Seawater carbonate chemistry and physiology of Baltic blue mussels (Mytilus edulis)

Increased maintenance costs at cellular, and consequently organism level, are thought to be involved in shaping the sensitivity of marine calcifiers to ocean acidification (OA). Yet, knowledge of the capacity of marine calcifiers to undergo metabolic adaptation is sparse. In Kiel Fjord, blue mussels thrive despite periodically high seawater PCO2, making this population interesting for studying metabolic adaptation under OA. Consequently, we conducted a multi-generation experiment and compared physiological responses of F1 mussels from 'tolerant' and 'sensitive' families exposed to OA for 1 year. Family classifications were based on larval survival; tolerant families settled at all PCO2 levels (700, 1120, 2400 µatm) while sensitive families did not settle at the highest PCO2 (>=99.8% mortality). We found similar filtration rates between family types at the control and intermediate PCO2 level. However, at 2400 µatm, filtration and metabolic scope of gill tissue decreased in tolerant families, indicating functional limitations at the tissue level. Routine metabolic rates (RMR) and summed tissue respiration (gill and outer mantle tissue) of tolerant families were increased at intermediate PCO2, indicating elevated cellular homeostatic costs in various tissues. By contrast, OA did not affect tissue and routine metabolism of sensitive families. However, tolerant mussels were characterised by lower RMR at control PCO2 than sensitive families, which had variable RMR. This might provide the energetic scope to cover increased energetic demands under OA, highlighting the importance of analysing intra-population variability. The mechanisms shaping such difference in RMR and scope, and thus species' adaptation potential, remain to be identified.人们认为，细胞乃至个体层面的维持成本上升，参与塑造了海洋钙化生物（marine calcifiers）对海洋酸化（ocean acidification, OA）的敏感性。然而，目前关于海洋钙化生物代谢适应能力的认知仍较为匮乏。在基尔峡湾（Kiel Fjord），蓝贻贝（blue mussels）能够在周期性高海水二氧化碳分压（PCO₂）的环境中存活繁衍，因此该种群成为研究海洋酸化下代谢适应的理想对象。为此，我们开展了一项多代实验，将暴露于海洋酸化环境长达1年的子一代（F₁）蓝贻贝，按“耐受型家系”与“敏感型家系”进行生理响应对比。家系分类依据幼虫存活率：耐受型家系可在所有二氧化碳分压水平（700、1120、2400 µatm）下完成幼虫附着，而敏感型家系在最高二氧化碳分压（死亡率≥99.8%）下无法完成附着。我们发现，在对照组与中等二氧化碳分压组中，两类家系的滤水率无显著差异。但在2400 µatm条件下，耐受型家系的鳃组织滤水率与代谢范围（metabolic scope）均出现下降，提示其组织层面存在功能限制。耐受型家系的日常代谢率（routine metabolic rates, RMR）以及鳃、外套膜外层两类组织的总呼吸速率，在中等二氧化碳分压条件下均有所升高，表明其多种组织的细胞稳态成本有所上升。与之相反，海洋酸化并未对敏感型家系的组织代谢与日常代谢率产生影响。不过，在对照组二氧化碳分压下，耐受型蓝贻贝的日常代谢率低于敏感型家系，而敏感型家系的日常代谢率存在较大变异。这或许为耐受型家系应对海洋酸化下的能量需求上升提供了能量余量，凸显了种群内变异研究的重要性。而决定日常代谢率与代谢范围差异的机制，以及由此决定的物种适应潜力，仍有待进一步阐明。