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

Authors: TAKIZAWA, YUKO - Hokkaido University; TAKANO, YOSHINORI - Japan Agency For Marine-Earth Science And Technology (JAMSTEC); CHOI, BOHYUNG - Hokkaido University; DHARAMPAL, PRARTHANA - University Of Wisconsin; Steffan, Shawn; OGAWA, NANAKO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC); OHKOUCHI, NAOHIKO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC); CHIKARAISHI, YOSHITO - Hokkaido University

Submitted to: Progress in Earth and Planetary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2020
Publication Date: 8/18/2020
Citation: Takizawa, Y., Takano, Y., Choi, B., Dharampal, P., Steffan, S.A., Ogawa, N., Ohkouchi, N., Chikaraishi, Y. 2020. A new insight into isotopic fractionation associated with de-carboxylation in organisms: implications for amino acid isotope approaches in biogeoscience. Progress in Earth and Planetary Science. 7. Article 50. https://doi.org/10.1186/s40645-020-00364-w.
DOI: https://doi.org/10.1186/s40645-020-00364-w

Interpretive Summary: The stable isotopes of nitrogen and carbon within specific amino acids are remarkably informative (and predictable) as tracers of biomass flow within food-webs. Because food-webs drive many of the goods and services that we derive from agroecosystems, understanding energy and biomass flow in a food-web provides a basis to characterize the roles of organisms within agroecosystems. Whether microbes or insects, it's critical to know what is eating what. Any line of inquiry requiring interpretation of stable isotopic signatures (agriculture, forensics, paleontology) will benefit from the content of this paper. For example, scientists or growers who would like to know if a given pollinator has visited the intended crop flowers will be able to discern where the insect has been/foraged.

Technical 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 the 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 trophic discrimination of nitrogen and carbon isotopes within amino acids ('d15NAA and 'd13CAA, respectively) usingin four pairs of consumer-diet species to illustrate a metabolic perspective for isotopic fractionation during degradation and synthesis of amino acids. The 'd15NAA values in these combinations reveal a trend con-sistent 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 'd15NAA values thus mirror the activity of amino acid deamination in consumers. In contrast, the trends in the 'd13CAA suggest a different metabolic fate for the amino acid carbon isotope. Based on our results we predict the following scenario: decarboxylation preferentially removes 12C a-carbon (i.e., carbonyl-carbon) from pyruvic acid in glycolysis, and from a-ketoglutaric acid in the tricarboxylic acid cycle, leaving behind the 13C-enriched both pyruvic and a-ketoglutaric acids. The 13C is then transferred to amino acids that are re-synthesized from the 13C-enriched precursor molecules within consumers. The 'd13CAA values therefore mirror the pathways of synthetic activity of amino acid de novo amino acid synthesis production in consumers. The proposed bridged perspective between nitrogen and carbon isotopes can refine of our knowledge on the potential processes affecting the isotopic fractionation within diet and consumer compartments as well as environmental samples