Intra‐trophic isotopic discrimination of 15N/14N for amino acids in autotrophs: Implications for nitrogen dynamics in ecological studies. Issue 9 (22nd March 2017)

• Record Type: Journal Article
• Authors: Takizawa, Yuko; Dharampal, Prarthana S.; Steffan, Shawn A.; Takano, Yoshinori; Ohkouchi, Naohiko; Chikaraishi, Yoshito
• Is Part Of: Ecology and evolution. Volume 7:Issue 9(2017:May)
• Page Start: 2916
• Page End: 2924
• Publication Date: 2017-03-22
• Subjects: food web -- isotopic fractionation -- plant phenology -- trophic position -- winter dormancy -- δ15N; Ecology -- Periodicals; Evolution -- Periodicals; 577.05
• DOI: 10.1002/ece3.2866
• Languages: English
• ISSNs: 2045-7758

Abstract

Abstract: The differential discrimination of nitrogen isotopes ( 15 N/ 14 N) within amino acids in consumers and their diets has been routinely used to estimate organismal tropic position (TP). Analogous isotopic discrimination can occur within plants, particularly in organs lacking chloroplasts. Such discrimination likely arises from the catabolic deamination of amino acids, resulting in a numerical elevation of estimated TP, within newly synthesized biomass. To investigate this phenomenon, we examined the 15 N/ 14 N of amino acids (δ 15 NAA ) in spring leaves and flowers from eight deciduous and two annual plants. These plants were classified on the basis of their time of bloom, plants that bloomed when their leaves were absent (Type I) versus plants that bloomed while leaves were already present (Type II). Based on the δ 15 NAA values from leaves, both plant types occupied comparable and ecologically realistic mean TPs (=1.0 ± 0.1, mean ± 1σ). However, the estimated TPs of flowers varied significantly (Type I: 2.2 ± 0.2; Type II: 1.0 ± 0.1). We hypothesize that these results can be interpreted by the following sequence of events: (1) Type I floral biomass is synthesized in absence of active photosynthesis; (2) the catabolic deamination of amino acids in particular, leaves behind 15 N in the residual pool of amino acids; and (3) the incorporation of these 15 N‐enriched amino acids within the biomass of Type I flowers results in the numerical elevation of the TPs. In contrast, the actively photosynthesizing Type II leaves energetically sustain the synthesis of Type II flower biomass, precluding any reliance on catabolic deamination of amino acids. Amino acids within Type II flowers are therefore isotopically comparable to the Type II leaves. These findings demonstrate the idiosyncratic nature of the δ 15 NAA values within autotrophic organs and have implications for interpreting trophic hierarchies using primary producers and their consumers. Abstract : Ecologists have exploited "intertrophic" discrimination in 15N/14N of amino acids between a consumer and its diet, to estimate the trophic positions of heterotrophic animals. However, "intratrophic" isotopic discrimination within a single autotroph is found in plant floral biomass that bloomed when their leaves were absent, leading to an elevation of the estimated trophic position of the plants.