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

Abstract

Abstract. Compound-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 δ 15 N value at the base of the food web (δ 15 N baseline ). Together, the CSIA-AA approach provides an unprecedented ability to disentangle the influences of δ 15 N baseline 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 TDF Glu-Phe (Δ 15 N Glu − Δ 15 N Phe ), 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 (Δ 15 N C-D ) and resulting TDF Glu-Phe values. We found tremendous variability in TDF Glu-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 TDF Glu-Phe values than consumers feeding on low-quality diets. Similarly, urea/uric acid-producing consumers also exhibit significantly lower TDF Glu-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 TDF Glu-Phe variability. We provide an overview of the biochemical and physiological mechanisms underpinning AA Δ 15 N C-D to explain these patterns. There are several seemingly unique systems, including the remarkably consistent TDF Glu-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 TDF Glu-Phe values. This likely requires developing new models of trophic transfer dynamics for some applications, including multi-TDF Glu-Phe equations that directly incorporate variability in TDF Glu-Phe value.

‘Trophic’ and ‘source’ amino acids in trophic estimation: a likely metabolic explanation

“…The majority of studies have used the difference between two amino acids, glutamate and phenylalanine (Glu and Phe), as the two ‘canonical’ ‘trophic’ and ‘source’ amino acids, but some have suggested using a different combination, or a multiple amino acid approach (Popp et al 2007; Nielsen et al 2015; McMahon and McCarthy 2016). …”

Use of compound‐specific nitrogen isotope analysis of amino acids in trophic ecology: assumptions, applications, and implications

“…In contrast, values of TDF Glu higher than 8.0& are expected for consumers with a PE lower than 28% (i.e., lower production flux and/or higher assimilation flux) (McMahon et al 2015). According to published data on controlled feeding experiments that were summarized in McMahon and McCarthy (2016), the TDF Glu value ranged from 0.3 to 19.4& among various combinations of consumer and prey, which is equivalent with 0.5-95% in PE (Goto et al 2018), if the microbe-mediated effect is not considered. It should be noted that consumers (dobsonfly larvae) with no ingestion flux (i.e., starved) show a nearly normal TDF Glu value of 7.5 ± 0.5& (n = 5), suggesting that their metabolic energy is compensated by catabolizing storage lipids or carbohydrates (Ishikawa et al 2017b).…”

“…2) (Goto et al 2018). Values of TDF Glu lower than 8.0& are expected for consumers with a higher production flux (i.e., PE's numerator) (McMahon and McCarthy 2016) or consumers with a lower assimilation flux (i.e., PE's denominator) . In contrast, values of TDF Glu higher than 8.0& are expected for consumers with a PE lower than 28% (i.e., lower production flux and/or higher assimilation flux) (McMahon et al 2015).…”

“…Figure 4 summarizes an example of the flow diagram with the relevant references for interpreting the TP estimate using CSIA-AA. The methodology will be improved with further exploration of the universality/variability of some key components such as TDF and b values in different ecosystems (Nielsen et al 2015;McMahon and McCarthy 2016). For example, it has been suggested that cultivated leguminous plants have an unusual metabolic pathway of amino acid biosynthesis, which may result in a different b value from that of other autotrophs (Styring et al 2014).…”

“…Detailed comments on the analytical procedures and biochemical basis for both methods is beyond the scope of this paper, and is holistically overviewed in elsewhere (e.g., Wada et al 2013;Ohkouchi et al 2015Ohkouchi et al , 2017McMahon and McCarthy 2016). In this paper, only the three most critical features of both the methods in the context of food web science were highlighted, as follows.…”