アミノ酸の窒素同位体比を用いた水棲生物の栄養段階の解析(<特集>有機物・微生物・生態系の地球化学)

概要

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Nitrogen isotopic composition (δ^<15>N) of individual amino acids has recently been employed as a potential powerful method for estimating the trophic level of organisms in food webs. In metabolic processes, one group of amino acids has little change in their nitrogen isotopic composition (e.g., 0.4‰ for phenylalanine), although another group has a large isotopic fractionation (e.g., 8.0‰ for glutamic acid). This fractionation could be associated with the cleavage of carbon-nitrogen bond in the metabolic processes (e.g., transamination) of amino acids. Therefore, a comparison between δ^<15>N values of these two types of amino acids would provide the trophic level of organisms. In fact, we can estimate the trophic levels of aquatic organisms with a small error (1σ = 0.12), employing the equation: [Trophic level] = (δ^<15>N_<glutamic acid>-δ^<15>N_<phenylalanine>-3.4)/7.6+1. Thus, a key advantage of this method is that the trophic level can be obtained based on the δ^<15>N values of two amino acids from a single organism; consequently, unlike the bulk method, it is not necessary to characterize the δ^<15>N values of primary producers. Here, we review the principle of this amino acid method and its application to natural organisms in marine and freshwater environments.
2010-12-25