Trophic biomarker or metabolic differences: An exploration in nitrogen and hydrogen isotopes
Hydrogen and nitrogen isotopes have been used to interpret trophic levels in a wide variety of environments and animals. The basis for these applications is empirical, and foundational understanding of the biochemical and physiological underpinnings of the isotopic differences in trophic systems is scant or lacking entirely. Although transamination of amino acids is thought to impart an enrichment in d15N biosynthate in vivo (e.g. Macko et al. 1986; Miura and Goto, 2012), the relative importance of enzymatic, anabolic and catabolic variations on isotopic fractionation at tissue sites such as the enterocyte of the intestinal lining, the microbiome of the colon or the liver are unknown. Even less is known about the systematics of hydrogen isotope fractionation in animal tissues, and the impact that the sum of these reactions may have on trophic level interpretations.
We present data from rodents and humans, as well as in vitro data from individual amino acids that address the issues of enzymatic fractionation in the small intestine, the impact of a microbiome consortia on both nitrogen and hydrogen isotopes, and the difficulty of ascribing a relative trophic level in omnivores, particularly humans.
Macko et al. 1986 Geochim. Cosmochim Acta 50, 2143-2146.
Miura K. and Goto A., 2012 Res. Org. Geochem. 28, 13-17.