Microbial Biogeography along an Estuarine Salinity Gradient: Combined Influences of Bacterial Growth and Residence Time

TLDR

Bacterioplankton composition shifts along the Parker River estuary salinity gradient are linked to residence time and doubling time, with most taxa belonging to common freshwater and marine clusters within Proteobacteria, Bacteroidetes, and Actinobacteria. The study used DGGE of PCR‑amplified 16S rDNA, combined with 14C‑leucine incorporation and direct counts to calculate doubling times, and sequenced DGGE bands to confirm single phylotypes. Freshwater and marine populations dominate the estuary throughout the year, but a distinct estuarine community emerges at intermediate salinities during summer and fall when doubling time is shorter than residence time, and these estuarine taxa span Proteobacteria, Bacteroidetes, and Actinobacteria and relate to diverse environmental clones.

Abstract

ABSTRACT Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([ 14 C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria , Bacteroidetes , and Actinobacteria . Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.