Active nitrogen fixing heterotrophic bacteria at and below the chemocline of the central Baltic Sea

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  • Author: Farnelid, Hanna

    Linnaeus University, Sweden

  • Author: Bentzon-Tilia, Mikkel

    University of Copenhagen

  • Author: Andersson, Anders F.

    KTH - Royal Institute of Technology, Sweden

  • Author: Bertilsson, Stefan

    Uppsala University, Sweden

  • Author: Jost, Günter

    Leibniz Institute for Baltic Sea Research, Germany

  • Author: Labrenz, Matthias

    Leibniz Institute for Baltic Sea Research, Germany

  • Author: Jürgens, Klaus

    Leibniz Institute for Baltic Sea Research, Germany

  • Author: Riemann, Lasse

    University of Copenhagen, Denmark

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The Baltic Sea receives large nitrogen inputs by diazotrophic (N2-fixing) heterocystous cyanobacteria but the significance of heterotrophic N2 fixation has not been studied. Here, the diversity, abundance and transcription of the nifH fragment of the nitrogenase enzyme in two basins of the Baltic Sea proper was examined. N2 fixation was measured at the surface (5 m) and in anoxic water (200 m). Vertical sampling profiles of >10 and <10 μm size fractions were collected in 2007, 2008 and 2011 at the Gotland Deep and in 2011 in the Bornholm Basin. Both of these stations are characterized by permanently anoxic bottom water. The 454-pyrosequencing nifH analysis revealed a diverse assemblage of nifH genes related to alpha-, beta- and gammaproteobacteria (nifH cluster I) and anaerobic bacteria (nifH cluster III) at and below the chemocline. Abundances of genes and transcripts of seven diazotrophic phylotypes were investigated using quantitative polymerase chain reaction revealing abundances of heterotrophic nifH phylotypes of up to 2.1 × 107 nifH copies l−1. Abundant nifH transcripts (up to 3.2 × 104 transcripts l−1) within nifH cluster III and co-occurring N2 fixation (0.44±0.26 nmol l−1 day−1) in deep water suggests that heterotrophic diazotrophs are fixing N2 in anoxic ammonium-rich waters. Our results reveal that N2 fixation in the Baltic Sea is not limited to illuminated N-deplete surface waters and suggest that N2 fixation could also be of importance in other suboxic regions of the world’s oceans.
Original languageEnglish
JournalI S M E Journal
Volume7
Pages (from-to)1413-1423
Number of pages11
ISSN1751-7362
DOIs
Publication statusPublished - 2013
Externally publishedYes
CitationsWeb of Science® Times Cited: No match on DOI

    Research areas

  • Baltic Sea, Chemocline, nifH, Nitrogen, Fixation, Oxic-anoxic interface, Pyrosequencing

ID: 110643483