Impact of ocean acidification and warming on the Mediterranean mussel (Mytilus galloprovincialis)

In order to assess the effects of ocean acidification and warming on the Mediterranean mussel (Mytilus galloprovincialis), specimens were reared in aquarium tanks and exposed to elevated conditions of temperature (+3°C) and acidity (-0.3 pH units) for a period of 10 months. The whole system comprised a factorial experimental design with 4 treatments (3 aquaria per treatment): control, lowered pH, elevated temperature, and lowered pH/elevated temperature. Mortality was estimated on a weekly basis and every 2 months, various biometrical parameters and physiological processes were measured: somatic and shell growth, metabolic rates and body fluid acid-base parameters. Mussels were highly sensitive to warming, with 100% mortality observed under elevated temperature at the end of our experiment in October. Mortality rates increased drastically in summer, when water temperature exceeded 25°C. In contrast, our results suggest that survival of this species will not be affected by a pH decrease of 0.3 in the Mediterranean Sea. Somatic and shell growth did not appear very sensitive to ocean acidification and warming during most of the experiment, but were reduced, after summer, in the lowered pH treatment. This was consistent with measured shell net dissolution and observed loss of periostracum, as well as uncompensated extracellular acidosis in the lowered pH treatment indicating a progressive insufficiency in acid-base regulation capacity. However, based on the present dataset, we cannot elucidate if these decreases in growth and regulation capacities after summer are a consequence of lower pH levels during that period or a consequence of a combined effect of acidification and warming. To summarize, while ocean acidification will potentially contribute to lower growth rates, especially in summer when mussels are exposed to sub-optimal conditions, ocean warming will likely pose more serious threats to Mediterranean mussels in this region in the coming decades.

为探究海洋酸化与海水升温对地中海贻贝（Mytilus galloprovincialis）的影响，本研究将实验个体饲养于水族箱内，分别置于升温3℃、酸度降低0.3 pH单位的极端培养条件下，为期10个月。实验整体采用析因实验设计，设置4个处理组，每组设置3个重复水族箱：对照组、低pH组、升温组以及低pH/升温联合处理组。死亡率每周统计一次；每2个月则测定多项生物计量参数与生理过程指标，包括体生长、壳生长、代谢速率以及体液酸碱参数。地中海贻贝对海水升温表现出极高的敏感性：至10月实验结束时，升温组个体死亡率达100%；夏季水温超过25℃时，各组死亡率均大幅攀升。与之相对，本研究结果显示，地中海海域pH降低0.3个单位不会对该物种的存活率造成显著影响。实验大部分周期内，体生长与壳生长对海洋酸化和升温的响应并不显著，但夏季过后低pH组的生长速率出现明显下降。该结果与观测到的贝壳净溶蚀、壳角质层丢失现象，以及低pH组出现的未代偿性细胞外酸中毒相一致，提示贻贝的酸碱调节能力随实验进程逐渐不足。不过基于本数据集，尚无法明确夏季过后生长速率与调节能力的下降，是单纯由该时期的低pH环境所致，还是海洋酸化与升温的联合效应导致。综上，尽管海洋酸化可能会降低贻贝的生长速率，尤其是在贻贝处于亚适宜环境的夏季，但在未来数十年间，海水升温或将对该区域的地中海贻贝构成更为严峻的生存威胁。