Olson et al. (2010) also compared δ^15 N and gut-content TLYFT results to those froma more recent δ^15 N technique: compound-specific isotope analysis (CSIA) of
constituent amino acids (McClelland & Montoya 2002; Popp et al. 2007; Hannides et
al. 2009). Stable nitrogen isotopes in different amino acids (AA) have different
enrichment responses to trophic transfers because they have strongly different
metabolic pathways. Some AAs (most strongly and consistently glutamic acid,
alanine, aspartamine) progressively accumulate ^15 N at each trophic transfer and are
termed “trophic AAs”. These are those most frequently deaminated and then
metabolized, with strong fractionation. Other “source AAs” (mostly consistently
glycine and phenylalanine) are incorporated from food without significant
fractionation, retaining the ^15 N signature of the base of the food chain. The distinction
does not parallel the difference between essential amino acids (synthesized only by
autotrophs and some bacteria, and thus a dietary requirement for metazoans) and
those synthesized by metazoans. It is definite that some AAs, including glycine and
phenylalanine, retain the δ^15 N of regional primary producers through multiple trophic
transfers. Glutamic acid accumulates δ^15 N at a recurring rate at each trophic transfer
∼7‰ per level (McClelland & Montoya 2002).
(^) The analysis for CSIA involves hydrolysis of protein, conversion of the amino acids
to heavier derivatives, gas-chromatographic separation of individual amino acid
derivatives, and finally mass spectrometry to determine the specific δ^15 N for each.
Using glutamic acid and glycine, the trophic level of tuna muscle was estimated from: