Food chain analysis by nitrogen isotopic composition of amino acids : Application to terrestrial environments

Abstract

Recent evidences have suggested that compound-specific stable isotope analysis (CSIA) of amino acids has been employed as a new powerful method with that enables the estimation of trophic level of organisms in not only aquatic but also terrestrial food webs. This CSIA approach is based on contrasting the ¹⁵N-enrichment with each trophic level between two common amino acids: glutamic acid shows significant enrichment of +8.0‰ with each trophic level, whereas phenylalanine shows little enrichment of +0.4‰. These ¹⁵N-enrichments are well consistent in both aquatic and terrestrial organisms. The trophic level of organisms can be estimated within a small error as 1σ=0.12 for aquatic and 0.17 for terrestrial food webs, employing the eq.: [Trophic level]=(δ¹⁵N_{glutamic acid}-δ¹⁵N_{phenylalanine}+β)/7.6+1, where β represents the isotopic difference between these two amino acids in primary producers (-3.4‰ for aquatic cyanobacteria and algae, +8.4‰ for terrestrial C3 higher plants, and -0.4‰ for terrestrial C4 higher plants). Here, we briefly review this new method (i.e. CSIA of amino acids) and its application to natural organisms in terrestrial environments.