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A new analytical method for determination of the nitrogen isotopic composition of methionine: Its application to aquatic ecosystems with mixed resources
Author(s) -
Ishikawa Naoto F.,
Chikaraishi Yoshito,
Takano Yoshinori,
Sasaki Yoko,
Takizawa Yuko,
Tsuchiya Masashi,
Tayasu Ichiro,
Nagata Toshi,
Ohkouchi Naohiko
Publication year - 2018
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.1002/lom3.10272
Subject(s) - trophic level , methionine , phenylalanine , food web , isotope analysis , aquatic ecosystem , algae , amino acid , chemistry , ecosystem , nitrogen , primary producers , food chain , biology , ecology , environmental chemistry , biochemistry , nutrient , organic chemistry , phytoplankton
Compound‐specific nitrogen isotope ( δ 15 N) analysis of amino acids is a powerful tool for estimating the trophic positions (TPs) of animals. The TP of an animal can be represented as a linear function of the isotopic difference between glutamic acid ( δ 15 N Glu ) and phenylalanine ( δ 15 N Phe ). However, the method using δ 15 N Glu and δ 15 N Phe cannot be applied to animals in mixed food webs where basal resources are derived from both terrestrial and aquatic primary producers, because the mean value of δ 15 N Phe relative to δ 15 N Glu differs greatly between terrestrial plants (+8.4‰) and aquatic algae (−3.4‰). To resolve this problem, the δ 15 N of methionine ( δ 15 N Met ) is useful. Because the C–N bond of methionine is not cleaved in its initial metabolic step, theoretically there should be little diversity in δ 15 N Met relative to δ 15 N Glu among primary producers and a small trophic discrimination factor for methionine in animal metabolism. We developed a dual‐column‐coupled GC‐C‐IRMS method to determine δ 15 N Met . Data collected from controlled feeding experiments and wild samples demonstrated that the isotopic difference between methionine and phenylalanine in terrestrial food webs (Δ Met−Phe = −16.5 ± 0.5‰) is clearly distinguishable from that in aquatic food webs (Δ Met−Phe = −5.0 ± 0.5‰). This approach allowed us to determine ecologically reasonable TP values for carnivores in a stream food web, which were substantially underestimated with the conventional method. This method has potential utility in assessing TP for animals that rely on varying proportions of both terrestrial‐ and aquatic‐derived resources, with no requirement to characterize δ 15 N in their basal resources.