Compound-specific δ15N composition of free amino acids in moss as indicators of atmospheric nitrogen sources

Kinetic nitrogen isotope effects of 18 amino acids degradation during burning processes

Understanding the nitrogen isotopic variations of individual amino acids (AAs) is essential for utilizing the nitrogen isotope values of individual amino acids (δ15N-AA) as source indicators to identify proteinaceous matter originating from biomass combustion processes. However, the nitrogen isotope effects (ε) associated with the degradation of individual amino acids during combustion processes have not been previously explored. In this study, we measured the nitrogen isotope values of residual free amino acids -following a series of controlled combustion experiments at temperatures of 160-240 °C and durations of 2 min to 8 h, as described in Part 1. δ15N values of proline, aspartate, alanine, valine, glycine, leucine, and isoleucine are more positive than their initial δ15N values after prolonged combustion. Variations in δ15N values of the most AAs conform to the Rayleigh fractionation during combustion and their nitrogen isotope effects (ε) are greatly impacted by their respective combustion degradation pathways. This is the first time the ε values associated with the degradation pathways of AAs during combustion have been characterized. Only the ε values associated with Pathway 1 (dehydration to form dipeptide) and 2 (simultaneous deamination and decarboxylation) are found to be significant and temperature-dependent, ranging from + 2.9 to 6.4‰ and + 0.9‰ to + 3.8‰, respectively. Conversely, ε values associated with other pathways are minor. This improves the current understanding on the degradation mechanisms of protein nitrogen during biomass burning.

Foliar stable isotope ratios of carbon and nitrogen in boreal forest plants exposed to long-term pollution from the nickel-copper smelter at Monchegorsk, Russia

Long-term exposure to primary air pollutants, such as sulphur dioxide (SO2) and nitrogen oxides (NOx), alters the structure and functions of forest ecosystems. Many biochemical and biogeochemical processes discriminate against the heavier isotopes in a mixture; thus, the values of δ13C and δ15N (i.e. the ratio of stable isotopes 13C to 12C and that of 15 N to 14 N, respectively) may give insights into changes in ecosystem processes and identify the immediate drivers of these changes. We studied sources of variation in the δ13C and δ15N values in the foliage of eight boreal forest C3 plants at 10 sites located at the distance of 1-40 km from the Monchegorsk nickel-copper smelter in Russia. From 1939‒2019, this smelter emitted over 14,000,000 metric tons (t) of SO2, 250,000 t of metals, primarily nickel and copper, and 140,000 t of NOx. The δ13C value in evergreen plants and the δ15N value in all plants increased near the smelter independently of the plant mycorrhizal type. We attribute the pollution-related increase in the foliar δ13C values of evergreen species mainly to direct effects of SO2 on stomatal conductance, in combination with pollution-related water stress, which jointly override the potential opposite effect of increasing ambient CO2 concentration on δ13C values. Stomatal uptake of NOx and root uptake of 15N-enriched organic N compounds and NH4+ may explain the increased foliar δ15N values and elevated foliar N concentrations, especially in the evergreen trees (Pinus sylvestris), close to Monchegorsk, where the soil inorganic N supply is reduced due to the impact of long-term SO2 and heavy metal emissions on plant biomass. We conclude that, despite the uncertainties in interpreting δ13C and δ15N responses to pollution, the Monchegorsk smelter has imposed and still imposes a great impact on C and N cycling in the surrounding N-limited subarctic forest ecosystems.

Compound-Specific 14N/15N Analysis of Amino Acid Trimethylsilylated Derivatives from Plant Seed Proteins

Isotopic analyses of plant samples are now of considerable importance for food certification and plant physiology. In fact, the natural nitrogen isotope composition (δ¹⁵N) is extremely useful to examine metabolic pathways of N nutrition involving isotope fractionations. However, δ¹⁵N analysis of amino acids is not straightforward and involves specific derivatization procedures to yield volatile derivatives that can be analysed by gas chromatography coupled to isotope ratio mass spectrometry (GC-C-IRMS). Derivatizations other than trimethylsilylation are commonly used since they are believed to be more reliable and accurate. Their major drawback is that they are not associated with metabolite databases allowing identification of derivatives and by-products. Here, we revisit the potential of trimethylsilylated derivatives via concurrent analysis of δ¹⁵N and exact mass GC-MS of plant seed protein samples, allowing facile identification of derivatives using a database used for metabolomics. When multiple silylated derivatives of several amino acids are accounted for, there is a good agreement between theoretical and observed N mole fractions, and δ¹⁵N values are satisfactory, with little fractionation during derivatization. Overall, this technique may be suitable for compound-specific δ¹⁵N analysis, with pros and cons.

Methylmercury biomagnification in aquatic food webs of Poyang Lake, China: Insights from amino acid signatures

As the dominant mercury species in fish, methylmercury (MeHg) biomagnifies during its trophic transfer through aquatic food webs. MeHg is known to bind to cysteine, forming the complex of MeHg-cysteine. However, relationship between MeHg and cysteine in large-scale food webs has not been explored and contrasted with MeHg biomagnification models. Here, we quantified the compound-specific nitrogen isotopic analysis of amino acids (CSIA-AA), MeHg, and amino acid composition in aquatic organisms of Poyang Lake, the largest freshwater lake in China. The trophic positions (TP_AA) of organisms ranged from 1.0 ± 0.1-3.7 ± 0.2 based on CSIA-AA approach. The trophic magnification factor (TMF) of MeHg, derived from the regression slope of Log-transformed MeHg in organisms upon their TP_AA for the entire food web was 9.5 ± 0.5. Significantly positive regression between MeHg and cysteine (R² = 0.64, p < 0.01) was documented, suggesting MeHg-cysteine complex may potentially play a critical role in the bioaccumulation of MeHg. Furthermore, TMFs of MeHg calculated with and without cysteine normalization compared well (7.7-8.7) when excluding primary producers. Our results implied that MeHg may biomagnify as the complex of MeHg-cysteine and contribute to our understanding of MeHg trophic transfer at the molecular level.