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

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

Chemical ecology of an apex predator life cycle

“…The mean TDF glu-phe across both the nematodes and bacteria in this food-chain was 7.18 ± 0.16‰, which did not represent a significant departure from the generalized 7.2‰ TDF glu-phe benchmark ( t22 = −0.14, P = 0.893). Given the degree of inter-trophic enrichment exhibited in the nematodes and bacteria, these consumer groups were consistent with the enrichment patterns of heterotrophs across terrestrial, marine, and freshwater systems, allowing for trophic position estimation using established isotopic protocols (Chikaraishi, 2007; Chikaraishi, 2009; Chikaraishi, 2011; Steffan, 2013; Steffan, 2015; Steffan, 2017; Takizawa, 2020).…”

“…This past work relied upon compound-specific isotopic analyses of select amino acid pools, particularly the degree of 15 N-enrichment between two amino acids— glutamic acid (glu) and phenylalanine (phe). The differential enrichment between these two amino acids provides a measure of inter-trophic enrichment, which is largely attributable to an organism’s assimilation of dietary amino acids (Chikaraishi, 2007; Chikaraishi, 2009; Takizawa, 2020). Such inter-trophic enrichment has been referred to as the trophic discrimination factor (TDF glu-phe ), and in carefully controlled feeding studies among diverse consumer groups in the Animalia, Fungi, and Bacteria, the TDF glu-phe has averaged approximately 7.2‰ (Steffan, 2015; Steffan, 2017).…”

Experimentally derived trophic enrichment and discrimination factors for Chinook salmon, Oncorhynchus tshawytscha

“…As with bulk SIA, accurate AA‐specific TEFs are critical to applying CSIA‐AA 25 . The utility of CSIA is derived from the recognition that there can be consistent fractionation in certain AAs, even though the sources of variation in AA TEFs are still under study 24,26,27 . With respect to nitrogen, AAs can be classified into three categories: source, trophic and other or metabolic 24,26,27 .…”

“…The utility of CSIA is derived from the recognition that there can be consistent fractionation in certain AAs, even though the sources of variation in AA TEFs are still under study 24,26,27 . With respect to nitrogen, AAs can be classified into three categories: source, trophic and other or metabolic 24,26,27 . Nitrogen fractionation is believed to occur during AA transamination and deamination and therefore TEFs for source AAs should be ca 0‰ as these AAs experience little fractionation due to a lack of deamination or transamination reactions, whereas trophic AAs, which do experience these reactions, should have TEFs not equal to 0‰ 24,26,27 .…”

“…With respect to nitrogen, AAs can be classified into three categories: source, trophic and other or metabolic 24,26,27 . Nitrogen fractionation is believed to occur during AA transamination and deamination and therefore TEFs for source AAs should be ca 0‰ as these AAs experience little fractionation due to a lack of deamination or transamination reactions, whereas trophic AAs, which do experience these reactions, should have TEFs not equal to 0‰ 24,26,27 . Because transamination and deamination reactions can replace the nitrogen atom but leave the AA carbon skeleton intact, nitrogen and carbon do not share the same AA categories 24,27 .…”