Characterizing Organic Matter Composition Along Oxygen Gradients In The Eastern Tropical Pacific Oxygen Deficient Zone

Irina Koester1, Margot Elizabeth White1, Daniel Petras2, Sonia Vargas3, Pieter Dorrestein4, Michael Beman5 and Lihini Aluwihare6, (1)University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (2)University of California San Diego, Scripps Institution of Oceanography, La Jolla, United States, (3)University of California, Merced, Quantitative and Systems Biology, Merced, CA, United States, (4)University of California San Diego, Collaborative Mass Spectrometry Innovation Center, La Jolla, CA, United States, (5)University of California, Merced, Environmental Systems, Merced, CA, United States, (6)Scripps Institution of Oceanography, La Jolla, United States
Biogeochemical processes that take place within marine oxygen deficient zones (ODZ) influence and are influenced by the quantity and quality of organic matter moving through these regions. For example, the identity and rate of nitrogen loss processes occurring in ODZs are likely affected by the amount and composition of organic nitrogen that is available. The unique metabolisms of microorganisms within oxygen deficient environments may alter organic matter composition in unexpected ways. Thorough examination of organic matter composition will therefore improve our understanding of the cycling of multiple elements in ODZs. This study characterizes organic matter and its relationship to microbial processes along vertical and horizontal oxygen gradients in the Eastern Tropical North Pacific (ETNP) ODZ. Samples (n=207) were collected from the center of the ETNP ODZ to stations outside the ODZ in April 2017 and June 2018. We quantified and characterized organic matter in the form of total, dissolved, and particulate organic carbon, nitrogen, and phosphorus, as well as fluorescent and chromophoric DOM. Organic matter was characterized on a molecular level by high-resolution liquid chromatography tandem mass spectrometry (LC MS/MS). This approach coupled a 2D-LC separation and untargeted mass spectrometry with novel bioinformatics tools—including molecular networking, in-silico elemental formula annotation, and machine learning based prediction of compound categories (e.g., functional groups). Bulk measurements of DOM composition showed marked changes within the ODZ that were evident at the molecular level with the untargeted MS/MS data. Paired measurements of the microbial community (16S and metagenomics) and carbon and nitrogen transformation rates demonstrated the role of organic matter composition in shaping the observed biogeochemical gradients. Collectively these results are indicative of dynamic organic matter cycling within the ETNP ODZ.