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 δ¹⁵ N _AA and Δ δ¹³ C _AA , respectively) using four consumer–diet pairs. Each pairing illustrates a metabolic perspective of isotopic fractionation of amino acids. The Δ δ¹⁵ 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 δ¹⁵ N _AA values thus mirror the activity of amino acid deamination in consumers. In contrast, the trends in the Δ δ¹³ 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 δ¹³ 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