Determination of aquatic food‐web structure based on compound‐specific nitrogen isotopic composition of amino acids

Abstract

The nitrogen isotopic composition (δ 15 N) of amino acids is potentially useful as an alternative method for estimating the trophic levels of organisms in food webs. However, because this "amino acid method" has been constructed from the observations of only a few case studies of food-consumer combinations in previous studies, the universality of the approach remains unclear. In this study, we investigated the δ 15 N signatures of amino acids in 17 photoautotrophs and the trophic relationships during four controlled feeding experiments using green algae, zooplankton, and fish. The results are consistent with those reported in previous studies, implying that the amino acid method can be applied to a variety of organisms. From these and previously published data, we estimate the two factors (β, isotope differences among amino acids in primary producers; Δ, the 15 N-enrichment factor for each trophic level) required to calculate the trophic level. Based on the lowest error (1σ = 0.12) in the estimated trophic level, we conclude that a comparison of the δ 15 N values for glutamic acid and phenylalanine is most useful in calculating precise estimates of the trophic level, using the following equation: trophic level (TL Glu/Phe ) = (δ 15 N Glu -δ 15 N Phe -3.4)/7.6 + 1.

Nutrient regime shift in the western North Atlantic indicated by compound-specific δ 15 N of deep-sea gorgonian corals

“…4). The Tr-AA are substantially enriched in δ 15 N versus the Src-AA, reflecting both autotrophic base metabolic patterns, as well as subsequent trophic transfers (4)(5)(6)(7)(8)23). We note that within the Src-AA, Thr appears to constitute an outlier, with very negative values.…”

15N/14N ratios of amino acids as a tool for studying terrestrial food webs: a case study of terrestrial insects (bees, wasps, and hornets)

“…Consequently, unlike bulk isotopic methods (i.e., determination of the trophic position from bulk nitrogen isotope analysis; e.g., Minagawa and Wada 1984), this method does not require characterization of the d 15 N values of primary producers or baseline consumers. Second, small accuracy is expected; the standard deviation (1r) of accuracy in the trophic position (=[actual TP] À [TP Glu/Phe ]) is only 0.12 units for aquatic organisms (Chikaraishi et al 2009) and 0.20 units for terrestrial organisms (Chikaraishi et al 2010a). …”

“…This approach is based on isotopic fractionations associated with the metabolic processes of two common amino acids: glutamic acid shows significant increase of d 15 N values during reactions (e.g., transamination and deamination) that cleaves the carbon-nitrogen bond, whereas phenylalanine shows little change in d 15 N values during conversion to tyrosine that neither forms nor cleaves the carbon-nitrogen bond (Chikaraishi et al 2007). Chikaraishi et al (2009Chikaraishi et al ( , 2010a established that the trophic position (TP Glu/Phe ) of organisms could be estimated from the nitrogen isotopic compositions of glutamic acid (d 15 N Glu ) and phenylalanine (d 15 N Phe ), as follows:…”

“…Compound-specific stable isotope analysis (CSIA) of amino acids is a new method that enables estimates of the trophic position of organisms in food webs (e.g., McClelland and Montoya 2002;Popp et al 2007;Chikaraishi et al 2009). It has been proposed that a comparison between large 15 N-enrichment (+8.0&) in glutamic acid and little change (+0.4&) in phenylalanine, with each increase in trophic level, provides a precise estimate of the trophic position of organisms in food webs (Fig.…”

Microbes are trophic analogs of animals

“…Using amino acid stable isotope fingerprinting (17)(18)(19)(20), we measure with high accuracy the trophic positions (TPs) of cultured and freeroaming organisms. In past work, this method has produced accurate TP estimates of zooplankton (18,21), fish (17), gastropods (17), amphibians (22), and insects (19,20), ranging from simple herbivores to higher-order carnivores.…”