Co‐occurrence of denitrification and nitrogen fixation in a meromictic lake, Lake Cadagno (Switzerland)

Abstract

Summary 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 ∼3.4 μM nitrate at the base of the oxic zone), while in the lower anoxic zone there were high concentrations of ammonium (up to 40 μM). Between these zones, a narrow zone was characterized by no measurable inorganic nitrogen, but high microbial biomass (up to 4 × 10 7 cells ml −1 ). Incubation experiments with 15 N‐nitrite revealed nitrogen loss occurring in the chemocline through denitrification (∼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.

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