced by the serpulid tubeworm, Hydroides elegans

As a consequence of anthropogenic CO2-driven ocean acidification (OA), coastal waters are becoming increasingly challenging for calcifiers due to reductions in saturation states of calcium carbonate (CaCO3) minerals. The response of calcification rate is one of the most frequently investigated symptoms of OA. However, OA may also result in poor quality calcareous products through impaired calcification processes despite there being no observed change in calcification rate. The mineralogy and ultrastructure of the calcareous products under OA conditions may be altered, resulting in changes to the mechanical properties of calcified structures. Here, the warm water biofouling tubeworm, Hydroides elegans, was reared from larva to early juvenile stage at the aragonite saturation state (Omega A) for the current pCO2 level (ambient) and those predicted for the years 2050, 2100 and 2300. Composition, ultrastructure and mechanical strength of the calcareous tubes produced by those early juvenile tubeworms were examined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and nanoindentation. Juvenile tubes were composed primarily of the highly soluble CaCO3 mineral form, aragonite. Tubes produced in seawater with aragonite saturation states near or below one had significantly higher proportions of the crystalline precursor, amorphous calcium carbonate (ACC) and the calcite/aragonite ratio dramatically increased. These alterations in tube mineralogy resulted in a holistic deterioration of the tube hardness and elasticity. Thus, in conditions where Omega A is near or below one, the aragonite-producing juvenile tubeworms may no longer be able to maintain the integrity of their calcification products, and may result in reduced survivorship due to the weakened tube protection.

受人为二氧化碳驱动的海洋酸化（OA）影响，近岸海域碳酸钙（CaCO3）矿物的饱和态持续降低，使得钙化生物的生存环境愈发严苛。钙化速率响应是海洋酸化研究中最常被探究的表征之一，但即便钙化速率未出现可观测变化，海洋酸化也可能通过损伤钙化过程，导致钙质产物的质量下降。海洋酸化环境下，钙质产物的矿物学特征与超微结构可能发生改变，进而引发钙化结构力学性能的变化。
本研究以暖水型生物污损华美盘管虫（Hydroides elegans）为实验材料，将其从幼虫阶段培育至早期幼体阶段，培养设置了当前二氧化碳分压（pCO2）水平（背景组），以及预测的2050、2100、2300年对应的文石饱和状态（Ω_A）四种条件。采用X射线衍射（XRD）、傅里叶变换红外光谱（FT-IR）、扫描电子显微镜（SEM）及纳米压痕技术，对上述早期幼体华美盘管虫所分泌的钙质管的组成、超微结构与力学强度进行了检测分析。
实验结果显示，幼体钙质管主要由高溶解性碳酸钙矿物相——文石构成。在文石饱和态接近或低于1的海水中培育得到的钙质管，其结晶前驱体无定形碳酸钙（ACC）的占比显著升高，且方解石/文石比值大幅上升。钙质管矿物学特征的上述改变，导致管的硬度与弹性全面下降。
综上，当文石饱和态Ω_A接近或低于1时，产文石的幼体华美盘管虫可能无法维持其钙化产物的结构完整性，进而因防护管弱化导致存活率降低。