A new method to analyze δ15N of phenylalanine and glutamic acid to reconstruct spatial and temporal variations in N sources and trophic structures

Lin Zhang, Texas A&M University Corpus Christi, Corpus Christi, TX, United States, Charlotte Wing Man Lee, Texas A&M University-Corpus Christi, Corpus Christi, United States, Chongxiao Ji, Texas A&M University Corpus Christi, Corpus Christi, United States, Rupsa Roy, University of Massachusetts Dartmouth, New Bedford, MA, United States, Ava Kreider-Mueller, University of Massachusetts Dartmouth, School for Marine Science and Technology, New Bedford, MA, United States and Mark A Altabet, Univ Massachusetts Darmouth, New Bedford, MA, United States
Compound-specific nitrogen (N) isotope analysis (δ15N) of individual amino acids (AA) is a powerful tool for tracing N source, N utilization patterns, and trophic fractionation in biogeochemical cycles and foodwebs. The δ15N of phenylalanine (Phe) retains the baseline δ15N values, decoupled from the effects of trophic transfer, while glutamic acid (Glu) demonstrates significant 15N enrichment with each trophic transfer. The δ15N of Glu and Phe in one single sample can thus yield information of both the N source and trophic position, which provides a better interpretive power than bulk δ15N.

We developed a method using Ion Exchange Chromatography –Pulsed Amperometric Detector to separate and collect underivatized AAs. Collected AAs are directly oxidized to nitrite by hypochlorite, which skips the evaporation to dryness step required by the Gas Chromatography-Combustion-Isotopic Ratio Mass Spectrometry (GC-C-IRMS) approach. Hypochlorite specifically oxidize the amino-N only, greatly reducing the potential interferences from other N species. Resulted nitrite can be converted to N2O via azide and δ15 N2O is measured using Purge Trap-IRMS. Our method can analyze 17 AAs with high precision (<±0.5‰). The accuracy of our method was tested by analyzing 4 individual glutamic acid standards (USGS 40 and 41a) and 4 in-house phenylalanine standards with a wide range of δ15N at natural abundance levels. Their δ15N values measured by our method are linearly correlated with the values provided by USGS with slopes of 1, proving that δ15N can be retained with high reliability after multi-preparation steps. This method allows a precise and sensitive δ15N measurement of most amino acids, especially the two important “source (Phe)” and “trophic (Glu)” AAs. Gulf of California, Equatorial Pacific, and Sargasso Sea have different spatial and temporal N sources and utilization patterns caused by wind-driven upwelling and/or El Niño. Analyzing the δ15N of Phe and Glu in sinking particles and sediments in these regions allow to reconstruct the variations of N sources and trophic structures.