title: Nitrogenase expression in estuarine bacterioplankton influenced by organic carbon and availability of oxygen.
abstract: The genetic capacity to fix gaseous nitrogen (N) is distributed among diverse diazotrophs belonging to the Bacteria and Archaea. However, only a subset of the putative diazotrophs present actively fix N at any given time in the environment. We experimentally tested whether the availability of carbon and inhibition by oxygen constrain N fixation by diazotrophs in coastal seawater. The goal was to test whether by alleviating these constraints an increased overlap between nitrogenase (nifH)-gene-carrying and -expressing organisms could be achieved. We incubated water from a eutrophic but N-limited fjord in Denmark under high-carbon/low-oxygen conditions and determined bacterial growth and production, diazotrophic community composition (Illumina nifH amplicon sequencing), and nifH gene abundance and expression [quantitative PCR (qPCR) and quantitative reverse transcriptase PCR (qRT-PCR)]. Bacterial abundances and production increased under high-carbon/low-oxygen conditions as did the similarity between present and active diazotrophic communities. This was caused by the loss of specific abundant yet non-active gammaproteobacterial phylotypes and increased expression by others. The prominent active gamma- and epsilonproteobacterial diazotrophs did not, however, respond to these conditions in a uniform way, highlighting the difficulty to assess how a change in environmental conditions may affect a diverse indigenous diazotrophic community.
authors.affiliation: Marine Biological Section, Department of Biology, University of Copenhagen, DK-3000 Helsingør, Denmark severin@igb-berlin.de.
authors.forename: Ina
authors.initials: I
authors.lastname: Severin
authors.affiliation: Marine Biological Section, Department of Biology, University of Copenhagen, DK-3000 Helsingør, Denmark.
authors.forename: Mikkel
authors.initials: M
authors.lastname: Bentzon-Tilia
authors.affiliation: Department of Biology, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, USA.
authors.forename: Pia H
authors.initials: PH
authors.lastname: Moisander
authors.affiliation: Marine Biological Section, Department of Biology, University of Copenhagen, DK-3000 Helsingør, Denmark.
authors.forename: Lasse
authors.initials: L
authors.lastname: Riemann
mesh_terms: D001419:Bacteria / Q000235:genetics*; D002244:Carbon / Q000378:metabolism*; D020565:Epsilonproteobacteria / Q000235:genetics / Q000502:physiology; D063366:Estuaries; D020563:Gammaproteobacteria / Q000235:genetics / Q000502:physiology; D059013:Microbial Consortia / Q000502:physiology*; D009586:Nitrogen Fixation; D009591:Nitrogenase / Q000235:genetics*; D010088:Oxidoreductases / Q000235:genetics; D010100:Oxygen / Q000378:metabolism*; D010802:Phylogeny; D010933:Plankton / Q000235:genetics* / Q000502:physiology; D060888:Real-Time Polymerase Chain Reaction; D012623:Seawater / Q000382:microbiology
chemical_list: D002244:Carbon; D010088:Oxidoreductases; D009591:Nitrogenase; C029130:nitrogenase reductase; D010100:Oxygen
keywords: bacterial activity; limiting factors; nitrogen fixation