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PubMed1996

Comparison of aquatic food chains using nitrogen isotopes.

Recent studies have shown the utility of delta(15)N to model trophic structure and contaminant bioaccumulation in aquatic food webs. However, cross-system comparisons in delta(15)N can be complicated by differences in delta(15)N at the base of the food chain. Such baseline variation in delta(15)N is difficult to resolve using plankton because of the large temporal variability in the delta(15)N of small organisms that have fast nitrogen turnover. Comparisons using large primary consumers, which have stable tissue isotopic signatures because of their slower nitrogen turnover, show that delta(15)N increases markedly with the human population density in the lake watershed. This shift in delta(15)N likely reflects the high delta(15)N of human sewage. Correcting for this baseline variation in delta(15)N, we report that, contrary to expectations based on previous food-web analysis, the food chains leading up to fish varied by about only one trophic level among the 40 lakes studied. Our results also suggest that the delta(15)N signatures of nitrogen at the base of the food chain will provide a useful tool in the assessment of anthropogenic nutrient inputs.

Animals↗
PubMed2003

Site-selective nitrogen isotopic ratio measurement of nitrous oxide using 2 microm diode lasers.

We demonstrate a high-precision measurement of the isotopomer abundance ratio 14N(15)N(16)O/15N(14)N(16)O/14N(14)N(16)O (approximately 0.37/0.37/100) using three wavelength-modulated 2 microm diode lasers combined with a multipass cell which provides different optical pathlengths of 100 and 1 m to compensate the large abundance difference. A set of absorption lines for which the absorbances have almost the same temperature dependence are selected so that the effect of a change in gas temperature is minimized. The test experiment using pure nearly natural-abundance N(2)O samples showed that the site-selective 15N/14N ratios can be measured relative to a reference material with a precision of +/-3 x 10(-4) (+/-0.3 per thousand) in approximately 2 h.

Lasers↗
PubMed1990

[Use of the stable nitrogen isotope 15N in assessing liver metabolism in hormonal contraception].

In 37 women with long-term use of oral contraceptives (average use 13.4 years) and 12 women who had not taken oral contraceptives for at least 5 years we determined the serum enzymes ALAT, ASAT, Gamma-GT and conducted the 15N-ammonium test as a marker of partial metabolic performance of the liver. In addition we studied total protein, protein fractions in the serum and haematologic parameter. 18 out of 37 women showed a morbid result in the 15N-ammonium test. In 4 of these 18 women morphological changes representing hepatic damage caused by contraceptives could be detected. On the average ALAT, Gamma-GT and 15N-ammonium test in both groups are clearly different. Because the enzymic levels are being in the normal range, only the 15N ammonium test enables to give a hepatotoxic evidence. It is possible to conclude that long-term use of oral contraceptives influences the liver metabolism.

Adult↗
PubMed1994

Nitrogen isotopic analyses by isotope-ratio-monitoring gas chromatography/mass spectrometry.

Amino acids containing natural-abundance levels of 15N were derivatized and analyzed isotopically using a technique in which individual compounds are separated by gas chromatography, combusted on-line, and the product stream sent directly to an isotope-ratio mass spectrometer. For samples of N2 gas, standard deviations of ratio measurement were better than 0.1% (Units for delta are parts per thousand or per million (%).) for samples larger than 400 pmol and better than 0.5% for samples larger than 25 pmol (0.1% 15N is equivalent to 0.00004 atom % 15N). Results duplicated those of conventional, batchwise analyses to within 0.05%. For combustion of organic compounds yielding CO2/N2 ratios between 14 and 28, in particular for N-acetyl n-propyl derivatives of amino acids, delta values were within 0.25% of results obtained using conventional techniques and standard deviations were better than 0.35%. Pooled data for measurements of all amino acids produced an accuracy and precision of 0.04 and 0.23%, respectively, when 2 nmol of each amino acid was injected on column and 20% of the stream of combustion products was delivered to the mass spectrometer.

Amino Acids↗
PubMed2005

Removal of nitrogen in different wetland filter materials: use of stable nitrogen isotopes to determine factors controlling denitrification and DNRA.

Laboratory incubations with varying O2 and NO3 concentrations were performed with a range of filter materials used in constructed wetlands (CWs). The study included material sampled from functioning CWs as well as raw materials subjected to laboratory pre-incubation. 15N-tracer techniques were used to assess the rates of denitrification versus dissimilatory nitrate reduction to ammonium (DNRA), and the relative role of nitrification versus denitrification in producing N2O. The N2O/(N2 + N2O) product ratio was assessed for the different materials. Sand, shell sand, and peat sustained high rates of denitrification. Raw light-weight aggregates (LWA) had a very low rate, while in LWA sampled from a functioning CW, the rate was similar to the one found in the other materials. The N2O/(N2 + N2O) ratio was very low for sand, shell sand and LWA from functioning CWs, but very high for raw LWA. The ratio was intermediate but variable for peat. The N2O produced by nitrification accounted for a significant percentage of the N2O accumulated during the incubation, but was dependent on the initial oxygen concentration. DNRA was significant only for shell sand taken from a functioning CW, suggesting that the establishment of active DNRA is a slower process than the establishment of a denitrifying flora.

Biodegradation, Environmental↗
PubMed2004

Nitrogen isotope ratios and fatty acid composition as indicators of animal diets in belowground systems.

This study analyses trophic interactions between soil fungi, micro- and mesofauna in microcosm experiments. The trophic shift of 15N and fatty acids (FAs) was investigated in different food chains, which comprised either two (fungi and grazers) or three (fungi, nematodes and Collembola) levels. Contrary to the widely accepted assumption of 15N enrichment in trophic cascades the experiments revealed enrichment, depletion or no change in 15N of consumers compared to their diet. Factors responsible for this pattern were suggested to be: (1) the main metabolic pathway used for N excretion in ammonotelic nematodes to be similar or depleted in the heavier isotope, and uricotelic Collembola mostly enriched in the heavier isotope; (2) a higher shift in 15N with a high-protein diet (e.g. for predators); (3) compensation due to low-quality food altering the fractionation of 15N. Analysis of the lipid composition showed phospholipids to be generally unaffected and neutral lipids closely related to the FA pattern of the food source. Dietary routing of FAs into neutral lipids occurred, as evidenced by corresponding frequencies of FAs in host and consumer profiles. Additionally, several FAs were only detected in the grazer when present in the food source. Oleic acid showed a shift over three trophic levels, from fungi to nematodes to Collembola. The assimilation of dietary FAs resulted in a more diverse neutral lipid profile, i.e. animals higher in the food chain contained more individual FAs compared to animals lower in the food chain. The results indicate that monoenoic C18 and monoenoic C20 FAs have the potential to act as tools for the bioindication of feeding strategies in belowground systems. We suggest that primary consumers will have no or only trace amounts of monoenoic C20 acids in their neutral lipid profile, whereas consumers feeding on a eukaryote diet will show a considerably higher frequency.

Analysis of Variance↗
PubMedDate not supplied

Perturbation and interpretation of nitrogen isotope distribution patterns in proteomics.

This study provides a discussion on the applications and limitations of (15)NH(4)(+) metabolic labeling in proteomic studies. The hyperthemophilic crenarchaeon Sulfolobus solfataricus was used as a model organism throughout this study. The distribution of nitrogen was studied in four different experiments in which this distribution was manipulated in a unique way. The experiments included full adaptation to media with relative isotope abundances (RIA) of 0.36%, 50%, and >98% (15)NH(4)(+). The incorporation efficiency was calculated on the basis of a comparison between theoretical and experimental spectra. In the case of full adaptation, incorporation efficiencies reflected the RIA (0.36%, 47.5% and 99% respectively). Labeling efficiencies were calculated on the basis of peak areas in TOF-MS spectra. It is shown that in the case of full adaptation, labeling efficiencies are 100%. In addition, we demonstrate that (15)NH(4)(+) labeling can be used in protein turnover studies, even when labeling is incomplete. In this case, incorporation efficiencies of 88-93% (lower than the RIA) were measured, providing evidence for amino acid recycling. Labeling efficiencies were always between 63% and 94% providing evidence for protein degradation. Finally, it was shown that isotope distributions can be useful in peptide identification.

Ammonia↗
PubMed1999

Mass spectrometry of the intramolecular nitrogen isotope distribution of environmental nitrous oxide using fragment-ion analysis

Mass spectrometry is applied to measure the intramolecular distribution of (15)N in N(2)O samples of near natural isotopic composition. The method is relatively straightforward and based on the analysis of the (14)NO and (15)NO fragment ion beams at mass 30 and 31, respectively, in combination with the standard analysis of the masses 44, 45, and 46 of the non-fragmented N(2)O. Various complications in the application, not all of which are resolved at present, are discussed. Copyright 1999 John Wiley & Sons, Ltd.

Journal Article↗
PubMed1983

Effect of rhizobial strain and host plant on nitrogen isotopic fractionation in legumes.

Lotus pedunculatus L., Medicago sativa L., Macroptilium atropurpureum, Glycine max, and Trifolium repens L. were grown in a N-free medium and inoculated with one of ten Rhizobium strains. Dry matter, N content, and delta(15)N values were determined for various plant parts.Nodules, with the exception of those from lucerne, were enriched in (15)N relative to atmospheric N. Considerable variation was found in delta(15)N values of plant herbage (-4.5 to +0.8). The extent of isotopic discrimination was dependent on both the host plant and the infecting rhizobial strain. This further complicates, but does not invalidate, the use of small variations in the natural abundance of (15)N to estimate the contribution of symbiotically fixed N(2) to the N in legume herbage. Some other implications of the observed differences are also discussed.

Journal Article↗