Spatial and temporal variability of bacterioplankton communities across river to ocean environmental gradients

Bacterioplankton communities are deeply diverse within and across environments, yet also display repeatable patterns over seasonal and annual time scales. We assessed patterns of bacterioplankton community variability across the Columbia River coastal margin over space and time. Coastal zones encompass a complex spectrum of environmental gradients, which impact the composition of bacterioplankton communities. Few studies have attempted to address these gradients comprehensively, especially across large spatial and long temporal scales. We generated a synoptic, 16S rRNA gene-based bacterioplankton community profile of a coastal zone from water samples collected from the Columbia River, estuary, plume, and along coastal transects covering 360 km of the Oregon and Washington coasts and extending to the deep ocean (>2000 m). Over 600 water samples were collected across four consecutive years and eleven research cruises. Spatially, bacterioplankton communities separated into seven environments across the coastal zone (ANOSIM, p<0.001): river, estuary, plume, epipelagic, mesopelagic, shelf bottom (depth<350 m), and slope bottom (depth>850 m). When comparing communities to an extensive database of environmental factors, communities correlated strongly with the structuring physical factors of salinity, temperature, and depth. Within each environment, community variability correlated with factors important to primary and secondary production. In the freshwater-influenced environments of the Columbia River, estuary, and plume, communities varied seasonally and reassembled annually. Freshwater SAR11, Oceanospirillales, and Flavobacteria taxa were indicators of changing seasonal conditions in these environments. In contrast, seasonal change in communities was not detected in the coastal ocean but instead varied spatially with environmental conditions. Each coastal ocean environment had distinct taxa including SAR406 and SUP05 taxa from the deep ocean and Prochlorococcus and SAR11 taxa from the upper water column. A survey of metabolic potential (metagenomics) and gene expression (metatranscriptomics) across the salinity gradient showed that although communities were taxonomically distinct, the metabolic potential of these communities was highly similar. Additionally, gene expression patterns were extremely different and reflected the short-time scales on which microbial processes persist in an environment. Across the coastal zone, bacterioplankton communities were taxonomically distinct but metabolically similar, structured by physical factors, and predictable across seasons from river to ocean.