title: Winter bloom of a rare betaproteobacterium in the Arctic Ocean.
abstract: Extremely low abundance microorganisms (members of the "rare biosphere") are believed to include dormant taxa, which can sporadically become abundant following environmental triggers. Yet, microbial transitions from rare to abundant have seldom been captured in situ, and it is uncertain how widespread these transitions are. A bloom of a single ribotype (≥99% similarity in the 16S ribosomal RNA gene) of a widespread betaproteobacterium (Janthinobacterium sp.) occurred over 2 weeks in Arctic marine waters. The Janthinobacterium population was not detected microscopically in situ in January and early February, but suddenly appeared in the water column thereafter, eventually accounting for up to 20% of bacterial cells in mid February. During the bloom, this bacterium was detected at open water sites up to 50 km apart, being abundant down to more than 300 m. This event is one of the largest monospecific bacterial blooms reported in polar oceans. It is also remarkable because Betaproteobacteria are typically found only in low abundance in marine environments. In particular, Janthinobacterium were known from non-marine habitats and had previously been detected only in the rare biosphere of seawater samples, including the polar oceans. The Arctic Janthinobacterium formed mucilagenous monolayer aggregates after short (ca. 8 h) incubations, suggesting that biofilm formation may play a role in maintaining rare bacteria in pelagic marine environments. The spontaneous mass occurrence of this opportunistic rare taxon in polar waters during the energy-limited season extends current knowledge of how and when microbial transitions between rare and abundant occur in the ocean.
authors.affiliation: Limnology and Science for Life Laboratory, Department of Ecology and Genetics, Uppsala University Uppsala, Sweden.
authors.forename: Laura
authors.initials: L
authors.lastname: Alonso-Sáez
authors.affiliation: Limnological Station, Institute of Plant Biology, University of Zurich Kilchberg, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Zeder
authors.affiliation: Département de Biologie, Université Laval Québec, QC, Canada.
authors.forename: Tommy
authors.initials: T
authors.lastname: Harding
authors.affiliation: Limnological Station, Institute of Plant Biology, University of Zurich Kilchberg, Switzerland.
authors.forename: Jakob
authors.initials: J
authors.lastname: Pernthaler
authors.affiliation: Département de Biologie, Université Laval Québec, QC, Canada.
authors.forename: Connie
authors.initials: C
authors.lastname: Lovejoy
authors.affiliation: Limnology and Science for Life Laboratory, Department of Ecology and Genetics, Uppsala University Uppsala, Sweden.
authors.forename: Stefan
authors.initials: S
authors.lastname: Bertilsson
authors.affiliation: Departament de Biologia Marina i Oceanografia, Institut de Ciències del Mar, CSIC Barcelona, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Pedrós-Alió
keywords: Arctic; Janthinobacterium; betaproteobacteria; biofilm; bloom; rare biosphere