Bacterioplankton Community Shifts in an Arctic Lake Correlate with Seasonal Changes in Organic Matter Source

TLDR

Seasonal shifts in Toolik Lake bacterioplankton composition are driven by changes in dissolved organic matter source and lability, with communities comprising persistent and transient populations that appear and disappear throughout the year. Community shifts were detected by DGGE of 16S rRNA genes, first at 4 °C beneath ice and snow in spring, then again after terrestrial DOM flux ceased in early summer as ice melted and phytoplankton developed. The shifts coincided with an annual peak in bacterial productivity driven by labile terrestrial DOM from snow meltwater, and sequencing revealed that transient populations were either advected with terrestrial DOM or grew from low concentrations during phytoplankton development, with most bands representing single ribotypes belonging to globally distributed freshwater α‑, β‑, Cytophaga‑Flavobacteria‑Bacteroides, and Actinobacteria clusters.

Abstract

ABSTRACT Seasonal shifts in bacterioplankton community composition in Toolik Lake, a tundra lake on the North Slope of Alaska, were related to shifts in the source (terrestrial versus phytoplankton) and lability of dissolved organic matter (DOM). A shift in community composition, measured by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes, occurred at 4°C in near-surface waters beneath seasonal ice and snow cover in spring. This shift was associated with an annual peak in bacterial productivity ([ 14 C]leucine incorporation) driven by the large influx of labile terrestrial DOM associated with snow meltwater. A second shift occurred after the flux of terrestrial DOM had ended in early summer as ice left the lake and as the phytoplankton community developed. Bacterioplankton communities were composed of persistent populations present throughout the year and transient populations that appeared and disappeared. Most of the transient populations could be divided into those that were advected into the lake with terrestrial DOM in spring and those that grew up from low concentrations during the development of the phytoplankton community in early summer. Sequencing of DNA in DGGE bands demonstrated that most bands represented single ribotypes and that matching bands from different samples represented identical ribotypes. Bacteria were identified as members of globally distributed freshwater phylogenetic clusters within the α- and β- Proteobacteria , the Cytophaga-Flavobacteria-Bacteroides group, and the Actinobacteria .