Ecological applications of compound-specific stable isotope analysis (CSIA) of amino acids (AAs) include 1) tracking carbon pathways in food webs using essential AA (AAESS) δ13C values, and 2) estimating consumer trophic position (TP) by comparing relative differences of 'trophic' and 'source' AA δ15N values. Despite the significance of these applications, few studies have examined AA-specific SI patterns among tissues with different AA compositions and metabolism/turnover rates, which could cause differential drawdown of body AA pools and impart tissue-specific isotopic fractionation. To address this knowledge gap, especially in the absence of controlled diet studies examining this issue in captive marine mammals, we used a paired-sample design to compare δ13C and δ15N values of 11 AAs in commonly sampled tissues (skin, muscle, and dentine) from wild beluga whales (Delphinapterus leucas). δ13C of two AAs, glutamic acid/glutamine (Glx, a non-essential AA) and, notably, threonine (an essential AA), differed between skin and muscle. Furthermore, δ15N of three AAs (alanine, glycine, and proline) differed significantly among the three tissues, with glycine δ15N differences of approximately 10 ‰ among tissues supporting recent findings it is unsuitable as a source AA. Significant δ15N differences in AAs such as proline, a trophic AA used as an alternative to Glx in TP estimation, highlight tissue selection as a potential source of error in ecological applications of CSIA-AA. Amino acids that differed among tissues play key roles in metabolic pathways (e.g., ketogenic and gluconeogenic AAs), pointing to potential physiological applications of CSIA-AA in studies of free-ranging animals. These findings underscore the complexity of isotopic dynamics within tissues and emphasize the need for a nuanced approach when applying CSIA-AA in ecological research.
Keywords: Delphinapterus leucas; Carbon; Compound-specific stable isotope analysis; Essential; Glutamic acid; Nitrogen; Phenylalanine; Trophic.
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