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.

We compiled published data on hydrogen isotope values for leaf wax n-alkanes (C 27 , C 29 ,

We investigated the nitrogen isotopic compositions of individual amino acids from brown and red macroalgae and gastropods in a natural marine coastal environment, to further evaluate them as a tool for ecological studies and to understand the factor(s) controlling the isotopic compositions in terms of biosynthetic and metabolic processes. The isotopic compositions of 12 amino acids range from -2.1 to + 8.4 ‰ for brown algae, from -3.3 to +12.9 ‰ for red algae, and from -0.6 to +16.6 ‰ for gastropods. The isotopic distributions between algae and gastropods and within algal classes are consistent with those in previous reports, suggesting them to be quite useful for studying food web structure. The nitrogen isotopic variation between amino acids strongly reflects their biosynthetic and metabolic processes. In particular, we suggest here that metabolic fate is an important factor in producing distinct trophic relationships in nitrogen isotopic compositions between amino acids. KEY WORDS: Nitrogen isotopic composition · Amino acids · Food web · Trophic level · Biosynthesis Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 342: [85][86][87][88][89][90] 2007 nation is generally applicable to other sets of producers and consumers. Particularly, few other data are available for primary producers, even though they provide essential information on the base of the food web. Moreover, the biochemical mechanism responsible for the distinct trophic relationship of δ 15 N value between amino acids (i.e. significant 15 N-enrichment in some amino acids vs. little change in others) is not well understood. Therefore, in the present study, we investigated the nitrogen isotopic compositions of individual amino acids in natural marine macroalgae and gastropods: (1) to ascertain the trophic relationship of the δ 15 N of amino acids in a marine coastal ecosystem and (2) to discuss controlling factor(s) of the isotopic signature of each amino acid from both biosynthetic and metabolic viewpoints. MATERIALS AND METHODSTwo brown (Heterokontophyta: Sargassum filicinum and Undaria pinnatifida) and 2 red (Rhodophyta: Binghamia californica and Gelidium japonica) macroalgae were collected from 2 to 4 m depth along the seacoast near Yokohama, Japan (35°08' N, 139°07' E) (Chikaraishi 2006). About 20 to 30 ind. for each alga were collected. This area is dominated by the brown algae and small colonies of the red algae, which are found in the vicinity of the brown algal colonies; other macroalgae are not common in this coastal area. We also collected 3 gastropods (Gastropoda: Batillus cornutus, Haliotis discus, and Omphalius pfeifferi) from the brown algal colonies. One individual from each gastropod species was collected. It has been shown that these gastropods specifically feed on brown macroalgae (e.g. Harada et al. 1984, Tutschulte & Connell 1988, Fallu 1991. This was also confirmed in this environment by a previous investigation into the carbon and hydrogen isotopic compositions of ster...

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Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well-understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher-order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free-roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.