Tissue-Specific Carbon Isotope Patterns of Amino Acids in Southern Sea Otters

Abstract The measurement of stable isotope values of individual compounds, such as amino acids (AAs), has quickly become a powerful tool in animal ecology and ecophysiology. As with any emerging technique, fundamental questions remain regarding the capability and limitations of this approach, including how metabolism and tissue synthesis impact AA isotope values and patterns. We measured carbon isotope (d13C) values of essential (AAESS) and nonessential (AANESS) AAs in bone collagen, whisker, muscle, and liver from ten southern sea otters (Enhydra lutris nereis) stranded in Monterey Bay, California. Sea otters in this population exhibit high degrees of individual dietary specialization, making this an excellent opportunity to explore differences in AA d13C values among tissues in a wild population. We found the d13C values of glutamic acid, proline, serine, lysine, and threonine differed significantly among tissues, indicating possible isotopic discrimination during tissue synthesis. Threonine d13C values were higher in liver relative to bone collagen and muscle, which may indicate catabolism of threonine for gluconeogenesis, an interpretation further supported by statistically significant correlations between the d13C values of threonine and its gluconeogenic products glycine and serine. For 6/10 of the sea otters analyzed, tissues from the same individual sea otter classified with different primary producers in multivariate space. We thus recommend caution when using data from multiple tissues in an AAESSd13C fingerprinting framework. Broadly, our results indicate that metabolism influences both AAESS and AANESSd13C values and highlight the importance of gluconeogenesis in driving compound-specific isotope patterns in this endangered apex marine consumer.