Award: OCE-1829834

Amino acids are an important component of all living things and the organic matter that is derived from them. Natural variations in their nitrogen stable isotope ratio (15N/14N; δ15N) have been used as powerful tools to reconstruct food webs as well as the source and fate of organic matter produced from them. However, methods used to date have been expensive and time consuming as well as limited in the kinds of amino acids that could be examined. We have developed a new and easier method for δ15N analysis of individual amino acids that extends the number of amino acids that can be analyzed as well as has the potential for determining variations in δ15N between the N atoms within an amino acid. An optimal procedure was developed for separation and collection of individual amino acids using ion chromatography and pretreatment of biological samples. Separated amino acids are then converted via nitrite to nitrous oxide for isotope analysis. The reliability and robustness of this method were verified by testing standard mixtures with known δ15N values and various samples including fish (Striped Mullet), cyanobacteria (Spirulina Pacifica), and sediments (Gulf of California). To apply these new methods, sinking/suspended particles and zooplankton samples were collected at the surface, deep chlorophyll maximum (DCM), anoxic core, and lower oxycline in the Eastern Tropical North Pacific in December 2020. These samples are analyzed for the δ15N of two amino acids (phenylalanine [Phe] and glutamic acid [Glu]) to retrieve information on both the base of the food web and number of trophic steps that produced the sample material. The preliminary results from suspended particles showed a low δ15NPhe which decreased to a minimum at the deep chlorophyll maximum (DCM), suggesting changes with depth in the phytoplankton communities contributing to these particles. δ15NPhe increased with depth below the DCM toward the lower oxycline. The vertical profile of δ15NGlu exhibited a similar trend to that of δ15NPhe, yielding relatively constant trophic stepping from DCM to the lower oxycline which suggests the planktonic origin of these suspended particles. The increasing trend in both δ15NPhe and δ15NGlu with depth implies that the protein materials in suspended particles experienced similar degradation processes, probably by heterotrophic microbes. The δ15NPhe and δ15NGlu data in sinking particles and zooplankton will further help elucidate the interactions between the two particle pools and zooplankton in the anoxic core. Last Modified: 01/19/2023 Submitted by: Mark A Altabet