Ocean acidification alters the material properties of Mytilus edulis shells

Ocean acidification (OA) and the resultant changing carbonate saturation states is threatening the formation of calcium carbonate shells and exoskeletons of marine organisms. The production of biominerals in such organisms relies on the availability of carbonate and the ability of the organism to biomineralize in changing environments. To understand how biomineralizers will respond to OA the common blue mussel, Mytilus edulis, was cultured at projected levels of pCO2 (380, 550, 750, 1000 µatm) and increased temperatures (ambient, ambient plus 2°C). Nanoindentation (a single mussel shell) and microhardness testing were used to assess the material properties of the shells. Young's modulus (E), hardness (H) and toughness (KIC) were measured in mussel shells grown in multiple stressor conditions. OA caused mussels to produce shell calcite that is stiffer (higher modulus of elasticity) and harder than shells grown in control conditions. The outer shell (calcite) is more brittle in OA conditions while the inner shell (aragonite) is softer and less stiff in shells grown under OA conditions. Combining increasing ocean pCO2 and temperatures as projected for future global ocean appears to reduce the impact of increasing pCO2 on the material properties of the mussel shell. OA may cause changes in shell material properties that could prove problematic under predation scenarios for the mussels; however, this may be partially mitigated by increasing temperature.

海洋酸化（Ocean Acidification, OA）及其引发的碳酸盐饱和度状态改变，正严重威胁海洋生物碳酸钙外壳与外骨骼的形成过程。此类海洋生物的生物矿化产物生成，依赖碳酸盐的可获取性，以及生物体在多变环境中完成生物矿化的生理能力。为探明生物矿化生物对海洋酸化的响应规律，研究团队将紫贻贝（Mytilus edulis）置于预测的未来二氧化碳分压（pCO2）水平（380、550、750、1000 µatm）与两种温度条件（环境基准温度、基准温度升高2℃）下进行培养。采用纳米压痕技术（针对单枚贻贝壳样本）与显微硬度测试，评估贻贝壳的材料力学特性，并测定了多胁迫因子交互条件下培育的贻贝壳的杨氏模量（E）、硬度（H）与断裂韧性（KIC）。研究结果显示：海洋酸化会使贻贝分泌的壳方解石相较对照组更为坚硬（弹性模量更高）；酸化环境下培育的贻贝外壳外层（方解石层）脆性显著提升，而内层（文石层）则表现出更柔软、刚度更低的特性。结合未来全球海洋预测的升高二氧化碳分压与升温的复合胁迫条件，似乎会削弱单一二氧化碳分压升高对贻贝壳材料力学特性的负面影响。海洋酸化可能通过改变贻贝壳的材料特性，使其在捕食压力情境下的生存面临更大挑战，但温度升高可部分缓解这一不利影响。