Literature Review Methods

We performed a structured literature search for primary producer amino acid (AA) δ¹⁵N data in Scopus and Google Scholar using the search terms nitrogen isotope OR 15N AND amino acid with each of the terms plant, *plankton, algae, bacteria, and autotroph. We also used the search terms trophic, diet, and food web to identify all studies that estimated trophic position via compound-specific stable isotope analysis (TP_CSIA) or that estimated AA-specific trophic discrimination factors (TDFs). We only included studies that reported natural abundance stable isotope data. The literature search yielded 15 studies that reported beta values (β) for individual primary producers, 44 studies that reported TDFs or paired consumer-diet data within a trophic ecology context (e.g., controlled feeding study designed to characterize AA fractionation), and 176 studies that applied the TP_CSIA equation (Figure 2 from Ramirez et al. 2021). The literature search yielded an additional 36 studies that reported AA δ¹⁵N data for autotrophs from which β values could be calculated and 9 additional studies from which TDFs could be calculated. The unit of replication for this meta-analysis was species-specific tissue within study. Therefore, if a study had multiple β values for a single primary producer species, a simple mean and standard deviation were calculated to consolidate the reported data into one estimate per species per study. Tissue-specific data were maintained separately whenever reported. This process resulted in a final dataset that consisted of 236 β values across ≥ 132 different primary producer genera in freshwater, marine, and terrestrial ecosystems (Table 1, Figure 3 in Ramirez et al. 2021). Our meta-analysis focused primarily on β values derived from Glx and Phe (β_Glx-Phe) given that they are the most commonly measured trophic and source AAs and applied to estimate TP_CSIA. However, we present β values for all combinations of trophic (Asx, Ala, Ile, Leu, Pro, Val) and source (Phe, Lys, Met, Tyr) AAs in Table 2 and Figures S1 (Ramirez et al. 2021). We also calculated β values for the “metabolic” AA Thr relative to the source AAs given its unique isotope dynamics with trophic transfer (McMahon & McCarthy, 2016). Primary producer Met and Tyr δ¹⁵N data were not routinely collected nor reported, therefore inferences were limited for these AAs.

A list of the publications is found in the Publications section below, and also in Ramirez et al. (2021).