title: Molecular evidence for sediment nitrogen fixation in a temperate New England estuary.
abstract: Primary production in coastal waters is generally nitrogen (N) limited with denitrification outpacing nitrogen fixation (N2-fixation). However, recent work suggests that we have potentially underestimated the importance of heterotrophic sediment N2-fixation in marine ecosystems. We used clone libraries to examine transcript diversity of nifH (a gene associated with N2-fixation) in sediments at three sites in a temperate New England estuary (Waquoit Bay, Massachusetts, USA) and compared our results to net sediment N2 fluxes previously measured at these sites. We observed nifH expression at all sites, including a site heavily impacted by anthropogenic N. At this N impacted site, we also observed mean net sediment N2-fixation, linking the geochemical rate measurement with nifH expression. This same site also had the lowest diversity (non-parametric Shannon = 2.75). At the two other sites, we also detected nifH transcripts, however, the mean N2 flux indicated net denitrification. These results suggest that N2-fixation and denitrification co-occur in these sediments. Of the unique sequences in this study, 67% were most closely related to uncultured bacteria from various marine environments, 17% to Cluster III, 15% to Cluster I, and only 1% to Cluster II. These data add to the growing body of literature that sediment heterotrophic N2-fixation, even under high inorganic nitrogen concentrations, may be an important yet overlooked source of N in coastal systems.
authors.affiliation: Department of Earth and Environment, Boston University, Boston, MA, USA; Department of Earth and Environmental Sciences, Wright State University, Dayton, OH, USA.
authors.forename: Silvia E
authors.initials: SE
authors.lastname: Newell
authors.affiliation: Department of Marine and Environmental Sciences, Northeastern University , Boston, MA , USA.
authors.forename: Kaitlyn R
authors.initials: KR
authors.lastname: Pritchard
authors.affiliation: Department of Earth and Environment, Boston University , Boston, MA , USA.
authors.forename: Sarah Q
authors.initials: SQ
authors.lastname: Foster
authors.affiliation: Department of Earth and Environment, Boston University, Boston, MA, USA; Department of Biology, Boston University, Boston, MA, USA.
authors.forename: Robinson W
authors.initials: RW
authors.lastname: Fulweiler
keywords: Denitrification; Heterotrophic nitrogen fixation; Sediments; Sulfate-reducing bacteria; nifH; nifH diversity