Metabolites are small organic compounds that are the products of cellular metabolism and the building blocks of macromolecules. The analysis of a multitude of metabolites in a sample simultaneously is known as metabolomics and is a powerful tool for understanding microbial interactions in the ocean. In particular, metabolomics provides a way to investigate how marine communities vary in composition during shifts in environmental conditions. The North Pacific Subtropical Gyre (NPSG) is a region where inorganic nitrogen availability limits phytoplankton productivity and microorganisms rely partially on diazotrophs for fixed nitrogen in the surface ocean. Because N2 fixation is often iron-limited, bioavailable iron should control fixed nitrogen levels in the gyre. Here, we tested this hypothesis by collecting metabolomic samples during a large-volume incubation in which tanks were amended with various nutrient combinations of iron, nitrogen, and phosphate during a month-long incubation. It was expected to stimulate a diazotroph bloom by limiting the incubation for nitrogen. In these nitrogen-limited tanks, we expect to see a strong metabolic response to the absence of fixed nitrogen, followed by the ingrowth of nitrogen fixers with their own metabolite fingerprints as the experiment progresses. I will compare metabolomes of incubations to those of phytoplankton cultures, including the nitrogen-fixing cyanobacteria UCYN-A and Trichodesmium. I will also use the incubation's nutrient concentration and microbial community metabolomics to test the hypothesis that altering nutrient supply ratios (Fe: N: P) in the NPSG microbial population will result in metabolite shifts. As the critical link between inorganic matter and the formation of the organic material that powers the ocean’s food chains and biological carbon pump, metabolomics provides a way to better understand the critical role that nitrogen fixation plays in regulating the taxonomy and biochemistry of the world’s largest biomes.

About one-quarter of photosynthetically fixed carbon is cycled through the marine microbial community in the form of metabolites, the intermediate compounds or products of metabolic processes. Marine heterotrophic bacteria are largely responsible for consuming these metabolites as a source of carbon, energy, and nutrients, yet little is known about transporter affinity and uptake kinetics of bacteria for abundant substrates. Homarine is a small, nitrogen-containing, zwitterionic metabolite that is produced by the cyanobacterium Synechococcus as well as some diatoms and haptophytes, where it is thought to function as an osmolyte. Dissolved homarine is present in the ocean at very low concentrations (~1.1 nM in Puget Sound). I hypothesize that these low concentrations are the result of high affinity bacterial transporters for homarine. Homarine can be used as a sole carbon and nitrogen source for OBi1, a marine bacterium isolated from Puget Sound. In this study, I investigate the uptake kinetics of homarine by OBi1 in the lab using the Michaelis-Menten model. I compare the uptake kinetics of OBi1 to similar homarine uptake experiments in the Salish Sea in June 2019. I expect that OBi1 will have a high affinity for homarine uptake and will take up homarine at nanomolar concentrations. I also anticipate that the marine microbial community in Puget Sound will have similar uptake kinetics to those observed with OBi1. Understanding the uptake kinetics of homarine by marine bacteria sheds light on the cycling of homarine in marine environments like Puget Sound and can help us understand the processes that keep the dissolved homarine concentration so low.