title: Co-occurrence of denitrification and nitrogen fixation in a meromictic lake, Lake Cadagno (Switzerland).
abstract: The nitrogen cycling of Lake Cadagno was investigated by using a combination of biogeochemical and molecular ecological techniques. In the upper oxic freshwater zone inorganic nitrogen concentrations were low (up to approximately 3.4 microM nitrate at the base of the oxic zone), while in the lower anoxic zone there were high concentrations of ammonium (up to 40 microM). Between these zones, a narrow zone was characterized by no measurable inorganic nitrogen, but high microbial biomass (up to 4 x 10(7) cells ml(-1)). Incubation experiments with (15)N-nitrite revealed nitrogen loss occurring in the chemocline through denitrification (approximately 3 nM N h(-1)). At the same depth, incubations experiments with (15)N(2)- and (13)C(DIC)-labelled bicarbonate, indicated substantial N(2) fixation (31.7-42.1 pM h(-1)) and inorganic carbon assimilation (40-85 nM h(-1)). Catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH) and sequencing of 16S rRNA genes showed that the microbial community at the chemocline was dominated by the phototrophic green sulfur bacterium Chlorobium clathratiforme. Phylogenetic analyses of the nifH genes expressed as mRNA revealed a high diversity of N(2) fixers, with the highest expression levels right at the chemocline. The majority of N(2) fixers were related to Chlorobium tepidum/C. phaeobacteroides. By using Halogen In Situ Hybridization-Secondary Ion Mass Spectroscopy (HISH-SIMS), we could for the first time directly link Chlorobium to N(2) fixation in the environment. Moreover, our results show that N(2) fixation could partly compensate for the N loss and that both processes occur at the same locale at the same time as suggested for the ancient Ocean.
authors.affiliation: Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany. hhalm@mpi-bremen.de
authors.forename: Hannah
authors.initials: H
authors.lastname: Halm
authors.forename: Niculina
authors.initials: N
authors.lastname: Musat
authors.forename: Phyllis
authors.initials: P
authors.lastname: Lam
authors.forename: Rebecca
authors.initials: R
authors.lastname: Langlois
authors.forename: Florin
authors.initials: F
authors.lastname: Musat
authors.forename: Sandro
authors.initials: S
authors.lastname: Peduzzi
authors.forename: Gaute
authors.initials: G
authors.lastname: Lavik
authors.forename: Carsten J
authors.initials: CJ
authors.lastname: Schubert
authors.forename: Bärbel
authors.initials: B
authors.lastname: Sinha
authors.forename: Bärbel
authors.initials: B
authors.lastname: Singha
authors.forename: Julie
authors.initials: J
authors.lastname: LaRoche
authors.forename: Marcel M M
authors.initials: MM
authors.lastname: Kuypers
mesh_terms: D002245:Carbon Dioxide / Q000032:analysis; D041883:Chlorobium / Q000145:classification / Q000302:isolation & purification / Q000378:metabolism; D002844:Chromatium / Q000302:isolation & purification / Q000378:metabolism; D005618:Fresh Water / Q000737:chemistry / Q000382:microbiology*; D017403:In Situ Hybridization; D009573:Nitrites / Q000032:analysis; D009584:Nitrogen / Q000032:analysis*; D009586:Nitrogen Fixation; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism; D010802:Phylogeny; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D012336:RNA, Ribosomal, 16S / Q000378:metabolism; D013557:Switzerland
chemical_list: D009573:Nitrites; D000644:Quaternary Ammonium Compounds; D012336:RNA, Ribosomal, 16S; D002245:Carbon Dioxide; D010088:Oxidoreductases; C029130:nitrogenase reductase; D009584:Nitrogen