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

Stable nitrogen ( 15 N/ 14 N) and carbon ( 13 C/ 12 C) 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 (Δ δ 15 N AA and Δ δ 13 C AA , respectively) using four consumer–diet pairs. Each pairing illustrates a metabolic perspective of isotopic fractionation of amino acids. The Δ δ 15 N AA 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 14 N amino group from diet-derived amino acids, leaving behind the 15 N-enriched amino acids in consumer biomass. The Δ δ 15 N AA values thus mirror the activity of amino acid deamination in consumers. In contrast, the trends in the Δ δ 13 C AA value suggest a different metabolic fate for the amino acid carbon isotope. Based on our results, we predict the following scenario: decarboxylation preferentially removes 12 C α -carbon (i.e., carbonyl-carbon) from pyruvic acid in glycolysis, and from α -ketoglutaric acid in the tricarboxylic acid cycle, leaving behind the 13 C-enriched both pyruvic and α -ketoglutaric acids. The 13 C is then transferred to amino acids that are synthesized from the 13 C-enriched precursor molecules within consumers. The Δ δ 13 C AA 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