Abstract
The fixation of dinitrogen (N2) and denitrification are two opposite processes in the nitrogen cycle. The former transfers atmospheric dinitrogen gas into bound nitrogen in the biosphere, while the latter returns this bound nitrogen back to atmospheric dinitrogen. It is unclear whether or not these processes are intimately connected in any microbial ecosystem or that they are spatially and/or temporally separated. Here, we measured seafloor nitrogen fixation and denitrification as well as pelagic nitrogen fixation by using the stable isotope technique. Alongside, we measured the diversity, abundance, and activity of nitrogen-fixing and denitrifying microorganisms at three stations in the southern North Sea. Nitrogen fixation ranged from undetectable to 2.4 nmol N L−1 d−1 and from undetectable to 8.2 nmol N g−1 d−1 in the water column and seafloor, respectively. The highest rates were measured in August at Doggersbank, both for the water column and for the seafloor. Denitrification ranged from 1.7 to 208.8 μmol m−2 d−1 and the highest rates were measured in May at the Oyster Grounds. DNA sequence analysis showed sequences of nifH, a structural gene for nitrogenase, related to sequences from anaerobic sulfur/iron reducers and sulfate reducers. Sequences of the structural gene for nitrite reductase, nirS, were related to environmental clones from marine sediments. Quantitative polymerase chain reaction (qPCR) data revealed the highest abundance of nifH and nirS genes at the Oyster Grounds. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) data revealed the highest nifH expression at Doggersbank and the highest nirS expression at the Oyster Grounds. The distribution of the diazotrophic and denitrifying communities seems to be subject to different selecting factors, leading to spatial and temporal separation of nitrogen fixation and denitrification. These selecting factors include temperature, organic matter availability, and oxygen concentration.
Highlights
The microbial biogeochemical cycle of nitrogen transfers atmospheric dinitrogen gas (N2) to bound nitrogen in the biosphere and back to N2 (Revsbech et al, 2006)
Dinitrogen fixation is the reduction of N2 to ammonia, which is subsequently assimilated into amino acids and proteins to synthesize biomass
The nitrogen fixation rates (0–2.4 nmol N L−1 d−1) in the water column of the southern North Sea that we report here were in the same range as those that have been reported for the tropical Atlantic Ocean (0.6–1.1 nmol N L−1 d−1) (Falcón et al, 2004), the subtropical and tropical eastern Atlantic Ocean (0–1.4 nmol N L−1 d−1) (Staal et al, 2007) and from some stations of the western Atlantic coastal waters (0.2–76.8 nmol N L−1 d−1) (Mulholland et al, 2012) but substantially lower than those reported for the western English Channel (18.9–20.0 nmol N L−1 d−1) (Rees et al, 2009)
Summary
The microbial biogeochemical cycle of nitrogen transfers atmospheric dinitrogen gas (N2) to bound nitrogen in the biosphere and back to N2 (Revsbech et al, 2006). The former seems to be quantitatively more important than the latter in most habitats, in certain environments anammox has been shown to out rate denitrification (Kuypers et al, 2003, 2005). It is unknown whether or not in any microbial ecosystem the nitrogen cycle is functional at the same spatial and temporal scales or that they are (partly) occurring separated. We investigated denitrification and dinitrogen fixation in the North Sea in order to answer this question
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