Seawater carbonate chemistry and biological processes of Mytilus edulis during experiments, 2011

Progressive ocean acidification due to anthropogenic CO2 emissions will alter marine ecosytem processes. Calcifying organisms might be particularly vulnerable to these alterations in the speciation of the marine carbonate system. While previous research efforts have mainly focused on external dissolution of shells in seawater under saturated with respect to calcium carbonate, the internal shell interface might be more vulnerable to acidification. In the case of the blue mussel Mytilus edulis, high body fluid pCO2 causes low pH and low carbonate concentrations in the extrapallial fluid, which is in direct contact with the inner shell surface. In order to test whether elevated seawater pCO2 impacts calcification and inner shell surface integrity we exposed Baltic M. edulis to four different seawater pCO2 (39, 142, 240, 405 Pa) and two food algae (310-350 cells mL-1 vs. 1600-2000 cells mL-1) concentrations for a period of seven weeks during winter (5°C). We found that low food algae concentrations and high pCO2 values each significantly decreased shell length growth. Internal shell surface corrosion of nacreous ( = aragonite) layers was documented via stereomicroscopy and SEM at the two highest pCO2 treatments in the high food group, while it was found in all treatments in the low food group. Both factors, food and pCO2, significantly influenced the magnitude of inner shell surface dissolution. Our findings illustrate for the first time that integrity of inner shell surfaces is tightly coupled to the animals' energy budget under conditions of CO2 stress. It is likely that under food limited conditions, energy is allocated to more vital processes (e.g. somatic mass maintenance) instead of shell conservation. It is evident from our results that mussels exert significant biological control over the structural integrity of their inner shell surfaces.人为二氧化碳排放引发的渐进式海洋酸化，将改变海洋生态系统过程。钙化生物（calcifying organisms）或许对海洋碳酸盐系统的化学形态变化尤为脆弱。过往研究多聚焦于碳酸钙饱和海水中壳体的外部溶蚀，而壳体内部界面或许对酸化更为易感。以蓝贻贝（Mytilus edulis）为例，较高的体液二氧化碳分压（pCO₂）会导致与壳体内表面直接接触的外套腔液出现pH降低与碳酸盐浓度下降的情况。为验证升高的海水二氧化碳分压是否会影响钙化作用与壳体内部表面完整性，我们在冬季（5℃）将波罗的海蓝贻贝暴露于四种不同海水二氧化碳分压（39、142、240、405 Pa）以及两种藻类饵料浓度（310~350 个·mL⁻¹ 与 1600~2000 个·mL⁻¹）中，持续七周。研究发现，低饵料浓度与高二氧化碳分压均会显著抑制壳体的长度生长。通过体式显微镜与扫描电镜（SEM）观察可知，在高饵料组的两种最高二氧化碳分压处理组中，出现了珍珠质（即文石，aragonite）层的壳体内部表面溶蚀；而在低饵料组的所有处理中均观察到该溶蚀现象。饵料与二氧化碳分压两个因素均会显著影响壳体内部表面的溶蚀程度。本研究首次表明，在二氧化碳胁迫条件下，壳体内部表面的完整性与动物的能量预算紧密相关。在饵料受限的条件下，能量大概率会被分配至更关键的生命过程（如躯体质量维持）而非壳体保护。本研究结果证实，贻贝可对其壳体内部表面的结构完整性实施显著的生物调控。