title: High rates of microbial dinitrogen fixation and sulfate reduction associated with the Mediterranean seagrass Posidonia oceanica.
abstract: Seagrass meadows of Posidonia oceanica represent hotspots of productivity in the oligotrophic Mediterranean Sea. The lack of dissolved inorganic nitrogen (DIN) in the seawater suggests that the N-demand of these meadows might be in part supported by microbial dinitrogen (N2) fixation. However, currently there are no direct N2fixation measurements available for this productive marine macrophyte. Here we investigated N2fixation activity associated with P. oceanica leaf, rhizome and root pieces. In 15N2incubations, the roots exhibited highest rates of N2fixation. The rates varied considerably between replicates, presumably due to a patchy microbial colonization of the roots. Additions of organic carbon compounds (acetate, glucose, sucrose or algal lysate) did not enhance the N2fixation rates. Sulfate reduction rates measured alongside were also highest in root incubations. Correspondingly, sequences of the nifH gene (a marker gene for the iron protein of the N2-fixing enzyme nitrogenase) related to known sulfate-reducing bacteria were retrieved from P. oceanica roots. Other nifH sequences clustered with known heterotrophic diazotrophs previously identified in other marine macrophytes. In particular, many sequences obtained from P. oceanica roots were similar (>94%) to a saltmarsh rhizosphere-associated heterotrophic diazotroph, indicating that heterotrophic lifestyle might be common among marine macrophyte-associated diazotrophs.
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany. Electronic address: nlehnen@mpi-bremen.de.
authors.forename: Nadine
authors.initials: N
authors.lastname: Lehnen
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Hannah K
authors.initials: HK
authors.lastname: Marchant
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Schwedt
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Jana
authors.initials: J
authors.lastname: Milucka
authors.affiliation: HYDRA Institute for Marine Sciences, Elba Field Station, 57034 Campo nell'Elba, Italy.
authors.forename: Christian
authors.initials: C
authors.lastname: Lott
authors.affiliation: HYDRA Institute for Marine Sciences, Elba Field Station, 57034 Campo nell'Elba, Italy.
authors.forename: Miriam
authors.initials: M
authors.lastname: Weber
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Julien
authors.initials: J
authors.lastname: Dekaezemacker
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Brandon K B
authors.initials: BK
authors.lastname: Seah
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Philipp F
authors.initials: PF
authors.lastname: Hach
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Wiebke
authors.initials: W
authors.lastname: Mohr
authors.affiliation: Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
authors.forename: Marcel M M
authors.initials: MM
authors.lastname: Kuypers
mesh_terms: D035961:Alismatales / Q000378:metabolism* / Q000382:microbiology*; D020564:Deltaproteobacteria / Q000378:metabolism; D020563:Gammaproteobacteria / Q000378:metabolism; D052836:Heterotrophic Processes / Q000502:physiology*; D019083:Mediterranean Region; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation / Q000502:physiology*; D010088:Oxidoreductases / Q000235:genetics; D018515:Plant Leaves / Q000378:metabolism / Q000382:microbiology; D018517:Plant Roots / Q000378:metabolism* / Q000382:microbiology*; D012623:Seawater / Q000737:chemistry; D013431:Sulfates / Q000378:metabolism
chemical_list: D013431:Sulfates; D010088:Oxidoreductases; C029130:nitrogenase reductase; D009584:Nitrogen
keywords: Heterotrophic nitrogen fixation; Posidonia oceanica; Sulfate reduction; nifH diversity