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

Abstract

Amino acid nitrogen isotopic analysis is a relatively new method for estimating trophic position. It uses the isotopic difference between an individual's 'trophic' and 'source' amino acids to determine its trophic position. So far, there is no accepted explanation for the mechanism by which the isotopic signals in 'trophic' and 'source' amino acids arise. Yet without a metabolic understanding, the utility of nitrogen isotopic analyses as a method for probing trophic relations, at either bulk tissue or amino acid level, is limited. I draw on isotopic tracer studies of protein metabolism, together with a consideration of amino acid metabolic pathways, to suggest that the 'trophic'/'source' groupings have a fundamental metabolic origin, to do with the cycling of amino-nitrogen between amino acids. 'Trophic' amino acids are those whose amino-nitrogens are interchangeable, part of a metabolic amino-nitrogen pool, and 'source' amino acids are those whose amino-nitrogens are not interchangeable with the metabolic pool. Nitrogen isotopic values of 'trophic' amino acids will reflect an averaged isotopic signal of all such dietary amino acids, offset by the integrated effect of isotopic fractionation from nitrogen cycling, and modulated by metabolic and physiological effects. Isotopic values of 'source' amino acids will be more closely linked to those of equivalent dietary amino acids, but also modulated by metabolism and physiology. The complexity of nitrogen cycling suggests that a single identifiable value for 'trophic discrimination factors' is unlikely to exist. Greater consideration of physiology and metabolism should help in better understanding observed patterns in nitrogen isotopic values.

Keywords: Food web; Isotope; Metabolism; Nitrogen isotopic analysis; Trophic discrimination factor; Trophic enrichment factor.

Fig. 1

The nitrogen isotopic difference between dietary and body amino acids in controlled feeding studies and well-constrained field collections. Data from McMahon and McCarthy (2016) are shown as boxplots for each amino acid, plotted by ecosystem type (marine, freshwater, terrestrial), where the shaded box represents the inter-quartile range (IQR), the whiskers represent the values within 1.5× IQR of the upper or lower quartiles, and outlier values beyond the end of the whiskers are plotted as points. ‘Trophic’ amino acids are marked with a hash, ‘source’ amino acids are marked with a dagger, and the ‘metabolic’ amino acid is marked with a double dagger. Glx glutamate, Asp aspartic acid, Ala alanine, Ile isoleucine, Leu leucine, Val valine, Pro proline, Gly glycine, Ser serine, Phe phenylalanine, Lys lysine, Met methionine, Thr threonine

Fig. 2

Schematic representation of the metabolic fate of nitrogen in ‘trophic’ and ‘source’ amino acids in mammals. Routes of nitrogen movement between ‘trophic’ and ‘source’ amino acids and other key nitrogen species are shown, with exchange represented by ⇌, and unidirectional movement (e.g. irreversible transamination or deamination) by →. Those amino acids whose nitrogen can be readily exchanged in a single chemical step, the ‘trophic’ amino acids (marked with a hash), are connected to other ‘trophic’ amino acids by ⇌, and together with free ammonia, they form the metabolic amino-nitrogen pool, outlined in the shaded box. ‘Source’ amino acids (marked with a dagger), and the ‘metabolic’ amino acid (marked with a double dagger) can also donate nitrogen to other amino acids and to the metabolic pool, but cannot readily exchange nitrogen (except between glycine and serine): all such amino acids are outside the metabolic amino-nitrogen pool. Glu glutamic acid, Gln glutamine, Asp aspartic acid, Ala alanine, Ile isoleucine, Leu leucine, Val valine, Pro proline, Gly glycine, Ser serine, Phe phenylalanine, Tyr tyrosine, Lys lysine, Met methionine, Thr threonine, His histidine