The Granger Lab studies Nitrogen in the marine environment, to understand the influence of this essential nutrient on ocean fertility and on the biological carbon pump. We strive to uncover how the combined activities of diverse microbes in the ocean contribute to the production, cycling, and loss of reactive N from the ocean, and thus influence ocean fertility. To this end, we measure the naturally occurring stable Nitrogen and Oxygen isotope ratios of nitrate and of other N species, which track the microbially-controlled transformations of reactive N, as well as its physical dispersion throughout the ocean. Current research activities focus on understanding the physiological bases of microbial N transformations and associated N isotopic imprints in laboratory cultures, to provide benchmarks from which to interpret N isotope distributions in the environment. We are also conducting regional and basin-scale surveys of nitrate isotope distributions in diverse ocean regions to investigate N cycling therein. Our work thus reflects the different scales of nitrogen biogeochemistry, from biochemical to global.