Assessing the potential of amino acid delta C-13 and delta N-15 analysis in terrestrial and freshwater ecosystems

1. Understanding the structure and dynamics of food webs requires accurate estimates of energy flow among organisms. Bulk tissue carbon (delta C-13) and nitrogen (delta N-15) isotope analysis is often used to this end; however, the limitations of this technique can outweigh the benefits. The isotope analysis of individual amino acids is being increasingly employed to trace energy flow and estimate consumer trophic level. Central to this compound-specific approach are the concepts of essential amino acid (AA(ESS)) delta C-13 fingerprinting and amino acid (AA) delta N-15 beta-values, both of which have been understudied and are poorly constrained in terrestrial and freshwater producers. 2. We present AA(ESS) delta C-13 data for 112 terrestrial and freshwater producers collected from two aridland habitats in the northern Chihuahuan Desert (New Mexico, USA) and AA delta N-15 data for a subset (n = 28) of these samples. We characterized AA(ESS) delta C-13 fingerprints by performing linear discriminant analysis on the delta C-13 values of isoleucine, leucine, lysine, phenylalanine, threonine and valine for four producer groups-C-3 plants, C-4 plants, CAM plants and filamentous green algae. We explored potential biochemical mechanisms underlying these AA(ESS) delta C-13 fingerprints by calculating differences between the delta C-13 values of AA(ESS) products and their AA precursors. This allowed us to estimate and compare isotopic discrimination for specific AA(ESS) synthesis pathways across producer groups. 3. We found a near-perfect separation of AA(ESS) delta C-13 fingerprints among producer groups; all groups reclassified with >95% success within our multivariate framework. We also found varied isotopic discrimination for specific AA(ESS) synthesis pathways among producer groups. Contrary to previous studies, we found no differences in beta-values between terrestrial C-3 and C-4 plants for any trophic-source AA pairing. Furthermore, we found that Lys delta N-15 values were less variable and more closely related to bulk tissue delta N-15 values than Phe delta N-15 values in terrestrial and freshwater producers. 4. Synthesis. We conclude that AA(ESS) delta C-13 fingerprints are a higher resolution tracer for freshwater food webs, where instream algae have overlapping bulk tissue delta C-13 values with terrestrial C-3 plants. Additionally, beta(Glx-Lys) and beta(Pro-Lys) are the best for AA delta N-15-based consumer trophic-level estimates in freshwater food webs containing both terrestrial and aquatic resources.