Abstract
Stable nitrogen isotopic composition of amino acids (δ15NAA) has recently been employed as a powerful tool in ecological food web studies, particularly for estimating the trophic position (TP) of animal species in food webs. However, the validity of these estimates depends on the consistency of the trophic discrimination factor (TDF; - Δδ15NAA at each shift of trophic level) among a suite of amino acids within the tissues of consumer species. In this study, we determined the TDF values of amino acids in tadpoles (the Japanese toad, Bufo japonicus) reared exclusively on one of three diets that differed in nutritional quality. The diets were commercial fish-food pellets (plant and animal biomass), bloodworms (animal biomass), and boiled white rice (plant carbohydrate), representing a balanced, protein-rich, and protein-poor diet, respectively. The TDF values of two “source amino acids” (Src-AAs), methionine and phenylalanine, were close to zero (0.3–0.5‰) among the three diets, typifying the values reported in the literature (∼0.5‰ and ∼0.4‰, respectively). However, TDF values of “trophic amino acids” (Tr-AAs) including alanine, valine, leucine, isoleucine, and glutamic acid varied by diet: for example, the glutamic acid TDF was similar to the standard value (∼8.0‰) when tadpoles were fed either the commercial pellets (8.0‰) or bloodworms (7.9‰), but when they were fed boiled rice, the TDF was significantly reduced (0.6‰). These results suggest that a profound lack of dietary protein may alter the TDF values of glutamic acid (and other Tr-AAs and glycine) within consumer species, but not the two Src-AAs (i.e., methionine and phenylalanine). Knowledge of how a nutritionally poor diet can influence the TDF of Tr- and Src-AAs will allow amino acid isotopic analyses to better estimate TP among free-roaming animals.
Highlights
Stable nitrogen isotopic composition of amino acids (d15NAA) has recently been employed as a new method in ecological food web studies, for understanding trophic linkage and energy flow among animal species in complex ecosystems, as well as estimation of background isotopic signals within and among habitats (e.g., McCarthy et al 2007; Popp et al 2007; Chikaraishi et al 2009, 2014). This method has been constructed based on the contrasting isotopic discrimination (Dd15N) associated with “source amino acids” (Src-AAs; e.g., methionine and phenylalanine) and “trophic amino acids” (Tr-AAs; e.g., alanine, valine, leucine, isoleucine, proline, and glutamic acid) (Fig. 1)
Ecology and Evolution published by John Wiley & Sons Ltd
The bloodworms are characterized by heterogeneous amino acid composition as a large amount of glutamic acid, glycine, and aspartic acid (153, 58, and 100 mg gÀ1, respectively) but a small amount of valine, threonine, alanine, and methionine (17, 18, 22, and 25 mg gÀ1, respectively)
Summary
Stable nitrogen isotopic composition of amino acids (d15NAA) has recently been employed as a new method in ecological food web studies, for understanding trophic linkage and energy flow among animal species in complex ecosystems, as well as estimation of background isotopic signals within and among habitats (e.g., McCarthy et al 2007; Popp et al 2007; Chikaraishi et al 2009, 2014). We determined the effect of diet quality on the TDF value of amino acids, based on controlled feeding experiments involving larvae (tadpoles) of the Japanese toad, Bufo japonicus, reared on three distinct diets: commercial pellets, bloodworms, and boiled rice, all of which represented a range of protein and carbohydrate concentrations.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
