Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TPCSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TPCSIAestimation is an understanding of beta ( This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources of We show that variation in Our results highlight that primary producer
Quantifying the fate of organic nitrogen in aquatic systems is important to improve understanding of its recycling efficiency and long‐term preservation. The fate of organic nitrogen can be investigated with15N labeling techniques, but relative amounts of15N in different chemical forms are difficult to quantify. We present a “streamlined” method by combining Ammonium Retention Time Shift‐High Performance Liquid Chromatography with zinc reduction, and UV oxidation. This method does not require a pre‐isolation step of different forms of nitrogen from the sample. At a sample volume of 50 mL, and a total N concentration in the range of 0.5–40
- NSF-PAR ID:
- 10458294
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography: Methods
- Volume:
- 18
- Issue:
- 2
- ISSN:
- 1541-5856
- Page Range / eLocation ID:
- p. 52-62
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract β ) values—the differences between trophic and source AA δ15N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue‐specificβ value variability than typically appreciated.β value variability and its contribution to TPCSIAuncertainty. We first synthesized all published primary producer AA δ15N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ15N andβ value variability. Lastly, we evaluated the sensitivity of TPCSIAestimates to uncertainty in meanβ Glx‐Phevalues and Glx‐Phe trophic discrimination factors (TDFGlx‐Phe).β Glx‐Phevalues is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non‐vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed toβ Glx‐Phevalue variability, but we found no clear distinction among C3, C4and CAM plantβ Glx‐Phevalues. Notably, we found that vascular plantβ Glx‐Lysvalues (+2.5 ± 1.6‰) are considerably less variable thanβ Glx‐Phevalues, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi‐trophic level sensitivity analyses demonstrate that TPCSIAestimates are highly sensitive to changes in bothβ Glx‐Pheand TDFGlx‐Phevalues but that the relative influence ofβ values dissipates at higher trophic levels.β values are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting forβ value variability to improve TPCSIAestimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ15N values to better understand the mechanistic drivers ofβ value variation. -
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Rationale Nitrogen isotopic compositions (δ15N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo‐sciences. The current analytical technique using gas chromatography‐combustion‐isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high‐performance liquid chromatography‐elemental analyzer‐IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano‐EA/IRMS instrumentations are not widely available.
Methods We present a method for high‐precision δ15N measurements of AAs (δ15N‐AA) optimized for canonical source AA‐phenylalanine (Phe) and trophic AA‐glutamic acid (Glu). This offline approach entails purification and separation via high‐pressure ion‐exchange chromatography (IC) with automated fraction collection, the sequential chemical conversion of AA to nitrite and then to nitrous oxide (N2O), and the final determination of δ15N of the produced N2O via purge‐and‐trap continuous‐flow isotope ratio mass spectrometry (PT/CF/IRMS).
Results The cross‐plots of δ15N of Glu and Phe standards (four different natural‐abundance levels) generated by this method and their accepted values have a linear regression slope of 1 and small intercepts demonstrating high accuracy. The precisions were 0.36‰–0.67‰ for Phe standards and 0.27‰–0.35‰ for Glu standards. Our method and the GC/C/IRMS approach produced equivalent δ15N values for two lab standards (McCarthy Lab AA mixture and cyanobacteria) within error. We further tested our method on a wide range of natural sample matrices and obtained reasonable results.
Conclusions Our method provides a reliable alternative to the current methods for δ15N‐AA measurement as IC or HPLC‐based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N2O can be easily implemented in laboratories currently analyzing δ15N of N2O using PT/CF/IRMS. This method will help promote the use of δ15N‐AA in important studies of N cycling and trophic ecology in a wide range of research areas.
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