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Title: Diet quality influences isotopic discrimination among amino acids in an aquatic vertebrate

Author
item CHIKARAISHI, YOSHITO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)
item Steffan, Shawn
item TAKANO, YOSHINORI - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)
item OHKOUCHI, NAOHIKO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)

Submitted to: Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2015
Publication Date: 4/25/2015
Publication URL: https://handle.nal.usda.gov/10113/60987
Citation: Chikaraishi, Y., Steffan, S.A., Takano, Y., Ohkouchi, N. 2015. Diet quality influences isotopic discrimination among amino acids in an aquatic vertebrate. Ecology and Evolution. 5(10):2048-2059.

Interpretive Summary: Isotopic analysis helps to decode what animals feed on, which is important to better understand the roles they play within their respective ecosystems. Whether an insect, a crustacean, or an amphibian, isotopes convey very precise information about where in the food chain an organism feeds. In the current study, we tested the hypothesis that diet quality can influence how isotopes “behave” within the tissues of the consumer. We show that a diet profoundly lacking in protein can change the fractionation of isotopes within the tissues of a consumer. Specifically, amino acids became differentially enriched in 15N within the model consumer (an amphibian), suggesting that there was different metabolic activity and/or isotopic routing associated with the amino acids. Although the enrichment effectively skews trophic position measurement, it does effectively simulate periods of hibernation or starvation during the life of an animal, thus improves our interpretations of isotopic data. Impact Statement: Compound-specific stable isotopic analysis has revolutionized food web ecology, and the current paper provides evidence of how diet quality impacts trophic position measurement. Our findings shed light on how isotopic composition should be interpreted.

Technical Abstract: Stable nitrogen isotopic composition of amino acids 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, d15N-AA) among a suite of amino acids within the tissues of consumer species. In the present 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 (0.0-0.6‰) 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) varied by diet: the glutamic acid TDF was similar to the standard value (~8.0‰) when tadpoles were fed either the commercial pellets (8.0‰) or bloodworms (7.8‰), but when they were fed boiled rice, the TDF was significantly reduced (~1.0‰). These results suggest that a profound lack of dietary protein may alter the TDF values of glutamic acid (and other Tr-AAs) within consumer species, but not the Src-AAs. Knowledge of how a nutritionally poor diet (e.g., prolonged starvation, winter hibernation) can influence the TDF of Tr- and Src-AA will allow amino acid isotopic analyses to better estimate trophic position among free-roaming animals.