Lower St. Lawrence Estuary bacterial 16S rRNA gene diversity

The Estuary and Gulf of St. Lawrence (EGSL) in eastern Canada is 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. Modeling 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 co-occurring 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 compared to 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.