Nitrogen isotope fractionation of amino acids from a controlled study on the green turtle (Chelonia mydas): expanding beyond Glx/Phe for trophic position

Compound specific nitrogen isotope analysis of amino acids (AA CSIA) has been used to study the trophic ecology of many marine organisms because of its ability to characterize the delta N-15 value at the base of a food web while simultaneously providing trophic position information from a single tissue sample of a consumer. Although application of this method is becoming more widely used in trophic studies, critical assumptions such as amino acid designation and trophic discrimination factors remain unresolved for many taxa and tissue types. In this study, we used captive green turtles (Chelonia mydas) in a controlled feeding study to determine and evaluate individual amino acid isotope fractionation patterns and trophic discrimination factors (TDFs) for skin and plasma. Our data build on findings from previous studies that show that the traditional paradigm of using glutamic acid and phenylalanine as trophic and source amino acids, respectively, for trophic position estimation is consistent for green turtles; we also provide evidence to support the potential utility of other combinations of amino acids for trophic position calculations. Green turtle TDFs for glutamic acid and phenylalanine were 4.0 +/- 0.5 parts per thousand and 6.6 +/- 0.7 parts per thousand for skin and plasma, respectively. We found serine to have the largest Delta delta N-15 (turtle-diet discrimination) value of all amino acids in both skin and plasma, while lysine Delta delta N-15 values were consistent for use as a source amino acid in skin and plasma tissue. We evaluated our TDFs by estimating trophic position of four sea turtle species using previously published amino acid isotope data. These results indicate that for green turtles, the traditional use of glutamic acid and phenylalanine along with a combination of trophic amino acids paired with lysine can provide robust trophic position estimates. Moreover, we highlight the taxa and tissue-specific nature of amino acid nitrogen isotope fractionation patterns in green turtle tissues and underscore that well-approximated TDFs can refine trophic position estimates for sea turtles.