Seawater carbonate chemistry and gastropod shell composition

Organisms, such as molluscs, that produce their hard parts from calcium carbonate are expected to show increased difficulties growing and maintaining their skeletons under ocean acidification (OA). Any loss of shell integrity increases vulnerability, as shells provide protection against predation, desiccation, and disease. Not all species show the same responses to OA, which may be due to the composition and microstructural arrangement of their shells. We explore the role of shell composition and microstructure in resisting dissolution caused by decreases in seawater pH using a combination of microCT scans, XRD analysis, and SEM imaging. Two gastropods with different shell compositions and microstructure, Tegula funebralis and Nucella ostrina, were exposed to simulated ocean acidification conditions for six months. Both species showed signs of dissolution on the exterior of their shells, but changes in density were significantly more pronounced in T. funebralis. XRD analysis indicated that the exterior layer of both shell types was made of calcite. T. funebralis may be more prone to dissolution because their outer fibrous calcite layer has more crystal edges and faces exposed, potentially increasing the surface area on which dissolution can occur. These results support a previous study where T. funebralis showed significant decreases in both shell growth and strength, but N. ostrina only showed slight reductions in shell strength, and unaffected growth. We suggest that microstructural arrangement of shell layers in molluscs, more so than their composition alone, is critical for determining the vulnerability of mollusc shells to OA.诸如软体动物（molluscs）这类以碳酸钙合成硬质贝壳组织的生物，在海洋酸化（ocean acidification, OA）环境中，其贝壳的生长与维持所需面临的困难将显著增加。贝壳完整性的任何受损都会提升生物的生存脆弱性，因为贝壳可为其提供抵御捕食、脱水与病害的保护屏障。并非所有物种对海洋酸化的响应均一致，这一差异可能源于其贝壳的组成成分与微观结构排布。本研究结合显微计算机断层扫描（microCT）、X射线衍射（XRD）分析与扫描电子显微镜（SEM）成像技术，探究了贝壳组成与微观结构在抵御海水pH降低引发的溶解效应中的作用。我们选取两种贝壳组成与微观结构存在差异的腹足类（gastropods）——黑钟螺（Tegula funebralis）与美洲骨螺（Nucella ostrina）——将其暴露于模拟海洋酸化的环境中长达六个月。两种生物的贝壳外部均出现了溶解迹象，但T. funebralis的贝壳密度变化更为显著。X射线衍射（XRD）分析结果显示，两种贝壳的外层均由方解石（calcite）构成。T. funebralis或许更易发生溶解，因其外层的纤维状方解石结构暴露了更多的晶体棱面，潜在地扩大了可发生溶解的表面积。本研究结果与此前一项研究的结论相符：T. funebralis的贝壳生长与强度均出现显著下降，而N. ostrina的贝壳强度仅出现轻微降低，生长未受明显影响。我们认为，相较于仅考量贝壳的组成成分，软体动物贝壳层的微观结构排布，是决定其贝壳对海洋酸化脆弱性的更为关键的因素。