Abstract
The estuarine nitrogen cycle can be substantially altered due to anthropogenic activities resulting in increased amounts of inorganic nitrogen (mainly nitrate). In the past, denitrification was considered to be the main ecosystem process removing reactive nitrogen from the estuarine ecosystem. However, recent reports on the contribution of dissimilatory nitrate reduction to ammonium (DNRA) to nitrogen removal in these systems indicated a similar or higher importance, although the ratio between both processes remains ambiguous. Compared to denitrification, DNRA has been underexplored for the last decades and the key organisms carrying out the process in marine environments are largely unknown. Hence, as a first step to better understand the interplay between denitrification, DNRA and reduction of nitrate to nitrite in estuarine sediments, nitrogen reduction potentials were determined in sediments of the Paulina polder mudflat (Westerschelde estuary). We observed high variability in dominant nitrogen removing processes over a short distance (1.6 m), with nitrous oxide, ammonium and nitrite production rates differing significantly between all sampling sites. Denitrification occurred at all sites, DNRA was either the dominant process (two out of five sites) or absent, while nitrate reduction to nitrite was observed in most sites but never dominant. In addition, novel nitrate-to-ammonium reducers assigned to Thalassospira, Celeribacter, and Halomonas, for which DNRA was thus far unreported, were isolated, with DNRA phenotype reconfirmed through nrfA gene amplification. This study demonstrates high small scale heterogeneity among dissimilatory nitrate reduction processes in estuarine sediments and provides novel marine DNRA organisms that represent valuable alternatives to the current model organisms.
Highlights
The rate of terrestrial nitrogen input has more than doubled over the past century, mostly through fossil fuel combustion and increased use of agricultural fertilizers
Song et al (2014) found benthic dissimilatory nitrate reduction to ammonium (DNRA) to be responsible for almost half of the nitrogen removal across the New River estuary, with DNRA rates exceeding those of denitrification (Lisa et al, 2014)
Potential nitrogen removal rates, i.e., nitrate reduction to nitrite, denitrification and DNRA, were measured across a 1.6 m scale, with five sampling sites approximately 9 cm from each other (Figure 1)
Summary
The rate of terrestrial nitrogen input has more than doubled over the past century, mostly through fossil fuel combustion and increased use of agricultural fertilizers When it is not biologically removed from streams and rivers, excess, anthropogenicallyderived nitrogen ends up in estuaries and coastal areas, where it is implicated in eutrophication, alteration of food webs, and hypoxia (Martinetto et al, 2006; Paerl et al, 2006; Diaz and Rosenberg, 2008). Nitrate can be lost from these systems via anaerobic ammonium oxidation (anammox) to dinitrogen gas or denitrification, i.e., the respiratory reduction of nitrate to either the potent greenhouse gas nitrous oxide or dinitrogen gas It can be retained in the system as biologically available ammonium via dissimilatory nitrate reduction to ammonium (DNRA), with possible nitrogen losses via trace amounts of nitrous oxide (Smith, 1982; Cruz-García et al, 2007; Giblin et al, 2013). In contrast to denitrification, DNRA has been underexplored for the last decades and, despite some attempts (Bonin, 1996; Yoon et al, 2015b), the key organisms carrying out this process in marine and estuarine environments, their response to varying environmental conditions and how the DNRA process itself relates to other nitrogen removing processes remain largely unknown
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