Abstract

The St. Lawrence Estuary (SLE) is a productive ecosystem characterized by steep
gradients in salinity and dissolved oxygen (DO). The SLE has been well studied in terms
of physical and chemical properties, however bacterial diversity and community structure
have not been documented in this large estuarine system. Here, we used 16S rRNA gene
surveys to assess the bacterial community structure of both the free-living (FL) and
particle-associated (PA) fractions of the bacterial community. Non-metric
multidimensional scaling (NMS) ordinations and ANOSIM analysis showed that
bacterial communities inhabiting particles were distinct from those found free-living in
the water column, and this structure held true for both surface communities and those
inhabiting the bottom waters. Taxonomic assignment of 16S DNA sequences revealed a
shift in the bacterial community structure along the surface salinity gradient of the SLE.
In general, Proteobacteria were more common in the free-living fraction, while
Bacteroidetes were more commonly associated with particles. Moreover, particleassociated
bacterial communities had an abundance of Alteromonadales in the deep
hypoxic waters and free-living bacteria exhibited a greater abundance of Marine Group A
and Delta-proteobacteria. These findings are shedding light on the diversity of bacterial
taxa inhabiting the SLE and further metagenomic analysis will elucidate the metabolic
contribution of these bacterial communities.