ALBERT

Description: In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes in the water column. The bulk isotope effect of nitrification is hard to predict: It is a two-step-process, where ammonia is oxidized via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it even more difficult to unravel the divergent isotope effects of both processes. However, during an exceptional flood in the Elbe River in June 2013, ammonium and nitrite accumulated in the water column for a short period, returning towards normal summer conditions within one week. Concentrations were sufficient for the analysis of δ15N-NH4+ and δ15N-NO2− evolution, which has not been studied before in a major European river like the Elbe River. In the concert with changes in SPM and δ15N-SPM, as well as nitrate concentration, δ15N-NO3− and δ18O-NO3−, we calculated the isotope fractionation effect during nitrification. We found that in the water column, ammonium and nitrite derived from internal recycling processes, whereas nitrate mainly leached from catchment area. Ammonium and nitrite concentrations increased to 3.4 µmol L−1 and 4.5 µmol L−1, respectively, due to remineralization and ammonium oxidation in the water column. δ15N-NH4+ values increased up to 12 ‰, and δ15N-NO2− ranged from −8.0 ‰ to −14.2 ‰. As water column nitrite concentration decreased, we calculated an isotope effect 15ε of −9.3 ‰ for nitrite oxidation. This isotope effect does not correspond to the inverse isotope fractionation with a positive 15ε proposed by pure culture studies. We hypothesize that the molecular mechanisms that lead to inverse fractionation also apply in natural environments, but that the resulting trend in δ15N-NO2− in this natural environment is masked by dilution with fresh nitrite stemming from ammonium oxidation. Our data are a first approximation of the isotope effect of nitrite oxidation in natural environments and highlight that pure culture results cannot readily be extrapolated to natural microbial assemblages or water bodies.

Description: In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes (e.g. remineralisation, nitrification) in the water column. Nitrification is a two-step process, where ammonia is oxidised via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it difficult to study the single isotope effect of ammonia oxidation or nitrite oxidation in natural systems. However, during an exceptional flood in the Elbe River in June 2013, we found a unique co-occurrence of ammonium, nitrite, and nitrate in the water column, returning towards normal summer conditions within 1 week. Over the course of the flood, we analysed the evolution of δ15N–NH4+ and δ15N–NO2− in the Elbe River. In concert with changes in suspended particulate matter (SPM) and δ15N SPM, as well as nitrate concentration, δ15N–NO3− and δ18O–NO3−, we calculated apparent isotope effects during net nitrite and nitrate consumption. During the flood event, 〉 97 % of total reactive nitrogen was nitrate, which was leached from the catchment area and appeared to be subject to assimilation. Ammonium and nitrite concentrations increased to 3.4 and 4.4 µmol L−1, respectively, likely due to remineralisation, nitrification, and denitrification in the water column. δ15N–NH4+ values increased up to 12 ‰, and δ15N–NO2− ranged from −8.0 to −14.2 ‰. Based on this, we calculated an apparent isotope effect 15ε of −10.0 ± 0.1 ‰ during net nitrite consumption, as well as an isotope effect 15ε of −4.0 ± 0.1 ‰ and 18ε of −5.3 ± 0.1 ‰ during net nitrate consumption. On the basis of the observed nitrite isotope changes, we evaluated different nitrite uptake processes in a simple box model. We found that a regime of combined riparian denitrification and 22 to 36 % nitrification fits best with measured data for the nitrite concentration decrease and isotope increase.

Description: Nitrification, the step-wise oxidation of ammonium to nitrite and nitrate, is important in the marine environment because it produces nitrate, the most abundant marine dissolved inorganic nitrogen (DIN) component and N-source for phytoplankton and microbes. This study focused on the second step of nitrification, which is carried out by a distinct group of organisms, nitrite-oxidizing bacteria (NOB). The growth of NOB is characterized by nitrite oxidation kinetics, which we investigated for 4 pure cultures of marine NOB (Nitrospina watsonii 347, Nitrospira sp. Ecomares 2.1, Nitrococcus mobilis 231, and Nitrobacter sp. 311). We further compared the kinetics to those of non-marine species because substrate concentrations in marine environments are comparatively low, which likely influences kinetics and highlights the importance of this study. We also determined the isotope effect during nitrite oxidation of a pure culture of Nitrospina (Nitrospina watsonii 347) belonging to one of the most abundant marine NOB genera, and for a Nitrospira strain (Nitrospira sp. Ecomares 2.1). The enzyme kinetics of nitrite oxidation, described by Michaelis-Menten kinetics, of 4 marine genera are rather narrow and fall in the low end of half-saturation constant (Km) values reported so far, which span over 3 orders of magnitude between 9 and 〉1000 µM NO2-. Nitrospina has the lowest Km (19 µM NO2-), followed by Nitrobacter (28 µM NO2-), Nitrospira (54 µM NO2-), and Nitrococcus (120 µM NO2-). The isotope effects during nitrite oxidation by Nitrospina watsonii 347 and Nitrospira sp. Ecomares 2.1 were 9.7 ± 0.8 and 10.2 ± 0.9‰, respectively. This confirms the inverse isotope effect of NOB described in other studies; however, it is at the lower end of reported isotope effects. We speculate that differences in isotope effects reflect distinct nitrite oxidoreductase (NXR) enzyme orientations.

Description: In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes (e.g. remineralisation, nitrification) in the water column. Nitrification is a two-step process, where ammonia is oxidised via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it difficult to study the single isotope effect of ammonia oxidation or nitrite oxidation in natural systems. However, during an exceptional flood in the Elbe River in June 2013, we found a unique co-occurrence of ammonium, nitrite, and nitrate in the water column, returning towards normal summer conditions within 1 week. Over the course of the flood, we analysed the evolution of d15N-[NH4]+ and d15N-[NO2]- in the Elbe River. In concert with changes in suspended particulate matter (SPM) and d15N SPM, as well as nitrate concentration, d15N-NO3 - and d18O-[NO3] -, we calculated apparent isotope effects during net nitrite and nitrate consumption. During the flood event, 〉 97 % of total reactive nitrogen was nitrate, which was leached from the catchment area and appeared to be subject to assimilation. Ammonium and nitrite concentrations increased to 3.4 and 4.4 µmol/l, respectively, likely due to remineralisation, nitrification, and denitrification in the water column. d15N-[NH4]+ values increased up to 12 per mil, and d15N-[NO2]- ranged from -8.0 to -14.2 per mil. Based on this, we calculated an apparent isotope effect 15-epsilon of -10.0 ± 0.1 per mil during net nitrite consumption, as well as an isotope effect 15-epsilon of -4.0 ± 0.1 per mil and 18-epsilon of -5.3 ± 0.1 per mil during net nitrate consumption. On the basis of the observed nitrite isotope changes, we evaluated different nitrite uptake processes in a simple box model. We found that a regime of combined riparian denitrification and 22 to 36 % nitrification fits best with measured data for the nitrite concentration decrease and isotope increase.

Description: We investigated nutrient input and retention in the Elbe River (Germany) at the river/estuarine transition with high agricultural loads of nitrogen. Surface water samples were taken at the weir Geesthacht (stream kilometre 585, 53°25'31''N, 10°20'10''E) from 2011 to 2021. In these samples, we analyzed nutrient concentrations, nitrate dual stable isotopes and suspended particulate matter composition. Usually, samples were taken once or twice per month. Aims of the study were to investigate 1) nitrate retention in the Elbe River and catchment, 2) seasonal dynamic of nitrate stable isotopes and 3) key nitrogen turnover processes and their respective controls over a ten year period.