Supporting Data for: Assessing the potential of amino acid δ13C and δ15N analysis in terrestrial and freshwater ecosystems

<p>Understanding the structure and dynamics of food webs requires accurate estimates of energy flow among organisms. Bulk tissue carbon (<em>δ</em>¹³C) and nitrogen (<em>δ</em>¹⁵N) 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) <em>δ</em>¹³C fingerprinting and amino acid (AA) <em>δ</em>¹⁵N beta-values, both of which have been understudied and are poorly constrained in terrestrial and freshwater producers.</p> <p>We present AA_ESS <em>δ</em>¹³C data for 112 terrestrial and freshwater producers collected from two aridland habitats in the northern Chihuahuan Desert (New Mexico, USA) and AA <em>δ</em>¹⁵N data for a subset (n=28) of these samples. We characterized AA_ESS <em>δ</em>¹³C fingerprints by performing linear discriminant analysis on the <em>δ</em>¹³C values of isoleucine, leucine, lysine, phenylalanine, threonine, and valine for four producer groups – C₃ plants, C₄ plants, CAM plants, and filamentous green algae. We explored potential biochemical mechanisms underlying these AA_ESS <em>δ</em>¹³C fingerprints by calculating differences between the <em>δ</em>¹³C 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.</p> <p>We found near perfect separation of AA_ESS <em>δ</em>¹³C fingerprints among producer groups; all groups reclassified with &gt;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 C₃ and C₄ plants for any trophic-source AA pairing. Furthermore, we found that Lys <em>δ</em>¹⁵N values were less variable and more closely related to bulk tissue <em>δ</em>¹⁵N values than Phe <em>δ</em>¹⁵N values in terrestrial and freshwater producers.</p> <p><span><span><span><span><span>We conclude that AA_ESS <em>δ</em></span></span></span></span></span>¹³<span><span><span><span><span>C fingerprints are a higher-resolution tracer for freshwater food webs where instream algae have overlapping bulk tissue <em>δ</em></span></span></span></span></span>¹³<span><span><span><span><span>C values with terrestrial C₃ plants. Additionally, </span></span></span></span></span><span><span><span><span><span>beta</span></span></span></span></span>_Glx-Lys<span><span><span><span><span> and </span></span></span></span></span><span><span><span><span><span>beta</span></span></span></span></span>_Pro-Lys<span><span><span><span><span> are the best for AA <em>δ</em></span></span></span></span></span>¹⁵<span><span><span><span><span>N-based consumer trophic level estimates in freshwater food webs containing both terrestrial and aquatic resources.</span></span></span></span></span></p>