Compound‐specific stable isotope analysis (
Ecologists increasingly determine the δ15N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ15N values in consumers. Thus, trophic‐source δ15N offsets (i.e., Δ15NT‐S) reflect trophic position in a food web. However, even minor variations in δ15Nsource AAvalues may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDFT‐S). Diet digestibility and protein content can influence the δ15N values of bulk animal tissue, but the effects of these factors on AA Δ15NT‐Sand TDFT‐Sin mammals are unknown.
We fed captive mice (
As dietary protein increased, Δ15NConsumer‐Dietslightly declined for bulk muscle tissue in both experiments; increased for AA in the low‐fat, high‐fiber diet (A); and remained the same or decreased for AA in the high‐fat, low‐fiber diet (B). The effects of dietary protein on Δ15NT‐Sand on TDFT‐Svaried by AA but were consistent between variables.
Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ15NConsumer‐Dietvalues. However, the similar responses of Δ15NT‐Sand of TDFT‐Sto diet variation suggest that if diet samples are available, Δ15NT‐Saccurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ15NT‐Svalues. The trophic‐source offset of Pro‐Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu‐Phe).
- NSF-PAR ID:
- 10452550
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Rapid Communications in Mass Spectrometry
- Volume:
- 35
- Issue:
- 11
- ISSN:
- 0951-4198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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 (
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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.
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δ 13C andδ 15N and amino acidδ 15N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna ( ) and squid (Thunnus albacares ) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species,Dosidicus gigas andCalanus pacificus , which were preserved in formaldehyde for 24–25 years.Eucalanus californicus Results Tissues in formaldehyde‐ethanol had higher bulk
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