A new insight into isotopic fractionation associated with decarboxylation in organisms: implications for amino acid isotope approaches in biogeoscience

Yuko Takizawa,Prarthana S Dharampal,Naohiko Ohkouchi,Shawn A Steffan,Yoshito Chikaraishi,Yoshinori Takano,Bohyung Choi,Nanako O Ogawa

doi:10.1186/s40645-020-00364-w

Abstract

Stable nitrogen (15N/14N) and carbon (13C/12C) isotopic compositions of amino acids in organisms have widely been employed as a powerful tool to evaluate resource utilization and trophic connection among organisms in diverse ecosystems. However, little is known about the physiological factors or mechanisms responsible for determining the isotopic discrimination (particularly for carbon) within amino acids of organisms. In the present study, we investigated the inter-trophic discrimination of nitrogen and carbon isotopes within amino acids (Δδ15NAA and Δδ13CAA, respectively) using four consumer–diet pairs. Each pairing illustrates a metabolic perspective of isotopic fractionation of amino acids. The Δδ15NAA values in these combinations reveal a trend consistent with those observed in many other combinations in previous studies. This further validates a standard scenario: the deamination preferentially removes 14N amino group from diet-derived amino acids, leaving behind the 15N-enriched amino acids in consumer biomass. The Δδ15NAA values thus mirror the activity of amino acid deamination in consumers. In contrast, the trends in the Δδ13CAA value suggest a different metabolic fate for the amino acid carbon isotope. Based on our results, we predict the following scenario: decarboxylation preferentially removes 12C α-carbon (i.e., carbonyl-carbon) from pyruvic acid in glycolysis, and from α-ketoglutaric acid in the tricarboxylic acid cycle, leaving behind the 13C-enriched both pyruvic and α-ketoglutaric acids. The 13C is then transferred to amino acids that are synthesized from the 13C-enriched precursor molecules within consumers. The Δδ13CAA values therefore mirror the pathways of de novo amino acid synthesis in consumers. The proposed link between nitrogen and carbon isotopes can refine our knowledge of the potential processes affecting the isotopic fractionation within diet and consumer compartments, as well as environmental samples.Graphical abstract

Highlights

During the past four decades, the analysis of stable isotope ratios (e.g., 15N/14N, 13C/12C) has been used widely in the study of biogeosciences (e.g., Fry 2006)
We determined the Δδ15NAA and Δδ13CAA values together with the concentration and molar balance of amino acids in four pairs of consumer– diet invertebrates: (1) sea slug–sponge, (2) ladybug beetle–aphid, (3) green lacewing–fall armyworm, and (4) green lacewing–green lacewing. These consumer–diet pairings allow for empirical measurement of inter-trophic discrimination factor for the nitrogen and carbon isotopes (i.e., Δδ15NAA and Δδ13CAA, respectively) in a single trophic transfer, and in the process provides a mechanistic model of the isotopic fractionation processes affecting the inter-TDF of amino acids
For both nitrogen and carbon, the combination of sea slug–sponge and ladybug beetle–aphid was analyzed by triplicate injections for each sample, and the combination of green lacewing–fall armyworm and green lacewing–green lacewing was analyzed by a single injection for each of four trophic groups

Summary

Introduction

During the past four decades, the analysis of stable isotope ratios (e.g., 15N/14N, 13C/12C) has been used widely in the study of biogeosciences (e.g., Fry 2006). The degree of deamination activity, results in a great diversity in the isotopic discrimination–large values for some amino acids (e.g., alanine, valine, isoleucine, proline, and glutamic acid) and small values for some other amino acids (e.g., methionine and phenylalanine) between consumer and diet-resource species, as shown in 15N-enrichment factors (Chikaraishi et al 2009) These two groups of amino acids are frequently called “trophic” and “source” amino acids, respectively (e.g., Popp et al 2007; reviewed in Ohkouchi et al 2017). These consumer–diet pairings allow for empirical measurement of inter-trophic discrimination factor (inter-TDF) for the nitrogen and carbon isotopes (i.e., Δδ15NAA and Δδ13CAA, respectively) in a single trophic transfer, and in the process provides a mechanistic model of the isotopic fractionation processes affecting the inter-TDF of amino acids

Materials and Methods

Analysis of nitrogen and carbon isotopic compositions in amino acids

Results

Discussion