Antioxidant response of the hard shelled mussel Mytilus coruscus exposed to reduced pH and oxygen concentration

Ocean acidification (OA) and hypoxic events are increasing worldwide problems, their interactive effects have not been well clarified, although their co-occurrence is prevalent. The East China Sea (the Yangtze River estuary area) suffers from not only coastal hypoxia but also pH fluctuation, representing an ideal study site to explore the combined effect of OA and hypoxia on marine bivalves. We experimentally evaluated the antioxidant response of the mussel Mytilus coruscus exposed to three pH levels (8.1, 7.7 and 7.3) at two dissolved oxygen (DO) levels (2.0 mg/L and 6.0 mg/L) for 72h. Activities of superoxide dismutase, catalase, glutathione peroxidase, acid phosphatase, and alkaline phosphatase and levels of malondialdehyde were measured in gills and hemolymph. All enzymatic activities in hemolymph and gills followed a similar pattern throughout the experiment duration. Generally, low DO showed greater effects on enzyme activities than elevated CO2. Significant interactions between DO, pH and time were only observed at superoxide dismutase and catalase in both tissues. PCA revealed positive relationships between most enzyme activities in both gills and hemolymph with the exception of alkaline phosphatase activity and the level of malondialdehyde in the hemolymph. Overall, our results suggested that decreased pH and low DO induced similar antioxidant responses in the hard shelled mussel, and showed an additive effect on most enzyme activities. The evaluation of multiple environmental stressors, a more realistic scenario than single ones, is crucial to predict the effect of future global changes on coastal species and our results supply some insights on the potential combined effects of reduced pH and DO on marine bivalves.

海洋酸化（Ocean Acidification, OA）与低氧事件已成为全球日益严峻的环境问题，尽管二者的共同发生极为普遍，但其交互影响尚未得到充分阐明。东海（长江口海域）不仅面临沿岸低氧问题，同时存在pH波动现象，是探究海洋酸化与低氧对海洋双壳类综合效应的理想研究区域。本实验以厚壳贻贝（Mytilus coruscus）为研究对象，设置3个pH梯度（8.1、7.7、7.3）与2个溶解氧（Dissolved Oxygen, DO）梯度（2.0 mg/L、6.0 mg/L），进行为期72小时的暴露处理，评估其抗氧化响应情况。本研究检测了鳃组织与血淋巴中超氧化物歧化酶（Superoxide Dismutase）、过氧化氢酶（Catalase）、谷胱甘肽过氧化物酶（Glutathione Peroxidase）、酸性磷酸酶（Acid Phosphatase）、碱性磷酸酶（Alkaline Phosphatase）的活性，以及丙二醛（Malondialdehyde）的含量。实验期间，血淋巴与鳃组织中的所有酶活性均呈现相似的变化趋势。总体而言，低溶解氧对酶活性的影响程度强于CO₂升高处理。仅在两种组织的超氧化物歧化酶与过氧化氢酶活性中，观测到溶解氧、pH与时间三者间存在显著交互效应。主成分分析（Principal Component Analysis, PCA）结果显示，除血淋巴中的碱性磷酸酶活性与丙二醛含量外，鳃组织与血淋巴中的多数酶活性之间均呈正相关关系。综合来看，本研究结果表明，pH降低与低溶解氧处理对厚壳贻贝诱导的抗氧化响应较为相似，且对多数酶活性表现出相加效应。相较于单一环境胁迫因子，多胁迫因子的评估更贴合自然现实场景，这对预测未来全球变化对沿岸物种的影响至关重要；本研究结果也为理解pH降低与溶解氧减少对海洋双壳类的潜在联合效应提供了新的认知参考。