The Estuary and Gulf of St. Lawrence (EGSL) in eastern Canada are among the largest and most productive coastal ecosystems in the world. Very little information on bacterial diversity exists, hampering our understanding of the relationships between bacterial community structure and biogeochemical function in the EGSL. During the productive spring period, we investigated free-living and particle-associated bacterial communities across the stratified waters of the Lower St. Lawrence Estuary, including the particle-rich surface and bottom boundary layers. Modelling of community structure based on 16S rRNA gene and transcript diversity identified bacterial assemblages specifically associated with four habitat types defined by water mass (upper water or lower water column) and size fraction (free living or particle associated). Assemblages from the upper waters represent sets of cooccurring bacterial populations that are widely distributed across Lower St. Lawrence Estuary surface waters and likely key contributors to organic matter degradation during the spring. In addition, we provide strong evidence that particles in deep hypoxic waters and the bottom boundary layer support a metabolically active bacterial community that is compositionally distinct from those of surface particles and the free-living communities. Among the distinctive features of the bacterial assemblage associated with lower-water particles was the presence of uncultivated lineages of Deltaproteobacteria, including marine myxobacteria. Overall, these results provide an important ecological framework for further investigations of the biogeochemical contributions of bacterial populations in this important coastal marine ecosystem.IMPORTANCE The Estuary and Gulf of St. Lawrence (EGSL) in eastern Canada is an appealing ecosystem for studying how microbial communities and metabolic processes are related to environmental change. Ocean and climate variability result in large spatiotemporal variations in environmental conditions and oceanographic processes. The EGSL is also exposed to a variety of additional human pressures that threaten its integrity and sustainable use, including shipping, aquiculture, coastal development, and oil exploration. To monitor and perhaps mitigate the impacts of these human activities on the EGSL, a comprehensive understanding of the biological communities is required. In this study, we provide the first comprehensive view of bacterial diversity in the EGSL and describe the distinct bacterial assemblages associated with different environmental habitats. This work therefore provides an important baseline ecological framework for bacterial communities in the EGSL useful for further studies on how these communities may respond to environmental change.

中文翻译：

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在深和低氧的下圣劳伦斯河口建模自由活动和与粒子相关的细菌集合。

加拿大东部的河口和圣劳伦斯湾（EGSL）是世界上最大，生产力最高的沿海生态系统之一。关于细菌多样性的信息很少，这妨碍了我们对EGSL中细菌群落结构与生物地球化学功能之间关系的理解。在春季生产期，我们调查了圣劳伦斯河下游河口分层水域中的自由生活和与颗粒相关的细菌群落，包括富含颗粒的表面和底部边界层。基于16S rRNA基因和转录本多样性的群落结构建模确定了与四种生境类型特别相关的细菌组合，这些生境类型由水量（上水或下水柱）和大小分数（自由生活或与颗粒相关）定义。来自上游水域的集合代表了共生细菌种群的集合，这些细菌种群广泛分布在圣劳伦斯河下游地表水域，并且可能是春季有机物降解的关键因素。此外，我们提供了有力的证据，表明深氧不足水域和底部边界层中的颗粒支持具有代谢活性的细菌群落，该细菌群落的成分不同于表面颗粒和自由生物群落。与较低水位颗粒相关的细菌集合体的显着特征之一是存在未经培养的三角洲变形杆菌，包括海洋黏菌。总体，这些结果为进一步研究这个重要的沿海海洋生态系统中细菌种群的生物地球化学贡献提供了重要的生态学框架。过程与环境变化有关。海洋和气候的变化导致环境条件和海洋学过程的时空变化很大。EGSL还面临着各种其他人类压力，威胁到其完整性和可持续利用，包括运输，水产养殖，沿海开发和石油勘探。为了监控并也许减轻这些人类活动对EGSL的影响，需要对生物群落有一个全面的了解。在这项研究中，我们提供了EGSL中细菌多样性的第一个综合视图，并描述了与不同环境栖息地相关的独特细菌组合。因此，这项工作为EGSL中的细菌群落提供了重要的基线生态框架，可用于进一步研究这些群落如何对环境变化做出反应。