Embracing variability in amino acid δ15N fractionation: mechanisms, implications, and applications for trophic ecology

Abstract

AbstractCompound‐specific stable isotope analysis (CSIA) of individual amino acids (AAs) has become a powerful analytical tool in trophic ecology. Heavily fractionating “trophic” AAs (e.g., glutamic acid: Glu) provide a robust indicator of trophic transfer, while minimally fractionating “source” AAs (e.g., phenylalanine: Phe) closely reflect the δ15N value at the base of the food web (δ15Nbaseline). Together, the CSIA‐AA approach provides an unprecedented ability to disentangle the influences of δ15Nbaseline values and trophic fractionation on consumer nitrogen isotope values. Perhaps the most important assumption underlying CSIA‐AA applications to trophic ecology is that trophic fractionation of Glu and Phe, and thus the trophic discrimination factor TDFGlu‐ Phe (Δ15NGlu − Δ15NPhe), is effectively constant across diverse consumer–resource relationships. To test this assumption, we conducted a comprehensive meta‐analysis of controlled feeding experiments that examined individual AA trophic fractionation (Δ15NC‐D) and resulting TDFGlu‐ Phe values. We found tremendous variability in TDFGlu‐ Phe values from 0‰ to >10‰ across 70 species (317 individuals) and 88 distinct consumer–diet combinations. However, this variability appears to follow predictable patterns driven by two dominant variables: diet quality and mode of nitrogen excretion. Consumers feeding on high‐quality diets (small diet–consumer AA imbalances) tend to have significantly lower TDFGlu‐ Phe values than consumers feeding on low‐quality diets. Similarly, urea/uric acid‐producing consumers also exhibit significantly lower TDFGlu‐ Phe values than their ammonia‐producing counterparts. While these patterns are certainly not universal, together these factors likely explain many of the observed patterns of TDFGlu‐ Phe variability. We provide an overview of the biochemical and physiological mechanisms underpinning AA Δ15NC‐D to explain these patterns. There are several seemingly unique systems, including the remarkably consistent TDFGlu‐ Phe values across insect food webs and the isotopically “invisible” trophic transfers in microbial food webs, that may provide additional insight into the influence of diet quality and nitrogen cycling on AA fractionation. In this review, we argue that to realize the full potential of CSIA‐AA approaches in trophic ecology, we must embrace the variability in TDFGlu‐ Phe values. This likely requires developing new models of trophic transfer dynamics for some applications, including multi‐TDFGlu‐ Phe equations that directly incorporate variability in TDFGlu‐ Phe value.