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Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal
Author(s) -
Whiteman John P.,
Rodriguez Curras Mauriel,
Feeser Kelli L.,
Newsome Seth D.
Publication year - 2021
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.9073
Subject(s) - trophic level , chemistry , isotopes of nitrogen , isotope , omnivore , amino acid , biochemistry , stable isotope ratio , δ15n , food science , nitrogen , δ13c , ecology , biology , organic chemistry , physics , quantum mechanics , predation
Rationale Ecologists increasingly determine the δ 15 N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ 15 N values in consumers. Thus, trophic‐source δ 15 N offsets (i.e., Δ 15 N T‐S ) reflect trophic position in a food web. However, even minor variations in δ 15 N source AA values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDF T‐S ). Diet digestibility and protein content can influence the δ 15 N values of bulk animal tissue, but the effects of these factors on AA Δ 15 N T‐S and TDF T‐S in mammals are unknown. Methods We fed captive mice ( Mus musculus ) either (A) a low‐fat, high‐fiber diet with low, intermediate, or high protein; or (B) a high‐fat, low‐fiber diet with low or intermediate protein. Mouse muscle and dietary protein were analyzed for bulk tissue δ 15 N using elemental analyzer‐isotope ratio mass spectrometry (EA‐IRMS), and were also hydrolyzed into free AA that were analyzed for δ 15 N using gas chromatography‐combustion‐IRMS. Results As dietary protein increased, Δ 15 N Consumer‐Diet slightly 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 Δ 15 N T‐S and on TDF T‐S varied by AA but were consistent between variables. Conclusions 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 Δ 15 N Consumer‐Diet values. However, the similar responses of Δ 15 N T‐S and of TDF T‐S to diet variation suggest that if diet samples are available, Δ 15 N T‐S accurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ 15 N T‐S values. 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).