Seawater carbonate chemistry and acid–base parameters, metabolic rate and health indicators of Mytilus edulis@en

Ocean acidification (OA) studies typically use stable open-ocean pH or CO2 values. However, species living within dynamic coastal environments can naturally experience wide fluctuations in abiotic factors, suggesting their responses to stable pH conditions may not be reflective of either present or near-future conditions. Here we investigate the physiological responses of the mussel Mytilus edulis to variable seawater pH conditions over short- (6 h) and medium-term (2 weeks) exposures under both current and near-future OA scenarios. Mussel haemolymph pH closely mirrored that of seawater pH over short-term changes of 1 pH unit with acidosis or recovery accordingly, highlighting a limited capacity for acid–base regulation. After 2 weeks, mussels under variable pH conditions had significantly higher metabolic rates, antioxidant enzyme activities and lipid peroxidation than those exposed to static pH under both current and near-future OA scenarios. Static near-future pH conditions induced significant acid–base disturbances and lipid peroxidation compared with the static present-day conditions but did not affect the metabolic rate. These results clearly demonstrate that living in naturally variable environments is energetically more expensive than living in static seawater conditions, which has consequences for how we extrapolate future OA responses in coastal species.海洋酸化（Ocean Acidification, OA）研究通常采用稳定的开阔大洋pH或CO₂值。然而，栖息于动态海岸环境中的生物天然会经历非生物因子的剧烈波动，这表明其对稳定pH条件的响应，既无法反映当前也无法反映近未来的实际环境状况。本研究以紫贻贝（Mytilus edulis）为研究对象，探究其在当前及近未来海洋酸化情景下，分别经短期（6小时）、中期（2周）的变动海水pH暴露后的生理响应。当海水pH发生1个pH单位的短期变动（酸化或恢复过程）时，紫贻贝的血淋巴pH会紧密跟随海水pH变化，凸显出其酸碱调节能力有限。暴露2周后，在当前及近未来海洋酸化情景下，经历变动pH条件的紫贻贝，其代谢速率、抗氧化酶活性及脂质过氧化水平均显著高于暴露于恒定pH条件的个体。与恒定当前pH条件相比，近未来恒定pH条件会引发显著的酸碱紊乱及脂质过氧化，但不会影响代谢速率。本研究结果明确显示，栖息于天然波动环境的代谢成本高于恒定海水环境，这将对我们推演海岸物种未来海洋酸化响应的方式产生重要影响。