Sustained periods of anoxia, driven by eutrophication, threaten coastal marine systems and can lead to mass mortalities of even resilient animals such as bivalves. While mortality rates under anoxia are well-studied, the specific mechanism(s) of mortality are less clear. We used a suite of complementary techniques (LT50, histology, 16S rRNA amplicon sequencing, and valvometry) to show that the proliferation of anaerobic bacteria within eastern oysters (Crassostrea virginica) accelerates mortality rate under anoxic conditions. Manipulative laboratory experiments revealed that oyster survival under anoxic conditions was halved when bacteria were present compared to when they were excluded by the broad-spectrum antibiotic chloramphenicol. Histological assessments supported this mechanism and showed infiltration of bacteria in oysters that were not treated with antibiotics compared to a general lack of bacteria when oysters were treated with antibiotics. 16S rRNA amplicon sequencing failed to identify any particular genera of bacteria responsible for mortality, rather a diversity of endogenous anaerobic and/or sulfate-reducing bacteria were common among oysters. In addition, monitoring of oyster valve gaping behavior in the field revealed that oysters showed remarkable valve closure synchrony when first exposed to anoxia. However, oysters periodically opened throughout anoxia/hypoxia in both the lab and field, suggesting that the infiltration of exogenous bacteria from the environment may also influence mortality rates under natural settings. Coupled with previous studies, we posit that mass mortality events in a wide range of coastal bivalves are likely the result of co-morbidity from asphyxiation and bacterial processes.

中文翻译：

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内部杀手：内源性细菌在持续缺氧期间加速牡蛎死亡率

由富营养化驱动的持续缺氧期威胁着沿海海洋系统，甚至可能导致双壳类等有弹性的动物大量死亡。虽然对缺氧条件下的死亡率进行了充分研究，但死亡率的具体机制尚不清楚。我们使用了一套互补技术（LT50、组织学、16S rRNA 扩增子测序和瓣膜测量法）来证明东部牡蛎（Crassostrea virginica）内厌氧菌的增殖) 加速缺氧条件下的死亡率。操作性实验室实验表明，与被广谱抗生素氯霉素排除时相比，存在细菌时在缺氧条件下的牡蛎存活率减半。组织学评估支持这种机制，并显示未用抗生素处理的牡蛎中细菌浸润，而用抗生素处理的牡蛎则普遍缺乏细菌。16S rRNA 扩增子测序未能识别出任何导致死亡的特定细菌属，而是牡蛎中常见的内源性厌氧菌和/或硫酸盐还原菌的多样性。此外，对田间牡蛎瓣膜张开行为的监测表明，牡蛎在首次暴露于缺氧时表现出显着的瓣膜关闭同步性。然而，在实验室和野外，牡蛎在缺氧/缺氧的情况下定期开放，这表明环境中外源细菌的渗透也可能影响自然环境下的死亡率。结合之前的研究，我们假设大量沿海双壳类动物的大规模死亡事件可能是窒息和细菌过程共病的结果。